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Updated SECSH-WG core drafts
Attached are updated versions of the SECSH-WG four core drafts.
The only changes are the addition of an 'Intellectual Property' section and
the removal of the previous 'Trademark Issues' section.
Since all four documents were revised the references have been udpated
as well.
--
Darren J Moffat
Network Working Group T. Ylonen
Internet-Draft T. Kivinen
Expires: March 21, 2003 SSH Communications Security Corp
M. Saarinen
University of Jyvaskyla
T. Rinne
S. Lehtinen
SSH Communications Security Corp
September 20, 2002
SSH Protocol Architecture
draft-ietf-secsh-architecture-13.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on March 21, 2003.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
SSH is a protocol for secure remote login and other secure network
services over an insecure network. This document describes the
architecture of the SSH protocol, as well as the notation and
terminology used in SSH protocol documents. It also discusses the
SSH algorithm naming system that allows local extensions. The SSH
protocol consists of three major components: The Transport Layer
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Protocol provides server authentication, confidentiality, and
integrity with perfect forward secrecy. The User Authentication
Protocol authenticates the client to the server. The Connection
Protocol multiplexes the encrypted tunnel into several logical
channels. Details of these protocols are described in separate
documents.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Specification of Requirements . . . . . . . . . . . . . . . . 3
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1 Host Keys . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2 Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3 Policy Issues . . . . . . . . . . . . . . . . . . . . . . . . 5
3.4 Security Properties . . . . . . . . . . . . . . . . . . . . . 6
3.5 Packet Size and Overhead . . . . . . . . . . . . . . . . . . . 6
3.6 Localization and Character Set Support . . . . . . . . . . . . 7
4. Data Type Representations Used in the SSH Protocols . . . . . 8
5. Algorithm Naming . . . . . . . . . . . . . . . . . . . . . . . 10
6. Message Numbers . . . . . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. Intellectual Property . . . . . . . . . . . . . . . . . . . . 12
10. Additional Information . . . . . . . . . . . . . . . . . . . . 12
References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
SSH is a protocol for secure remote login and other secure network
services over an insecure network. It consists of three major
components:
o The Transport Layer Protocol [SSH-TRANS] provides server
authentication, confidentiality, and integrity. It may optionally
also provide compression. The transport layer will typically be
run over a TCP/IP connection, but might also be used on top of any
other reliable data stream.
o The User Authentication Protocol [SSH-USERAUTH] authenticates the
client-side user to the server. It runs over the transport layer
protocol.
o The Connection Protocol [SSH-CONNECT] multiplexes the encrypted
tunnel into several logical channels. It runs over the user
authentication protocol.
The client sends a service request once a secure transport layer
connection has been established. A second service request is sent
after user authentication is complete. This allows new protocols to
be defined and coexist with the protocols listed above.
The connection protocol provides channels that can be used for a wide
range of purposes. Standard methods are provided for setting up
secure interactive shell sessions and for forwarding ("tunneling")
arbitrary TCP/IP ports and X11 connections.
2. Specification of Requirements
All documents related to the SSH protocols shall use the keywords
"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" to describe
requirements. They are to be interpreted as described in [RFC-2119].
3. Architecture
3.1 Host Keys
Each server host SHOULD have a host key. Hosts MAY have multiple
host keys using multiple different algorithms. Multiple hosts MAY
share the same host key. If a host has keys at all, it MUST have at
least one key using each REQUIRED public key algorithm (currently DSS
[FIPS-186]).
The server host key is used during key exchange to verify that the
client is really talking to the correct server. For this to be
possible, the client must have a priori knowledge of the server's
public host key.
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Two different trust models can be used:
o The client has a local database that associates each host name (as
typed by the user) with the corresponding public host key. This
method requires no centrally administered infrastructure, and no
third-party coordination. The downside is that the database of
name-to-key associations may become burdensome to maintain.
o The host name-to-key association is certified by some trusted
certification authority. The client only knows the CA root key,
and can verify the validity of all host keys certified by accepted
CAs.
The second alternative eases the maintenance problem, since
ideally only a single CA key needs to be securely stored on the
client. On the other hand, each host key must be appropriately
certified by a central authority before authorization is possible.
Also, a lot of trust is placed on the central infrastructure.
The protocol provides the option that the server name - host key
association is not checked when connecting to the host for the first
time. This allows communication without prior communication of host
keys or certification. The connection still provides protection
against passive listening; however, it becomes vulnerable to active
man-in-the-middle attacks. Implementations SHOULD NOT normally allow
such connections by default, as they pose a potential security
problem. However, as there is no widely deployed key infrastructure
available on the Internet yet, this option makes the protocol much
more usable during the transition time until such an infrastructure
emerges, while still providing a much higher level of security than
that offered by older solutions (e.g. telnet [RFC-854] and rlogin
[RFC-1282]).
Implementations SHOULD try to make the best effort to check host
keys. An example of a possible strategy is to only accept a host key
without checking the first time a host is connected, save the key in
a local database, and compare against that key on all future
connections to that host.
Implementations MAY provide additional methods for verifying the
correctness of host keys, e.g. a hexadecimal fingerprint derived
from the SHA-1 hash of the public key. Such fingerprints can easily
be verified by using telephone or other external communication
channels.
All implementations SHOULD provide an option to not accept host keys
that cannot be verified.
We believe that ease of use is critical to end-user acceptance of
security solutions, and no improvement in security is gained if the
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new solutions are not used. Thus, providing the option not to check
the server host key is believed to improve the overall security of
the Internet, even though it reduces the security of the protocol in
configurations where it is allowed.
3.2 Extensibility
We believe that the protocol will evolve over time, and some
organizations will want to use their own encryption, authentication
and/or key exchange methods. Central registration of all extensions
is cumbersome, especially for experimental or classified features.
On the other hand, having no central registration leads to conflicts
in method identifiers, making interoperability difficult.
We have chosen to identify algorithms, methods, formats, and
extension protocols with textual names that are of a specific format.
DNS names are used to create local namespaces where experimental or
classified extensions can be defined without fear of conflicts with
other implementations.
One design goal has been to keep the base protocol as simple as
possible, and to require as few algorithms as possible. However, all
implementations MUST support a minimal set of algorithms to ensure
interoperability (this does not imply that the local policy on all
hosts would necessary allow these algorithms). The mandatory
algorithms are specified in the relevant protocol documents.
Additional algorithms, methods, formats, and extension protocols can
be defined in separate drafts. See Section Algorithm Naming (Section
5) for more information.
3.3 Policy Issues
The protocol allows full negotiation of encryption, integrity, key
exchange, compression, and public key algorithms and formats.
Encryption, integrity, public key, and compression algorithms can be
different for each direction.
The following policy issues SHOULD be addressed in the configuration
mechanisms of each implementation:
o Encryption, integrity, and compression algorithms, separately for
each direction. The policy MUST specify which is the preferred
algorithm (e.g. the first algorithm listed in each category).
o Public key algorithms and key exchange method to be used for host
authentication. The existence of trusted host keys for different
public key algorithms also affects this choice.
o The authentication methods that are to be required by the server
for each user. The server's policy MAY require multiple
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authentication for some or all users. The required algorithms MAY
depend on the location where the user is trying to log in from.
o The operations that the user is allowed to perform using the
connection protocol. Some issues are related to security; for
example, the policy SHOULD NOT allow the server to start sessions
or run commands on the client machine, and MUST NOT allow
connections to the authentication agent unless forwarding such
connections has been requested. Other issues, such as which
TCP/IP ports can be forwarded and by whom, are clearly issues of
local policy. Many of these issues may involve traversing or
bypassing firewalls, and are interrelated with the local security
policy.
3.4 Security Properties
The primary goal of the SSH protocol is improved security on the
Internet. It attempts to do this in a way that is easy to deploy,
even at the cost of absolute security.
o All encryption, integrity, and public key algorithms used are
well-known, well-established algorithms.
o All algorithms are used with cryptographically sound key sizes
that are believed to provide protection against even the strongest
cryptanalytic attacks for decades.
o All algorithms are negotiated, and in case some algorithm is
broken, it is easy to switch to some other algorithm without
modifying the base protocol.
Specific concessions were made to make wide-spread fast deployment
easier. The particular case where this comes up is verifying that
the server host key really belongs to the desired host; the protocol
allows the verification to be left out (but this is NOT RECOMMENDED).
This is believed to significantly improve usability in the short
term, until widespread Internet public key infrastructures emerge.
3.5 Packet Size and Overhead
Some readers will worry about the increase in packet size due to new
headers, padding, and MAC. The minimum packet size is in the order
of 28 bytes (depending on negotiated algorithms). The increase is
negligible for large packets, but very significant for one-byte
packets (telnet-type sessions). There are, however, several factors
that make this a non-issue in almost all cases:
o The minimum size of a TCP/IP header is 32 bytes. Thus, the
increase is actually from 33 to 51 bytes (roughly).
o The minimum size of the data field of an Ethernet packet is 46
bytes [RFC-894]. Thus, the increase is no more than 5 bytes.
When Ethernet headers are considered, the increase is less than 10
percent.
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o The total fraction of telnet-type data in the Internet is
negligible, even with increased packet sizes.
The only environment where the packet size increase is likely to have
a significant effect is PPP [RFC-1134] over slow modem lines (PPP
compresses the TCP/IP headers, emphasizing the increase in packet
size). However, with modern modems, the time needed to transfer is
in the order of 2 milliseconds, which is a lot faster than people can
type.
There are also issues related to the maximum packet size. To
minimize delays in screen updates, one does not want excessively
large packets for interactive sessions. The maximum packet size is
negotiated separately for each channel.
3.6 Localization and Character Set Support
For the most part, the SSH protocols do not directly pass text that
would be displayed to the user. However, there are some places where
such data might be passed. When applicable, the character set for
the data MUST be explicitly specified. In most places, ISO 10646
with UTF-8 encoding is used [RFC-2279]. When applicable, a field is
also provided for a language tag [RFC-1766].
One big issue is the character set of the interactive session. There
is no clear solution, as different applications may display data in
different formats. Different types of terminal emulation may also be
employed in the client, and the character set to be used is
effectively determined by the terminal emulation. Thus, no place is
provided for directly specifying the character set or encoding for
terminal session data. However, the terminal emulation type (e.g.
"vt100") is transmitted to the remote site, and it implicitly
specifies the character set and encoding. Applications typically use
the terminal type to determine what character set they use, or the
character set is determined using some external means. The terminal
emulation may also allow configuring the default character set. In
any case, the character set for the terminal session is considered
primarily a client local issue.
Internal names used to identify algorithms or protocols are normally
never displayed to users, and must be in US-ASCII.
The client and server user names are inherently constrained by what
the server is prepared to accept. They might, however, occasionally
be displayed in logs, reports, etc. They MUST be encoded using ISO
10646 UTF-8, but other encodings may be required in some cases. It
is up to the server to decide how to map user names to accepted user
names. Straight bit-wise binary comparison is RECOMMENDED.
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For localization purposes, the protocol attempts to minimize the
number of textual messages transmitted. When present, such messages
typically relate to errors, debugging information, or some externally
configured data. For data that is normally displayed, it SHOULD be
possible to fetch a localized message instead of the transmitted
message by using a numerical code. The remaining messages SHOULD be
configurable.
4. Data Type Representations Used in the SSH Protocols
byte
A byte represents an arbitrary 8-bit value (octet) [RFC-1700].
Fixed length data is sometimes represented as an array of bytes,
written byte[n], where n is the number of bytes in the array.
boolean
A boolean value is stored as a single byte. The value 0
represents FALSE, and the value 1 represents TRUE. All non-zero
values MUST be interpreted as TRUE; however, applications MUST NOT
store values other than 0 and 1.
uint32
Represents a 32-bit unsigned integer. Stored as four bytes in the
order of decreasing significance (network byte order). For
example, the value 699921578 (0x29b7f4aa) is stored as 29 b7 f4
aa.
uint64
Represents a 64-bit unsigned integer. Stored as eight bytes in
the order of decreasing significance (network byte order).
string
Arbitrary length binary string. Strings are allowed to contain
arbitrary binary data, including null characters and 8-bit
characters. They are stored as a uint32 containing its length
(number of bytes that follow) and zero (= empty string) or more
bytes that are the value of the string. Terminating null
characters are not used.
Strings are also used to store text. In that case, US-ASCII is
used for internal names, and ISO-10646 UTF-8 for text that might
be displayed to the user. The terminating null character SHOULD
NOT normally be stored in the string.
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For example, the US-ASCII string "testing" is represented as 00 00
00 07 t e s t i n g. The UTF8 mapping does not alter the encoding
of US-ASCII characters.
mpint
Represents multiple precision integers in two's complement format,
stored as a string, 8 bits per byte, MSB first. Negative numbers
have the value 1 as the most significant bit of the first byte of
the data partition. If the most significant bit would be set for
a positive number, the number MUST be preceded by a zero byte.
Unnecessary leading bytes with the value 0 or 255 MUST NOT be
included. The value zero MUST be stored as a string with zero
bytes of data.
By convention, a number that is used in modular computations in
Z_n SHOULD be represented in the range 0 <= x < n.
Examples:
value (hex) representation (hex)
---------------------------------------------------------------
0 00 00 00 00
9a378f9b2e332a7 00 00 00 08 09 a3 78 f9 b2 e3 32 a7
80 00 00 00 02 00 80
-1234 00 00 00 02 ed cc
-deadbeef 00 00 00 05 ff 21 52 41 11
name-list
A string containing a comma separated list of names. A name list
is represented as a uint32 containing its length (number of bytes
that follow) followed by a comma-separated list of zero or more
names. A name MUST be non-zero length, and it MUST NOT contain a
comma (','). Context may impose additional restrictions on the
names; for example, the names in a list may have to be valid
algorithm identifier (see Algorithm Naming below), or [RFC-1766]
language tags. The order of the names in a list may or may not be
significant, also depending on the context where the list is is
used. Terminating NUL characters are not used, neither for the
individual names, nor for the list as a whole.
Examples:
value representation (hex)
---------------------------------------
(), the empty list 00 00 00 00
("zlib") 00 00 00 04 7a 6c 69 62
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("zlib", "none") 00 00 00 09 7a 6c 69 62 2c 6e 6f 6e 65
5. Algorithm Naming
The SSH protocols refer to particular hash, encryption, integrity,
compression, and key exchange algorithms or protocols by names.
There are some standard algorithms that all implementations MUST
support. There are also algorithms that are defined in the protocol
specification but are OPTIONAL. Furthermore, it is expected that
some organizations will want to use their own algorithms.
In this protocol, all algorithm identifiers MUST be printable US-
ASCII non-empty strings no longer than 64 characters. Names MUST be
case-sensitive.
There are two formats for algorithm names:
o Names that do not contain an at-sign (@) are reserved to be
assigned by IETF consensus (RFCs). Examples include `3des-cbc',
`sha-1', `hmac-sha1', and `zlib' (the quotes are not part of the
name). Names of this format MUST NOT be used without first
registering them. Registered names MUST NOT contain an at-sign
(@) or a comma (,).
o Anyone can define additional algorithms by using names in the
format name@domainname, e.g. "ourcipher-cbc%ssh.com@localhost". The format
of the part preceding the at sign is not specified; it MUST
consist of US-ASCII characters except at-sign and comma. The part
following the at-sign MUST be a valid fully qualified internet
domain name [RFC-1034] controlled by the person or organization
defining the name. It is up to each domain how it manages its
local namespace.
6. Message Numbers
SSH packets have message numbers in the range 1 to 255. These
numbers have been allocated as follows:
Transport layer protocol:
1 to 19 Transport layer generic (e.g. disconnect, ignore, debug,
etc.)
20 to 29 Algorithm negotiation
30 to 49 Key exchange method specific (numbers can be reused for
different authentication methods)
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User authentication protocol:
50 to 59 User authentication generic
60 to 79 User authentication method specific (numbers can be
reused for different authentication methods)
Connection protocol:
80 to 89 Connection protocol generic
90 to 127 Channel related messages
Reserved for client protocols:
128 to 191 Reserved
Local extensions:
192 to 255 Local extensions
7. IANA Considerations
Allocation of the following types of names in the SSH protocols is
assigned by IETF consensus:
o encryption algorithm names,
o MAC algorithm names,
o public key algorithm names (public key algorithm also implies
encoding and signature/encryption capability),
o key exchange method names, and
o protocol (service) names.
These names MUST be printable US-ASCII strings, and MUST NOT contain
the characters at-sign ('@'), comma (','), or whitespace or control
characters (ASCII codes 32 or less). Names are case-sensitive, and
MUST NOT be longer than 64 characters.
Names with the at-sign ('@') in them are allocated by the owner of
DNS name after the at-sign (hierarchical allocation in [RFC-2343]),
otherwise the same restrictions as above.
Each category of names listed above has a separate namespace.
However, using the same name in multiple categories SHOULD be avoided
to minimize confusion.
Message numbers (see Section Message Numbers (Section 6)) in the
range of 0..191 should be allocated via IETF consensus; message
numbers in the 192..255 range (the "Local extensions" set) are
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reserved for private use.
8. Security Considerations
Special care should be taken to ensure that all of the random numbers
are of good quality. The random numbers SHOULD be produced with safe
mechanisms discussed in [RFC-1750].
When displaying text, such as error or debug messages to the user,
the client software SHOULD replace any control characters (except
tab, carriage return and newline) with safe sequences to avoid
attacks by sending terminal control characters.
Not using MAC or encryption SHOULD be avoided. The user
authentication protocol is subject to man-in-the-middle attacks if
the encryption is disabled. The SSH protocol does not protect
against message alteration if no MAC is used.
9. Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification can
be obtained from the IETF Secretariat.
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
10. Additional Information
The current document editor is: Darren.Moffat%Sun.COM@localhost. Comments on
this internet draft should be sent to the IETF SECSH working group,
details at: http://ietf.org/html.charters/secsh-charter.html
References
[FIPS-186] Federal Information Processing Standards Publication,
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., "FIPS PUB 186, Digital Signature Standard", May
1994.
[RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol
Specification", STD 8, RFC 854, May 1983.
[RFC0894] Hornig, C., "Standard for the transmission of IP
datagrams over Ethernet networks", STD 41, RFC 894,
Apr 1984.
[RFC1034] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, Nov 1987.
[RFC1134] Perkins, D., "Point-to-Point Protocol: A proposal for
multi-protocol transmission of datagrams over Point-
to-Point links", RFC 1134, Nov 1989.
[RFC1282] Kantor, B., "BSD Rlogin", RFC 1282, December 1991.
[RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD
2, RFC 1700, October 1994.
[RFC1750] Eastlake, D., Crocker, S. and J. Schiller,
"Randomness Recommendations for Security", RFC 1750,
December 1994.
[RFC1766] Alvestrand, H., "Tags for the Identification of
Languages", RFC 1766, March 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
2434, October 1998.
[SSH-ARCH] Ylonen, T., "SSH Protocol Architecture", I-D draft-
ietf-architecture-13.txt, September 2002.
[SSH-TRANS] Ylonen, T., "SSH Transport Layer Protocol", I-D
draft-ietf-transport-15.txt, September 2002.
[SSH-USERAUTH] Ylonen, T., "SSH Authentication Protocol", I-D draft-
ietf-userauth-16.txt, September 2002.
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Internet-Draft SSH Protocol Architecture September 2002
[SSH-CONNECT] Ylonen, T., "SSH Connection Protocol", I-D draft-
ietf-connect-16.txt, September 2002.
Authors' Addresses
Tatu Ylonen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: ylo%ssh.com@localhost
Tero Kivinen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: kivinen%ssh.com@localhost
Markku-Juhani O. Saarinen
University of Jyvaskyla
Timo J. Rinne
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: tri%ssh.com@localhost
Sami Lehtinen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: sjl%ssh.com@localhost
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Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
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Internet organizations, except as needed for the purpose of
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English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Ylonen, et. al. Expires March 21, 2003 [Page 15]
Network Working Group T. Ylonen
Internet-Draft T. Kivinen
Expires: March 21, 2003 SSH Communications Security Corp
M. Saarinen
University of Jyvaskyla
T. Rinne
S. Lehtinen
SSH Communications Security Corp
September 20, 2002
SSH Connection Protocol
draft-ietf-secsh-connect-16.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on March 21, 2003.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
SSH is a protocol for secure remote login and other secure network
services over an insecure network.
This document describes the SSH Connection Protocol. It provides
interactive login sessions, remote execution of commands, forwarded
TCP/IP connections, and forwarded X11 connections. All of these
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channels are multiplexed into a single encrypted tunnel.
The SSH Connection Protocol has been designed to run on top of the
SSH transport layer and user authentication protocols.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Global Requests . . . . . . . . . . . . . . . . . . . . . . 3
3. Channel Mechanism . . . . . . . . . . . . . . . . . . . . . 3
3.1 Opening a Channel . . . . . . . . . . . . . . . . . . . . . 4
3.2 Data Transfer . . . . . . . . . . . . . . . . . . . . . . . 5
3.3 Closing a Channel . . . . . . . . . . . . . . . . . . . . . 6
3.4 Channel-Specific Requests . . . . . . . . . . . . . . . . . 7
4. Interactive Sessions . . . . . . . . . . . . . . . . . . . . 7
4.1 Opening a Session . . . . . . . . . . . . . . . . . . . . . 8
4.2 Requesting a Pseudo-Terminal . . . . . . . . . . . . . . . . 8
4.3 X11 Forwarding . . . . . . . . . . . . . . . . . . . . . . . 8
4.3.1 Requesting X11 Forwarding . . . . . . . . . . . . . . . . . 8
4.3.2 X11 Channels . . . . . . . . . . . . . . . . . . . . . . . . 9
4.4 Environment Variable Passing . . . . . . . . . . . . . . . . 10
4.5 Starting a Shell or a Command . . . . . . . . . . . . . . . 10
4.6 Session Data Transfer . . . . . . . . . . . . . . . . . . . 11
4.7 Window Dimension Change Message . . . . . . . . . . . . . . 11
4.8 Local Flow Control . . . . . . . . . . . . . . . . . . . . . 11
4.9 Signals . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.10 Returning Exit Status . . . . . . . . . . . . . . . . . . . 12
5. TCP/IP Port Forwarding . . . . . . . . . . . . . . . . . . . 13
5.1 Requesting Port Forwarding . . . . . . . . . . . . . . . . . 13
5.2 TCP/IP Forwarding Channels . . . . . . . . . . . . . . . . . 14
6. Encoding of Terminal Modes . . . . . . . . . . . . . . . . . 16
7. Summary of Message Numbers . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . 18
9. Intellectual Property . . . . . . . . . . . . . . . . . . . 19
10. Additional Information . . . . . . . . . . . . . . . . . . . 19
References . . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 20
Full Copyright Statement . . . . . . . . . . . . . . . . . . 22
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1. Introduction
The SSH Connection Protocol has been designed to run on top of the
SSH transport layer and user authentication protocols. It provides
interactive login sessions, remote execution of commands, forwarded
TCP/IP connections, and forwarded X11 connections. The service name
for this protocol (after user authentication) is "ssh-connection".
This document should be read only after reading the SSH architecture
document [SSH-ARCH]. This document freely uses terminology and
notation from the architecture document without reference or further
explanation.
2. Global Requests
There are several kinds of requests that affect the state of the
remote end "globally", independent of any channels. An example is a
request to start TCP/IP forwarding for a specific port. All such
requests use the following format.
byte SSH_MSG_GLOBAL_REQUEST
string request name (restricted to US-ASCII)
boolean want reply
... request-specific data follows
Request names follow the DNS extensibility naming convention outlined
in [SSH-ARCH].
The recipient will respond to this message with
SSH_MSG_REQUEST_SUCCESS or SSH_MSG_REQUEST_FAILURE if `want reply' is
TRUE.
byte SSH_MSG_REQUEST_SUCCESS
..... response specific data
Usually the response specific data is non-existent.
If the recipient does not recognize or support the request, it simply
responds with SSH_MSG_REQUEST_FAILURE.
byte SSH_MSG_REQUEST_FAILURE
3. Channel Mechanism
All terminal sessions, forwarded connections, etc. are channels.
Either side may open a channel. Multiple channels are multiplexed
into a single connection.
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Channels are identified by numbers at each end. The number referring
to a channel may be different on each side. Requests to open a
channel contain the sender's channel number. Any other channel-
related messages contain the recipient's channel number for the
channel.
Channels are flow-controlled. No data may be sent to a channel until
a message is received to indicate that window space is available.
3.1 Opening a Channel
When either side wishes to open a new channel, it allocates a local
number for the channel. It then sends the following message to the
other side, and includes the local channel number and initial window
size in the message.
byte SSH_MSG_CHANNEL_OPEN
string channel type (restricted to US-ASCII)
uint32 sender channel
uint32 initial window size
uint32 maximum packet size
... channel type specific data follows
The channel type is a name as described in the SSH architecture
document, with similar extension mechanisms. `sender channel' is a
local identifier for the channel used by the sender of this message.
`initial window size' specifies how many bytes of channel data can be
sent to the sender of this message without adjusting the window.
`Maximum packet size' specifies the maximum size of an individual
data packet that can be sent to the sender (for example, one might
want to use smaller packets for interactive connections to get better
interactive response on slow links).
The remote side then decides whether it can open the channel, and
responds with either
byte SSH_MSG_CHANNEL_OPEN_CONFIRMATION
uint32 recipient channel
uint32 sender channel
uint32 initial window size
uint32 maximum packet size
... channel type specific data follows
where `recipient channel' is the channel number given in the original
open request, and `sender channel' is the channel number allocated by
the other side, or
byte SSH_MSG_CHANNEL_OPEN_FAILURE
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uint32 recipient channel
uint32 reason code
string additional textual information (ISO-10646 UTF-8 [RFC2279])
string language tag (as defined in [RFC1766])
If the recipient of the SSH_MSG_CHANNEL_OPEN message does not support
the specified channel type, it simply responds with
SSH_MSG_CHANNEL_OPEN_FAILURE. The client MAY show the additional
information to the user. If this is done, the client software should
take the precautions discussed in [SSH-ARCH].
The following reason codes are defined:
#define SSH_OPEN_ADMINISTRATIVELY_PROHIBITED 1
#define SSH_OPEN_CONNECT_FAILED 2
#define SSH_OPEN_UNKNOWN_CHANNEL_TYPE 3
#define SSH_OPEN_RESOURCE_SHORTAGE 4
3.2 Data Transfer
The window size specifies how many bytes the other party can send
before it must wait for the window to be adjusted. Both parties use
the following message to adjust the window.
byte SSH_MSG_CHANNEL_WINDOW_ADJUST
uint32 recipient channel
uint32 bytes to add
After receiving this message, the recipient MAY send the given number
of bytes more than it was previously allowed to send; the window size
is incremented.
Data transfer is done with messages of the following type.
byte SSH_MSG_CHANNEL_DATA
uint32 recipient channel
string data
The maximum amount of data allowed is the current window size. The
window size is decremented by the amount of data sent. Both parties
MAY ignore all extra data sent after the allowed window is empty.
Additionally, some channels can transfer several types of data. An
example of this is stderr data from interactive sessions. Such data
can be passed with SSH_MSG_CHANNEL_EXTENDED_DATA messages, where a
separate integer specifies the type of the data. The available types
and their interpretation depend on the type of the channel.
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byte SSH_MSG_CHANNEL_EXTENDED_DATA
uint32 recipient_channel
uint32 data_type_code
string data
Data sent with these messages consumes the same window as ordinary
data.
Currently, only the following type is defined.
#define SSH_EXTENDED_DATA_STDERR 1
3.3 Closing a Channel
When a party will no longer send more data to a channel, it SHOULD
send SSH_MSG_CHANNEL_EOF.
byte SSH_MSG_CHANNEL_EOF
uint32 recipient_channel
No explicit response is sent to this message; however, the
application may send EOF to whatever is at the other end of the
channel. Note that the channel remains open after this message, and
more data may still be sent in the other direction. This message
does not consume window space and can be sent even if no window space
is available.
When either party wishes to terminate the channel, it sends
SSH_MSG_CHANNEL_CLOSE. Upon receiving this message, a party MUST
send back a SSH_MSG_CHANNEL_CLOSE unless it has already sent this
message for the channel. The channel is considered closed for a
party when it has both sent and received SSH_MSG_CHANNEL_CLOSE, and
the party may then reuse the channel number. A party MAY send
SSH_MSG_CHANNEL_CLOSE without having sent or received
SSH_MSG_CHANNEL_EOF.
byte SSH_MSG_CHANNEL_CLOSE
uint32 recipient_channel
This message does not consume window space and can be sent even if no
window space is available.
It is recommended that any data sent before this message is delivered
to the actual destination, if possible.
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3.4 Channel-Specific Requests
Many channel types have extensions that are specific to that
particular channel type. An example is requesting a pty (pseudo
terminal) for an interactive session.
All channel-specific requests use the following format.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient channel
string request type (restricted to US-ASCII)
boolean want reply
... type-specific data
If want reply is FALSE, no response will be sent to the request.
Otherwise, the recipient responds with either SSH_MSG_CHANNEL_SUCCESS
or SSH_MSG_CHANNEL_FAILURE, or request-specific continuation
messages. If the request is not recognized or is not supported for
the channel, SSH_MSG_CHANNEL_FAILURE is returned.
This message does not consume window space and can be sent even if no
window space is available. Request types are local to each channel
type.
The client is allowed to send further messages without waiting for
the response to the request.
request type names follow the DNS extensibility naming convention
outlined in [SSH-ARCH]
byte SSH_MSG_CHANNEL_SUCCESS
uint32 recipient_channel
byte SSH_MSG_CHANNEL_FAILURE
uint32 recipient_channel
These messages do not consume window space and can be sent even if no
window space is available.
4. Interactive Sessions
A session is a remote execution of a program. The program may be a
shell, an application, a system command, or some built-in subsystem.
It may or may not have a tty, and may or may not involve X11
forwarding. Multiple sessions can be active simultaneously.
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4.1 Opening a Session
A session is started by sending the following message.
byte SSH_MSG_CHANNEL_OPEN
string "session"
uint32 sender channel
uint32 initial window size
uint32 maximum packet size
Client implementations SHOULD reject any session channel open
requests to make it more difficult for a corrupt server to attack the
client.
4.2 Requesting a Pseudo-Terminal
A pseudo-terminal can be allocated for the session by sending the
following message.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient_channel
string "pty-req"
boolean want_reply
string TERM environment variable value (e.g., vt100)
uint32 terminal width, characters (e.g., 80)
uint32 terminal height, rows (e.g., 24)
uint32 terminal width, pixels (e.g., 640)
uint32 terminal height, pixels (e.g., 480)
string encoded terminal modes
The encoding of terminal modes is described in Section Encoding of
Terminal Modes (Section 6). Zero dimension parameters MUST be
ignored. The character/row dimensions override the pixel dimensions
(when nonzero). Pixel dimensions refer to the drawable area of the
window.
The dimension parameters are only informational.
The client SHOULD ignore pty requests.
4.3 X11 Forwarding
4.3.1 Requesting X11 Forwarding
X11 forwarding may be requested for a session by sending
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient channel
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string "x11-req"
boolean want reply
boolean single connection
string x11 authentication protocol
string x11 authentication cookie
uint32 x11 screen number
It is recommended that the authentication cookie that is sent be a
fake, random cookie, and that the cookie is checked and replaced by
the real cookie when a connection request is received.
X11 connection forwarding should stop when the session channel is
closed; however, already opened forwardings should not be
automatically closed when the session channel is closed.
If `single connection' is TRUE, only a single connection should be
forwarded. No more connections will be forwarded after the first, or
after the session channel has been closed.
The `x11 authentication protocol' is the name of the X11
authentication method used, e.g. "MIT-MAGIC-COOKIE-1".
The x11 authentication cookie MUST be hexadecimal encoded.
X Protocol is documented in [SCHEIFLER].
4.3.2 X11 Channels
X11 channels are opened with a channel open request. The resulting
channels are independent of the session, and closing the session
channel does not close the forwarded X11 channels.
byte SSH_MSG_CHANNEL_OPEN
string "x11"
uint32 sender channel
uint32 initial window size
uint32 maximum packet size
string originator address (e.g. "192.168.7.38")
uint32 originator port
The recipient should respond with SSH_MSG_CHANNEL_OPEN_CONFIRMATION
or SSH_MSG_CHANNEL_OPEN_FAILURE.
Implementations MUST reject any X11 channel open requests if they
have not requested X11 forwarding.
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4.4 Environment Variable Passing
Environment variables may be passed to the shell/command to be
started later. Uncontrolled setting of environment variables in a
privileged process can be a security hazard. It is recommended that
implementations either maintain a list of allowable variable names or
only set environment variables after the server process has dropped
sufficient privileges.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient channel
string "env"
boolean want reply
string variable name
string variable value
4.5 Starting a Shell or a Command
Once the session has been set up, a program is started at the remote
end. The program can be a shell, an application program or a
subsystem with a host-independent name. Only one of these requests
can succeed per channel.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient channel
string "shell"
boolean want reply
This message will request the user's default shell (typically defined
in /etc/passwd in UNIX systems) to be started at the other end.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient channel
string "exec"
boolean want reply
string command
This message will request the server to start the execution of the
given command. The command string may contain a path. Normal
precautions MUST be taken to prevent the execution of unauthorized
commands.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient channel
string "subsystem"
boolean want reply
string subsystem name
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This last form executes a predefined subsystem. It is expected that
these will include a general file transfer mechanism, and possibly
other features. Implementations may also allow configuring more such
mechanisms. As the user's shell is usually used to execute the
subsystem, it is advisable for the subsystem protocol to have a
"magic cookie" at the beginning of the protocol transaction to
distinguish it from arbitrary output generated by shell
initialization scripts etc. This spurious output from the shell may
be filtered out either at the server or at the client.
The server SHOULD not halt the execution of the protocol stack when
starting a shell or a program. All input and output from these
SHOULD be redirected to the channel or to the encrypted tunnel.
It is RECOMMENDED to request and check the reply for these messages.
The client SHOULD ignore these messages.
Subsystem names follow the DNS extensibility naming convention
outlined in [SSH-ARCH].
4.6 Session Data Transfer
Data transfer for a session is done using SSH_MSG_CHANNEL_DATA and
SSH_MSG_CHANNEL_EXTENDED_DATA packets and the window mechanism. The
extended data type SSH_EXTENDED_DATA_STDERR has been defined for
stderr data.
4.7 Window Dimension Change Message
When the window (terminal) size changes on the client side, it MAY
send a message to the other side to inform it of the new dimensions.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient_channel
string "window-change"
boolean FALSE
uint32 terminal width, columns
uint32 terminal height, rows
uint32 terminal width, pixels
uint32 terminal height, pixels
No response SHOULD be sent to this message.
4.8 Local Flow Control
On many systems, it is possible to determine if a pseudo-terminal is
using control-S/control-Q flow control. When flow control is
allowed, it is often desirable to do the flow control at the client
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end to speed up responses to user requests. This is facilitated by
the following notification. Initially, the server is responsible for
flow control. (Here, again, client means the side originating the
session, and server means the other side.)
The message below is used by the server to inform the client when it
can or cannot perform flow control (control-S/control-Q processing).
If `client can do' is TRUE, the client is allowed to do flow control
using control-S and control-Q. The client MAY ignore this message.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient channel
string "xon-xoff"
boolean FALSE
boolean client can do
No response is sent to this message.
4.9 Signals
A signal can be delivered to the remote process/service using the
following message. Some systems may not implement signals, in which
case they SHOULD ignore this message.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient channel
string "signal"
boolean FALSE
string signal name without the "SIG" prefix.
Signal names will be encoded as discussed in the "exit-signal"
SSH_MSG_CHANNEL_REQUEST.
4.10 Returning Exit Status
When the command running at the other end terminates, the following
message can be sent to return the exit status of the command.
Returning the status is RECOMMENDED. No acknowledgment is sent for
this message. The channel needs to be closed with
SSH_MSG_CHANNEL_CLOSE after this message.
The client MAY ignore these messages.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient_channel
string "exit-status"
boolean FALSE
uint32 exit_status
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The remote command may also terminate violently due to a signal.
Such a condition can be indicated by the following message. A zero
exit_status usually means that the command terminated successfully.
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient channel
string "exit-signal"
boolean FALSE
string signal name without the "SIG" prefix.
boolean core dumped
string error message (ISO-10646 UTF-8)
string language tag (as defined in [RFC1766])
The signal name is one of the following (these are from [POSIX])
ABRT
ALRM
FPE
HUP
ILL
INT
KILL
PIPE
QUIT
SEGV
TERM
USR1
USR2
Additional signal names MAY be sent in the format "sig-name@xyz",
where `sig-name' and `xyz' may be anything a particular implementor
wants (except the `@' sign). However, it is suggested that if a
`configure' script is used, the non-standard signal names it finds be
encoded as "SIG@xyz.config.guess", where `SIG' is the signal name
without the "SIG" prefix, and `xyz' be the host type, as determined
by `config.guess'.
The `error message' contains an additional explanation of the error
message. The message may consist of multiple lines. The client
software MAY display this message to the user. If this is done, the
client software should take the precautions discussed in [SSH-ARCH].
5. TCP/IP Port Forwarding
5.1 Requesting Port Forwarding
A party need not explicitly request forwardings from its own end to
the other direction. However, if it wishes that connections to a
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port on the other side be forwarded to the local side, it must
explicitly request this.
byte SSH_MSG_GLOBAL_REQUEST
string "tcpip-forward"
boolean want reply
string address to bind (e.g. "0.0.0.0")
uint32 port number to bind
`Address to bind' and `port number to bind' specify the IP address
and port to which the socket to be listened is bound. The address
should be "0.0.0.0" if connections are allowed from anywhere. (Note
that the client can still filter connections based on information
passed in the open request.)
Implementations should only allow forwarding privileged ports if the
user has been authenticated as a privileged user.
Client implementations SHOULD reject these messages; they are
normally only sent by the client.
If a client passes 0 as port number to bind and has want reply TRUE
then the server allocates the next available unprivileged port number
and replies with the following message, otherwise there is no
response specific data.
byte SSH_MSG_GLOBAL_REQUEST_SUCCESS
uint32 port that was bound on the server
A port forwarding can be cancelled with the following message. Note
that channel open requests may be received until a reply to this
message is received.
byte SSH_MSG_GLOBAL_REQUEST
string "cancel-tcpip-forward"
boolean want reply
string address_to_bind (e.g. "127.0.0.1")
uint32 port number to bind
Client implementations SHOULD reject these messages; they are
normally only sent by the client.
5.2 TCP/IP Forwarding Channels
When a connection comes to a port for which remote forwarding has
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been requested, a channel is opened to forward the port to the other
side.
byte SSH_MSG_CHANNEL_OPEN
string "forwarded-tcpip"
uint32 sender channel
uint32 initial window size
uint32 maximum packet size
string address that was connected
uint32 port that was connected
string originator IP address
uint32 originator port
Implementations MUST reject these messages unless they have
previously requested a remote TCP/IP port forwarding with the given
port number.
When a connection comes to a locally forwarded TCP/IP port, the
following packet is sent to the other side. Note that these messages
MAY be sent also for ports for which no forwarding has been
explicitly requested. The receiving side must decide whether to
allow the forwarding.
byte SSH_MSG_CHANNEL_OPEN
string "direct-tcpip"
uint32 sender channel
uint32 initial window size
uint32 maximum packet size
string host to connect
uint32 port to connect
string originator IP address
uint32 originator port
`Host to connect' and `port to connect' specify the TCP/IP host and
port where the recipient should connect the channel. `Host to
connect' may be either a domain name or a numeric IP address.
`Originator IP address' is the numeric IP address of the machine
where the connection request comes from, and `originator port' is the
port on the originator host from where the connection came from.
Forwarded TCP/IP channels are independent of any sessions, and
closing a session channel does not in any way imply that forwarded
connections should be closed.
Client implementations SHOULD reject direct TCP/IP open requests for
security reasons.
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6. Encoding of Terminal Modes
Terminal modes (as passed in a pty request) are encoded into a byte
stream. It is intended that the coding be portable across different
environments.
The tty mode description is a stream of bytes. The stream consists
of opcode-argument pairs. It is terminated by opcode TTY_OP_END (0).
Opcodes 1 to 159 have a single uint32 argument. Opcodes 160 to 255
are not yet defined, and cause parsing to stop (they should only be
used after any other data).
The client SHOULD put in the stream any modes it knows about, and the
server MAY ignore any modes it does not know about. This allows some
degree of machine-independence, at least between systems that use a
POSIX-like tty interface. The protocol can support other systems as
well, but the client may need to fill reasonable values for a number
of parameters so the server pty gets set to a reasonable mode (the
server leaves all unspecified mode bits in their default values, and
only some combinations make sense).
The following opcodes have been defined. The naming of opcodes
mostly follows the POSIX terminal mode flags.
0 TTY_OP_END Indicates end of options.
1 VINTR Interrupt character; 255 if none. Similarly for the
other characters. Not all of these characters are
supported on all systems.
2 VQUIT The quit character (sends SIGQUIT signal on POSIX
systems).
3 VERASE Erase the character to left of the cursor.
4 VKILL Kill the current input line.
5 VEOF End-of-file character (sends EOF from the terminal).
6 VEOL End-of-line character in addition to carriage return
and/or linefeed.
7 VEOL2 Additional end-of-line character.
8 VSTART Continues paused output (normally control-Q).
9 VSTOP Pauses output (normally control-S).
10 VSUSP Suspends the current program.
11 VDSUSP Another suspend character.
12 VREPRINT Reprints the current input line.
13 VWERASE Erases a word left of cursor.
14 VLNEXT Enter the next character typed literally, even if it
is a special character
15 VFLUSH Character to flush output.
16 VSWTCH Switch to a different shell layer.
17 VSTATUS Prints system status line (load, command, pid etc).
18 VDISCARD Toggles the flushing of terminal output.
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30 IGNPAR The ignore parity flag. The parameter SHOULD be 0 if
this flag is FALSE set, and 1 if it is TRUE.
31 PARMRK Mark parity and framing errors.
32 INPCK Enable checking of parity errors.
33 ISTRIP Strip 8th bit off characters.
34 INLCR Map NL into CR on input.
35 IGNCR Ignore CR on input.
36 ICRNL Map CR to NL on input.
37 IUCLC Translate uppercase characters to lowercase.
38 IXON Enable output flow control.
39 IXANY Any char will restart after stop.
40 IXOFF Enable input flow control.
41 IMAXBEL Ring bell on input queue full.
50 ISIG Enable signals INTR, QUIT, [D]SUSP.
51 ICANON Canonicalize input lines.
52 XCASE Enable input and output of uppercase characters by
preceding their lowercase equivalents with `\'.
53 ECHO Enable echoing.
54 ECHOE Visually erase chars.
55 ECHOK Kill character discards current line.
56 ECHONL Echo NL even if ECHO is off.
57 NOFLSH Don't flush after interrupt.
58 TOSTOP Stop background jobs from output.
59 IEXTEN Enable extensions.
60 ECHOCTL Echo control characters as ^(Char).
61 ECHOKE Visual erase for line kill.
62 PENDIN Retype pending input.
70 OPOST Enable output processing.
71 OLCUC Convert lowercase to uppercase.
72 ONLCR Map NL to CR-NL.
73 OCRNL Translate carriage return to newline (output).
74 ONOCR Translate newline to carriage return-newline
(output).
75 ONLRET Newline performs a carriage return (output).
90 CS7 7 bit mode.
91 CS8 8 bit mode.
92 PARENB Parity enable.
93 PARODD Odd parity, else even.
128 TTY_OP_ISPEED Specifies the input baud rate in bits per second.
129 TTY_OP_OSPEED Specifies the output baud rate in bits per second.
7. Summary of Message Numbers
#define SSH_MSG_GLOBAL_REQUEST 80
#define SSH_MSG_REQUEST_SUCCESS 81
#define SSH_MSG_REQUEST_FAILURE 82
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#define SSH_MSG_CHANNEL_OPEN 90
#define SSH_MSG_CHANNEL_OPEN_CONFIRMATION 91
#define SSH_MSG_CHANNEL_OPEN_FAILURE 92
#define SSH_MSG_CHANNEL_WINDOW_ADJUST 93
#define SSH_MSG_CHANNEL_DATA 94
#define SSH_MSG_CHANNEL_EXTENDED_DATA 95
#define SSH_MSG_CHANNEL_EOF 96
#define SSH_MSG_CHANNEL_CLOSE 97
#define SSH_MSG_CHANNEL_REQUEST 98
#define SSH_MSG_CHANNEL_SUCCESS 99
#define SSH_MSG_CHANNEL_FAILURE 100
8. Security Considerations
This protocol is assumed to run on top of a secure, authenticated
transport. User authentication and protection against network-level
attacks are assumed to be provided by the underlying protocols.
This protocol can, however, be used to execute commands on remote
machines. The protocol also permits the server to run commands on
the client. Implementations may wish to disallow this to prevent an
attacker from coming from the server machine to the client machine.
X11 forwarding provides major security improvements over normal
cookie-based X11 forwarding. The cookie never needs to be
transmitted in the clear, and traffic is encrypted and integrity-
protected. No useful authentication data will remain on the server
machine after the connection has been closed. On the other hand, in
some situations a forwarded X11 connection might be used to get
access to the local X server across security perimeters.
Port forwardings can potentially allow an intruder to cross security
perimeters such as firewalls. They do not offer anything
fundamentally new that a user could not do otherwise; however, they
make opening tunnels very easy. Implementations should allow policy
control over what can be forwarded. Administrators should be able to
deny forwardings where appropriate.
Since this protocol normally runs inside an encrypted tunnel,
firewalls will not be able to examine the traffic.
It is RECOMMENDED that implementations disable all the potentially
dangerous features (e.g. agent forwarding, X11 forwarding, and
TCP/IP forwarding) if the host key has changed.
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9. Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification can
be obtained from the IETF Secretariat.
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
10. Additional Information
The current document editor is: Darren.Moffat%Sun.COM@localhost. Comments on
this internet draft should be sent to the IETF SECSH working group,
details at: http://ietf.org/html.charters/secsh-charter.html
References
[RFC1766] Alvestrand, H., "Tags for the Identification of
Languages", RFC 1766, March 1995.
[RFC1884] Hinden, R., Deering, S. and Editors, "IP Version 6
Addressing Architecture", RFC 1884, December 1995.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[SCHEIFLER] Scheifler, R., "X Window System : The Complete
Reference to Xlib, X Protocol, Icccm, Xlfd, 3rd
edition.", Digital Press ISBN 1555580882, Feburary
1992.
[POSIX] ISO/IEC, 9945-1., "Information technology -- Portable
Operating System Interface (POSIX)-Part 1: System
Application Program Interface (API) C Language",
ANSI/IEE Std 1003.1, July 1996.
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Internet-Draft SSH Connection Protocol September 2002
[SSH-ARCH] Ylonen, T., "SSH Protocol Architecture", I-D draft-
ietf-architecture-13.txt, September 2002.
[SSH-TRANS] Ylonen, T., "SSH Transport Layer Protocol", I-D
draft-ietf-transport-15.txt, September 2002.
[SSH-USERAUTH] Ylonen, T., "SSH Authentication Protocol", I-D draft-
ietf-userauth-16.txt, September 2002.
[SSH-CONNECT] Ylonen, T., "SSH Connection Protocol", I-D draft-
ietf-connect-16.txt, September 2002.
Authors' Addresses
Tatu Ylonen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: ylo%ssh.com@localhost
Tero Kivinen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: kivinen%ssh.com@localhost
Markku-Juhani O. Saarinen
University of Jyvaskyla
Timo J. Rinne
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: tri%ssh.com@localhost
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Sami Lehtinen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: sjl%ssh.com@localhost
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Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Ylonen, et. al. Expires March 21, 2003 [Page 22]
Network Working Group T. Ylonen
Internet-Draft T. Kivinen
Expires: March 21, 2003 SSH Communications Security Corp
M. Saarinen
University of Jyvaskyla
T. Rinne
S. Lehtinen
SSH Communications Security Corp
September 20, 2002
SSH Transport Layer Protocol
draft-ietf-secsh-transport-15.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on March 21, 2003.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
SSH is a protocol for secure remote login and other secure network
services over an insecure network.
This document describes the SSH transport layer protocol which
typically runs on top of TCP/IP. The protocol can be used as a basis
for a number of secure network services. It provides strong
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encryption, server authentication, and integrity protection. It may
also provide compression.
Key exchange method, public key algorithm, symmetric encryption
algorithm, message authentication algorithm, and hash algorithm are
all negotiated.
This document also describes the Diffie-Hellman key exchange method
and the minimal set of algorithms that are needed to implement the
SSH transport layer protocol.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Connection Setup . . . . . . . . . . . . . . . . . . . . . . . 3
3.1 Use over TCP/IP . . . . . . . . . . . . . . . . . . . . . . . 3
3.2 Protocol Version Exchange . . . . . . . . . . . . . . . . . . 3
3.3 Compatibility With Old SSH Versions . . . . . . . . . . . . . 4
3.4 Old Client, New Server . . . . . . . . . . . . . . . . . . . . 4
3.5 New Client, Old Server . . . . . . . . . . . . . . . . . . . . 5
4. Binary Packet Protocol . . . . . . . . . . . . . . . . . . . . 5
4.1 Maximum Packet Length . . . . . . . . . . . . . . . . . . . . 6
4.2 Compression . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.3 Encryption . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.4 Data Integrity . . . . . . . . . . . . . . . . . . . . . . . . 9
4.5 Key Exchange Methods . . . . . . . . . . . . . . . . . . . . . 10
4.6 Public Key Algorithms . . . . . . . . . . . . . . . . . . . . 10
5. Key Exchange . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1 Algorithm Negotiation . . . . . . . . . . . . . . . . . . . . 13
5.2 Output from Key Exchange . . . . . . . . . . . . . . . . . . . 16
5.3 Taking Keys Into Use . . . . . . . . . . . . . . . . . . . . . 17
6. Diffie-Hellman Key Exchange . . . . . . . . . . . . . . . . . 17
6.1 diffie-hellman-group1-sha1 . . . . . . . . . . . . . . . . . . 19
7. Key Re-Exchange . . . . . . . . . . . . . . . . . . . . . . . 19
8. Service Request . . . . . . . . . . . . . . . . . . . . . . . 20
9. Additional Messages . . . . . . . . . . . . . . . . . . . . . 21
9.1 Disconnection Message . . . . . . . . . . . . . . . . . . . . 21
9.2 Ignored Data Message . . . . . . . . . . . . . . . . . . . . . 22
9.3 Debug Message . . . . . . . . . . . . . . . . . . . . . . . . 22
9.4 Reserved Messages . . . . . . . . . . . . . . . . . . . . . . 23
10. Summary of Message Numbers . . . . . . . . . . . . . . . . . . 23
11. Security Considerations . . . . . . . . . . . . . . . . . . . 23
12. Intellectual Property . . . . . . . . . . . . . . . . . . . . 25
13. Additional Information . . . . . . . . . . . . . . . . . . . . 25
References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 27
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 28
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1. Introduction
The SSH transport layer is a secure low level transport protocol. It
provides strong encryption, cryptographic host authentication, and
integrity protection.
Authentication in this protocol level is host-based; this protocol
does not perform user authentication. A higher level protocol for
user authentication can be designed on top of this protocol.
The protocol has been designed to be simple, flexible, to allow
parameter negotiation, and to minimize the number of round-trips.
Key exchange method, public key algorithm, symmetric encryption
algorithm, message authentication algorithm, and hash algorithm are
all negotiated. It is expected that in most environments, only 2
round-trips will be needed for full key exchange, server
authentication, service request, and acceptance notification of
service request. The worst case is 3 round-trips.
2. Conventions Used in This Document
The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
and "MAY" that appear in this document are to be interpreted as
described in [RFC2119]
The used data types and terminology are specified in the architecture
document [SSH-ARCH]
The architecture document also discusses the algorithm naming
conventions that MUST be used with the SSH protocols.
3. Connection Setup
SSH works over any 8-bit clean, binary-transparent transport. The
underlying transport SHOULD protect against transmission errors as
such errors cause the SSH connection to terminate.
The client initiates the connection.
3.1 Use over TCP/IP
When used over TCP/IP, the server normally listens for connections on
port 22. This port number has been registered with the IANA, and has
been officially assigned for SSH.
3.2 Protocol Version Exchange
When the connection has been established, both sides MUST send an
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identification string of the form "SSH-protoversion-softwareversion
comments", followed by carriage return and newline characters (ASCII
13 and 10, respectively). Both sides MUST be able to process
identification strings without carriage return character. No null
character is sent. The maximum length of the string is 255
characters, including the carriage return and newline.
The part of the identification string preceding carriage return and
newline is used in the Diffie-Hellman key exchange (see Section
Section 6).
The server MAY send other lines of data before sending the version
string. Each line SHOULD be terminated by a carriage return and
newline. Such lines MUST NOT begin with "SSH-", and SHOULD be
encoded in ISO-10646 UTF-8 [RFC2279] (language is not specified).
Clients MUST be able to process such lines; they MAY be silently
ignored, or MAY be displayed to the client user; if they are
displayed, control character filtering discussed in [SSH-ARCH] SHOULD
be used. The primary use of this feature is to allow TCP-wrappers to
display an error message before disconnecting.
Version strings MUST consist of printable US-ASCII characters, not
including whitespaces or a minus sign (-). The version string is
primarily used to trigger compatibility extensions and to indicate
the capabilities of an implementation. The comment string should
contain additional information that might be useful in solving user
problems.
The protocol version described in this document is 2.0.
Key exchange will begin immediately after sending this identifier.
All packets following the identification string SHALL use the binary
packet protocol, to be described below.
3.3 Compatibility With Old SSH Versions
During the transition period, it is important to be able to work in a
way that is compatible with the installed SSH clients and servers
that use an older version of the protocol. Information in this
section is only relevant for implementations supporting compatibility
with SSH versions 1.x.
3.4 Old Client, New Server
Server implementations MAY support a configurable "compatibility"
flag that enables compatibility with old versions. When this flag is
on, the server SHOULD identify its protocol version as "1.99".
Clients using protocol 2.0 MUST be able to identify this as identical
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to "2.0". In this mode the server SHOULD NOT send the carriage
return character (ASCII 13) after the version identification string.
In the compatibility mode the server SHOULD NOT send any further data
after its initialization string until it has received an
identification string from the client. The server can then determine
whether the client is using an old protocol, and can revert to the
old protocol if required. In the compatibility mode, the server MUST
NOT send additional data before the version string.
When compatibility with old clients is not needed, the server MAY
send its initial key exchange data immediately after the
identification string.
3.5 New Client, Old Server
Since the new client MAY immediately send additional data after its
identification string (before receiving server's identification), the
old protocol may already have been corrupted when the client learns
that the server is old. When this happens, the client SHOULD close
the connection to the server, and reconnect using the old protocol.
4. Binary Packet Protocol
Each packet is in the following format:
uint32 packet_length
byte padding_length
byte[n1] payload; n1 = packet_length - padding_length - 1
byte[n2] random padding; n2 = padding_length
byte[m] mac (message authentication code); m = mac_length
packet_length
The length of the packet (bytes), not including MAC or the
packet_length field itself.
padding_length
Length of padding (bytes).
payload
The useful contents of the packet. If compression has been
negotiated, this field is compressed. Initially, compression
MUST be "none".
random padding
Arbitrary-length padding, such that the total length of
(packet_length || padding_length || payload || padding) is a
multiple of the cipher block size or 8, whichever is larger.
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There MUST be at least four bytes of padding. The padding
SHOULD consist of random bytes. The maximum amount of padding
is 255 bytes.
mac
Message authentication code. If message authentication has
been negotiated, this field contains the MAC bytes. Initially,
the MAC algorithm MUST be "none".
Note that length of the concatenation of packet length, padding
length, payload, and padding MUST be a multiple of the cipher block
size or 8, whichever is larger. This constraint MUST be enforced
even when using stream ciphers. Note that the packet length field is
also encrypted, and processing it requires special care when sending
or receiving packets.
The minimum size of a packet is 16 (or the cipher block size,
whichever is larger) bytes (plus MAC); implementations SHOULD decrypt
the length after receiving the first 8 (or cipher block size,
whichever is larger) bytes of a packet.
4.1 Maximum Packet Length
All implementations MUST be able to process packets with uncompressed
payload length of 32768 bytes or less and total packet size of 35000
bytes or less (including length, padding length, payload, padding,
and MAC.). The maximum of 35000 bytes is an arbitrary chosen value
larger than uncompressed size. Implementations SHOULD support longer
packets, where they might be needed, e.g. if an implementation wants
to send a very large number of certificates. Such packets MAY be
sent if the version string indicates that the other party is able to
process them. However, implementations SHOULD check that the packet
length is reasonable for the implementation to avoid denial-of-
service and/or buffer overflow attacks.
4.2 Compression
If compression has been negotiated, the payload field (and only it)
will be compressed using the negotiated algorithm. The length field
and MAC will be computed from the compressed payload. Encryption
will be done after compression.
Compression MAY be stateful, depending on the method. Compression
MUST be independent for each direction, and implementations MUST
allow independently choosing the algorithm for each direction.
The following compression methods are currently defined:
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none REQUIRED no compression
zlib OPTIONAL ZLIB (LZ77) compression
The "zlib" compression is described in [RFC1950] and in [RFC1951].
The compression context is initialized after each key exchange, and
is passed from one packet to the next with only a partial flush being
performed at the end of each packet. A partial flush means that the
current compressed block is ended and all data will be output. If
the current block is not a stored block, one or more empty blocks are
added after the current block to ensure that there are at least 8
bits counting from the start of the end-of-block code of the current
block to the end of the packet payload.
Additional methods may be defined as specified in [SSH-ARCH].
4.3 Encryption
An encryption algorithm and a key will be negotiated during the key
exchange. When encryption is in effect, the packet length, padding
length, payload and padding fields of each packet MUST be encrypted
with the given algorithm.
The encrypted data in all packets sent in one direction SHOULD be
considered a single data stream. For example, initialization vectors
SHOULD be passed from the end of one packet to the beginning of the
next packet. All ciphers SHOULD use keys with an effective key
length of 128 bits or more.
The ciphers in each direction MUST run independently of each other,
and implementations MUST allow independently choosing the algorithm
for each direction (if multiple algorithms are allowed by local
policy).
The following ciphers are currently defined:
3des-cbc REQUIRED three-key 3DES in CBC mode
blowfish-cbc RECOMMENDED Blowfish in CBC mode
twofish256-cbc OPTIONAL Twofish in CBC mode,
with 256-bit key
twofish-cbc OPTIONAL alias for "twofish256-cbc" (this
is being retained for
historical reasons)
twofish192-cbc OPTIONAL Twofish with 192-bit key
twofish128-cbc RECOMMENDED Twofish with 128-bit key
aes256-cbc OPTIONAL AES (Rijndael) in CBC mode,
with 256-bit key
aes192-cbc OPTIONAL AES with 192-bit key
aes128-cbc RECOMMENDED AES with 128-bit key
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serpent256-cbc OPTIONAL Serpent in CBC mode, with
256-bit key
serpent192-cbc OPTIONAL Serpent with 192-bit key
serpent128-cbc OPTIONAL Serpent with 128-bit key
arcfour OPTIONAL the ARCFOUR stream cipher
idea-cbc OPTIONAL IDEA in CBC mode
cast128-cbc OPTIONAL CAST-128 in CBC mode
none OPTIONAL no encryption; NOT RECOMMENDED
The "3des-cbc" cipher is three-key triple-DES (encrypt-decrypt-
encrypt), where the first 8 bytes of the key are used for the first
encryption, the next 8 bytes for the decryption, and the following 8
bytes for the final encryption. This requires 24 bytes of key data
(of which 168 bits are actually used). To implement CBC mode, outer
chaining MUST be used (i.e., there is only one initialization
vector). This is a block cipher with 8 byte blocks. This algorithm
is defined in [SCHNEIER]
The "blowfish-cbc" cipher is Blowfish in CBC mode, with 128 bit keys
[SCHNEIER]. This is a block cipher with 8 byte blocks.
The "twofish-cbc" or "twofish256-cbc" cipher is Twofish in CBC mode,
with 256 bit keys as described [TWOFISH]. This is a block cipher
with 16 byte blocks.
The "twofish192-cbc" cipher. Same as above but with 192-bit key.
The "twofish128-cbc" cipher. Same as above but with 128-bit key.
The "aes256-cbc" cipher is AES (Advanced Encryption Standard),
formerly Rijndael, in CBC mode. This version uses 256-bit key.
The "aes192-cbc" cipher. Same as above but with 192-bit key.
The "aes128-cbc" cipher. Same as above but with 128-bit key.
The "serpent256-cbc" cipher in CBC mode, with 256-bit key as
described in the Serpent AES submission.
The "serpent192-cbc" cipher. Same as above but with 192-bit key.
The "serpent128-cbc" cipher. Same as above but with 128-bit key.
The "arcfour" is the Arcfour stream cipher with 128 bit keys. The
Arcfour cipher is believed to be compatible with the RC4 cipher
[SCHNEIER]. RC4 is a registered trademark of RSA Data Security Inc.
Arcfour (and RC4) has problems with weak keys, and should be used
with caution.
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The "idea-cbc" cipher is the IDEA cipher in CBC mode [SCHNEIER].
IDEA is patented by Ascom AG.
The "cast128-cbc" cipher is the CAST-128 cipher in CBC mode
[RFC2144].
The "none" algorithm specifies that no encryption is to be done.
Note that this method provides no confidentiality protection, and it
is not recommended. Some functionality (e.g. password
authentication) may be disabled for security reasons if this cipher
is chosen.
Additional methods may be defined as specified in [SSH-ARCH].
4.4 Data Integrity
Data integrity is protected by including with each packet a message
authentication code (MAC) that is computed from a shared secret,
packet sequence number, and the contents of the packet.
The message authentication algorithm and key are negotiated during
key exchange. Initially, no MAC will be in effect, and its length
MUST be zero. After key exchange, the selected MAC will be computed
before encryption from the concatenation of packet data:
mac = MAC(key, sequence_number || unencrypted_packet)
where unencrypted_packet is the entire packet without MAC (the length
fields, payload and padding), and sequence_number is an implicit
packet sequence number represented as uint32. The sequence number is
initialized to zero for the first packet, and is incremented after
every packet (regardless of whether encryption or MAC is in use). It
is never reset, even if keys/algorithms are renegotiated later. It
wraps around to zero after every 2^32 packets. The packet sequence
number itself is not included in the packet sent over the wire.
The MAC algorithms for each direction MUST run independently, and
implementations MUST allow choosing the algorithm independently for
both directions.
The MAC bytes resulting from the MAC algorithm MUST be transmitted
without encryption as the last part of the packet. The number of MAC
bytes depends on the algorithm chosen.
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The following MAC algorithms are currently defined:
hmac-sha1 REQUIRED HMAC-SHA1 (digest length = key
length = 20)
hmac-sha1-96 RECOMMENDED first 96 bits of HMAC-SHA1 (digest
length = 12, key length = 20)
hmac-md5 OPTIONAL HMAC-MD5 (digest length = key
length = 16)
hmac-md5-96 OPTIONAL first 96 bits of HMAC-MD5 (digest
length = 12, key length = 16)
none OPTIONAL no MAC; NOT RECOMMENDED
The "hmac-*" algorithms are described in [RFC2104] The "*-n" MACs use
only the first n bits of the resulting value.
The hash algorithms are described in [SCHNEIER].
Additional methods may be defined as specified in [SSH-ARCH].
4.5 Key Exchange Methods
The key exchange method specifies how one-time session keys are
generated for encryption and for authentication, and how the server
authentication is done.
Only one REQUIRED key exchange method has been defined:
diffie-hellman-group1-sha1 REQUIRED
This method is described later in this document.
Additional methods may be defined as specified in [SSH-ARCH].
4.6 Public Key Algorithms
This protocol has been designed to be able to operate with almost any
public key format, encoding, and algorithm (signature and/or
encryption).
There are several aspects that define a public key type:
o Key format: how is the key encoded and how are certificates
represented. The key blobs in this protocol MAY contain
certificates in addition to keys.
o Signature and/or encryption algorithms. Some key types may not
support both signing and encryption. Key usage may also be
restricted by policy statements in e.g. certificates. In this
case, different key types SHOULD be defined for the different
policy alternatives.
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o Encoding of signatures and/or encrypted data. This includes but
is not limited to padding, byte order, and data formats.
The following public key and/or certificate formats are currently defined:
ssh-dss REQUIRED sign Simple DSS
ssh-rsa RECOMMENDED sign Simple RSA
x509v3-sign-rsa OPTIONAL sign X.509 certificates (RSA key)
x509v3-sign-dss OPTIONAL sign X.509 certificates (DSS key)
spki-sign-rsa OPTIONAL sign SPKI certificates (RSA key)
spki-sign-dss OPTIONAL sign SPKI certificates (DSS key)
pgp-sign-rsa OPTIONAL sign OpenPGP certificates (RSA key)
pgp-sign-dss OPTIONAL sign OpenPGP certificates (DSS key)
Additional key types may be defined as specified in [SSH-ARCH].
The key type MUST always be explicitly known (from algorithm
negotiation or some other source). It is not normally included in
the key blob.
Certificates and public keys are encoded as follows:
string certificate or public key format identifier
byte[n] key/certificate data
The certificate part may have be a zero length string, but a public
key is required. This is the public key that will be used for
authentication; the certificate sequence contained in the certificate
blob can be used to provide authorization.
Public key / certifcate formats that do not explicitly specify a
signature format identifier MUST use the public key / certificate
format identifier as the signature identifier.
Signatures are encoded as follows:
string signature format identifier (as specified by the
public key / cert format)
byte[n] signature blob in format specific encoding.
The "ssh-dss" key format has the following specific encoding:
string "ssh-dss"
mpint p
mpint q
mpint g
mpint y
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Here the p, q, g, and y parameters form the signature key blob.
Signing and verifying using this key format is done according to the
Digital Signature Standard [FIPS-186] using the SHA-1 hash. A
description can also be found in [SCHNEIER].
The resulting signature is encoded as follows:
string "ssh-dss"
string dss_signature_blob
dss_signature_blob is encoded as a string containing r followed by s
(which are 160 bits long integers, without lengths or padding,
unsigned and in network byte order).
The "ssh-rsa" key format has the following specific encoding:
string "ssh-rsa"
mpint e
mpint n
Here the e and n parameters form the signature key blob.
Signing and verifying using this key format is done according to
[SCHNEIER] and [PKCS1] using the SHA-1 hash.
The resulting signature is encoded as follows:
string "ssh-rsa"
string rsa_signature_blob
rsa_signature_blob is encoded as a string containing s (which is an
integer, without lengths or padding, unsigned and in network byte
order).
The "spki-sign-rsa" method indicates that the certificate blob
contains a sequence of SPKI certificates. The format of SPKI
certificates is described in [RFC2693]. This method indicates that
the key (or one of the keys in the certificate) is an RSA-key.
The "spki-sign-dss". As above, but indicates that the key (or one of
the keys in the certificate) is a DSS-key.
The "pgp-sign-rsa" method indicates the certificates, the public key,
and the signature are in OpenPGP compatible binary format
([RFC2440]). This method indicates that the key is an RSA-key.
The "pgp-sign-dss". As above, but indicates that the key is a DSS-
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key.
5. Key Exchange
Key exchange begins by each side sending lists of supported
algorithms. Each side has a preferred algorithm in each category,
and it is assumed that most implementations at any given time will
use the same preferred algorithm. Each side MAY guess which
algorithm the other side is using, and MAY send an initial key
exchange packet according to the algorithm if appropriate for the
preferred method.
Guess is considered wrong, if:
o the kex algorithm and/or the host key algorithm is guessed wrong
(server and client have different preferred algorithm), or
o if any of the other algorithms cannot be agreed upon (the
procedure is defined below in Section Section 5.1).
Otherwise, the guess is considered to be right and the optimistically
sent packet MUST be handled as the first key exchange packet.
However, if the guess was wrong, and a packet was optimistically sent
by one or both parties, such packets MUST be ignored (even if the
error in the guess would not affect the contents of the initial
packet(s)), and the appropriate side MUST send the correct initial
packet.
Server authentication in the key exchange MAY be implicit. After a
key exchange with implicit server authentication, the client MUST
wait for response to its service request message before sending any
further data.
5.1 Algorithm Negotiation
Key exchange begins by each side sending the following packet:
byte SSH_MSG_KEXINIT
byte[16] cookie (random bytes)
string kex_algorithms
string server_host_key_algorithms
string encryption_algorithms_client_to_server
string encryption_algorithms_server_to_client
string mac_algorithms_client_to_server
string mac_algorithms_server_to_client
string compression_algorithms_client_to_server
string compression_algorithms_server_to_client
string languages_client_to_server
string languages_server_to_client
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boolean first_kex_packet_follows
uint32 0 (reserved for future extension)
Each of the algorithm strings MUST be a comma-separated list of
algorithm names (see ''Algorithm Naming'' in [SSH-ARCH]). Each
supported (allowed) algorithm MUST be listed in order of preference.
The first algorithm in each list MUST be the preferred (guessed)
algorithm. Each string MUST contain at least one algorithm name.
cookie
The cookie MUST be a random value generated by the sender. Its
purpose is to make it impossible for either side to fully
determine the keys and the session identifier.
kex_algorithms
Key exchange algorithms were defined above. The first
algorithm MUST be the preferred (and guessed) algorithm. If
both sides make the same guess, that algorithm MUST be used.
Otherwise, the following algorithm MUST be used to choose a key
exchange method: iterate over client's kex algorithms, one at a
time. Choose the first algorithm that satisfies the following
conditions:
+ the server also supports the algorithm,
+ if the algorithm requires an encryption-capable host key,
there is an encryption-capable algorithm on the server's
server_host_key_algorithms that is also supported by the
client, and
+ if the algorithm requires a signature-capable host key,
there is a signature-capable algorithm on the server's
server_host_key_algorithms that is also supported by the
client.
+ If no algorithm satisfying all these conditions can be
found, the connection fails, and both sides MUST disconnect.
server_host_key_algorithms
List of the algorithms supported for the server host key. The
server lists the algorithms for which it has host keys; the
client lists the algorithms that it is willing to accept.
(There MAY be multiple host keys for a host, possibly with
different algorithms.)
Some host keys may not support both signatures and encryption
(this can be determined from the algorithm), and thus not all
host keys are valid for all key exchange methods.
Algorithm selection depends on whether the chosen key exchange
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algorithm requires a signature or encryption capable host key.
It MUST be possible to determine this from the public key
algorithm name. The first algorithm on the client's list that
satisfies the requirements and is also supported by the server
MUST be chosen. If there is no such algorithm, both sides MUST
disconnect.
encryption_algorithms
Lists the acceptable symmetric encryption algorithms in order
of preference. The chosen encryption algorithm to each
direction MUST be the first algorithm on the client's list
that is also on the server's list. If there is no such
algorithm, both sides MUST disconnect.
Note that "none" must be explicitly listed if it is to be
acceptable. The defined algorithm names are listed in Section
Section 4.3.
mac_algorithms
Lists the acceptable MAC algorithms in order of preference.
The chosen MAC algorithm MUST be the first algorithm on the
client's list that is also on the server's list. If there is
no such algorithm, both sides MUST disconnect.
Note that "none" must be explicitly listed if it is to be
acceptable. The MAC algorithm names are listed in Section
Figure 1.
compression_algorithms
Lists the acceptable compression algorithms in order of
preference. The chosen compression algorithm MUST be the first
algorithm on the client's list that is also on the server's
list. If there is no such algorithm, both sides MUST
disconnect.
Note that "none" must be explicitly listed if it is to be
acceptable. The compression algorithm names are listed in
Section Section 4.2.
languages
This is a comma-separated list of language tags in order of
preference [RFC1766]. Both parties MAY ignore this list. If
there are no language preferences, this list SHOULD be empty.
first_kex_packet_follows
Indicates whether a guessed key exchange packet follows. If a
guessed packet will be sent, this MUST be TRUE. If no guessed
packet will be sent, this MUST be FALSE.
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After receiving the SSH_MSG_KEXINIT packet from the other side,
each party will know whether their guess was right. If the
other party's guess was wrong, and this field was TRUE, the
next packet MUST be silently ignored, and both sides MUST then
act as determined by the negotiated key exchange method. If
the guess was right, key exchange MUST continue using the
guessed packet.
After the KEXINIT packet exchange, the key exchange algorithm is run.
It may involve several packet exchanges, as specified by the key
exchange method.
5.2 Output from Key Exchange
The key exchange produces two values: a shared secret K, and an
exchange hash H. Encryption and authentication keys are derived from
these. The exchange hash H from the first key exchange is
additionally used as the session identifier, which is a unique
identifier for this connection. It is used by authentication methods
as a part of the data that is signed as a proof of possession of a
private key. Once computed, the session identifier is not changed,
even if keys are later re-exchanged.
Each key exchange method specifies a hash function that is used in
the key exchange. The same hash algorithm MUST be used in key
derivation. Here, we'll call it HASH.
Encryption keys MUST be computed as HASH of a known value and K as
follows:
o Initial IV client to server: HASH(K || H || "A" || session_id)
(Here K is encoded as mpint and "A" as byte and session_id as raw
data."A" means the single character A, ASCII 65).
o Initial IV server to client: HASH(K || H || "B" || session_id)
o Encryption key client to server: HASH(K || H || "C" || session_id)
o Encryption key server to client: HASH(K || H || "D" || session_id)
o Integrity key client to server: HASH(K || H || "E" || session_id)
o Integrity key server to client: HASH(K || H || "F" || session_id)
Key data MUST be taken from the beginning of the hash output. 128
bits (16 bytes) SHOULD be used for algorithms with variable-length
keys. For other algorithms, as many bytes as are needed are taken
from the beginning of the hash value. If the key length in longer
than the output of the HASH, the key is extended by computing HASH of
the concatenation of K and H and the entire key so far, and appending
the resulting bytes (as many as HASH generates) to the key. This
process is repeated until enough key material is available; the key
is taken from the beginning of this value. In other words:
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K1 = HASH(K || H || X || session_id) (X is e.g. "A")
K2 = HASH(K || H || K1)
K3 = HASH(K || H || K1 || K2)
...
key = K1 || K2 || K3 || ...
This process will lose entropy if the amount of entropy in K is
larger than the internal state size of HASH.
5.3 Taking Keys Into Use
Key exchange ends by each side sending an SSH_MSG_NEWKEYS message.
This message is sent with the old keys and algorithms. All messages
sent after this message MUST use the new keys and algorithms.
When this message is received, the new keys and algorithms MUST be
taken into use for receiving.
This message is the only valid message after key exchange, in
addition to SSH_MSG_DEBUG, SSH_MSG_DISCONNECT and SSH_MSG_IGNORE
messages. The purpose of this message is to ensure that a party is
able to respond with a disconnect message that the other party can
understand if something goes wrong with the key exchange.
Implementations MUST NOT accept any other messages after key exchange
before receiving SSH_MSG_NEWKEYS.
byte SSH_MSG_NEWKEYS
6. Diffie-Hellman Key Exchange
The Diffie-Hellman key exchange provides a shared secret that can not
be determined by either party alone. The key exchange is combined
with a signature with the host key to provide host authentication.
In the following description (C is the client, S is the server; p is
a large safe prime, g is a generator for a subgroup of GF(p), and q
is the order of the subgroup; V_S is S's version string; V_C is C's
version string; K_S is S's public host key; I_C is C's KEXINIT
message and I_S S's KEXINIT message which have been exchanged before
this part begins):
1. C generates a random number x (1 < x < q) and computes e = g^x
mod p. C sends "e" to S.
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2. S generates a random number y (0 < y < q) and computes f = g^y
mod p. S receives "e". It computes K = e^y mod p, H = hash(V_C
|| V_S || I_C || I_S || K_S || e || f || K) (these elements are
encoded according to their types; see below), and signature s on
H with its private host key. S sends "K_S || f || s" to C. The
signing operation may involve a second hashing operation.
3. C verifies that K_S really is the host key for S (e.g. using
certificates or a local database). C is also allowed to accept
the key without verification; however, doing so will render the
protocol insecure against active attacks (but may be desirable
for practical reasons in the short term in many environments). C
then computes K = f^x mod p, H = hash(V_C || V_S || I_C || I_S ||
K_S || e || f || K), and verifies the signature s on H.
Either side MUST NOT send or accept e or f values that are not in the
range [1, p-1]. If this condition is violated, the key exchange
fails.
This is implemented with the following messages. The hash algorithm
for computing the exchange hash is defined by the method name, and is
called HASH. The public key algorithm for signing is negotiated with
the KEXINIT messages.
First, the client sends the following:
byte SSH_MSG_KEXDH_INIT
mpint e
The server responds with the following:
byte SSH_MSG_KEXDH_REPLY
string server public host key and certificates (K_S)
mpint f
string signature of H
The hash H is computed as the HASH hash of the concatenation of the
following:
string V_C, the client's version string (CR and NL excluded)
string V_S, the server's version string (CR and NL excluded)
string I_C, the payload of the client's SSH_MSG_KEXINIT
string I_S, the payload of the server's SSH_MSG_KEXINIT
string K_S, the host key
mpint e, exchange value sent by the client
mpint f, exchange value sent by the server
mpint K, the shared secret
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This value is called the exchange hash, and it is used to
authenticate the key exchange. The exchange hash SHOULD be kept
secret.
The signature algorithm MUST be applied over H, not the original
data. Most signature algorithms include hashing and additional
padding. For example, "ssh-dss" specifies SHA-1 hashing; in that
case, the data is first hashed with HASH to compute H, and H is then
hashed with SHA-1 as part of the signing operation.
6.1 diffie-hellman-group1-sha1
The "diffie-hellman-group1-sha1" method specifies Diffie-Hellman key
exchange with SHA-1 as HASH, and the following group:
The prime p is equal to 2^1024 - 2^960 - 1 + 2^64 * floor( 2^894 Pi +
129093 ). Its hexadecimal value is:
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE65381
FFFFFFFF FFFFFFFF.
In decimal, this value is:
179769313486231590770839156793787453197860296048756011706444
423684197180216158519368947833795864925541502180565485980503
646440548199239100050792877003355816639229553136239076508735
759914822574862575007425302077447712589550957937778424442426
617334727629299387668709205606050270810842907692932019128194
467627007.
The generator used with this prime is g = 2. The group order q is (p
- 1) / 2.
This group was taken from the ISAKMP/Oakley specification, and was
originally generated by Richard Schroeppel at the University of
Arizona. Properties of this prime are described in [Orm96].
7. Key Re-Exchange
Key re-exchange is started by sending an SSH_MSG_KEXINIT packet when
not already doing a key exchange (as described in Section Section
5.1). When this message is received, a party MUST respond with its
own SSH_MSG_KEXINIT message except when the received SSH_MSG_KEXINIT
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already was a reply. Either party MAY initiate the re-exchange, but
roles MUST NOT be changed (i.e., the server remains the server, and
the client remains the client).
Key re-exchange is performed using whatever encryption was in effect
when the exchange was started. Encryption, compression, and MAC
methods are not changed before a new SSH_MSG_NEWKEYS is sent after
the key exchange (as in the initial key exchange). Re-exchange is
processed identically to the initial key exchange, except for the
session identifier that will remain unchanged. It is permissible to
change some or all of the algorithms during the re-exchange. Host
keys can also change. All keys and initialization vectors are
recomputed after the exchange. Compression and encryption contexts
are reset.
It is recommended that the keys are changed after each gigabyte of
transmitted data or after each hour of connection time, whichever
comes sooner. However, since the re-exchange is a public key
operation, it requires a fair amount of processing power and should
not be performed too often.
More application data may be sent after the SSH_MSG_NEWKEYS packet
has been sent; key exchange does not affect the protocols that lie
above the SSH transport layer.
8. Service Request
After the key exchange, the client requests a service. The service
is identified by a name. The format of names and procedures for
defining new names are defined in [SSH-ARCH].
Currently, the following names have been reserved:
ssh-userauth
ssh-connection
Similar local naming policy is applied to the service names, as is
applied to the algorithm names; a local service should use the
"servicename@domain" syntax.
byte SSH_MSG_SERVICE_REQUEST
string service name
If the server rejects the service request, it SHOULD send an
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appropriate SSH_MSG_DISCONNECT message and MUST disconnect.
When the service starts, it may have access to the session identifier
generated during the key exchange.
If the server supports the service (and permits the client to use
it), it MUST respond with the following:
byte SSH_MSG_SERVICE_ACCEPT
string service name
Message numbers used by services should be in the area reserved for
them (see Section 6 in [SSH-ARCH]). The transport level will
continue to process its own messages.
Note that after a key exchange with implicit server authentication,
the client MUST wait for response to its service request message
before sending any further data.
9. Additional Messages
Either party may send any of the following messages at any time.
9.1 Disconnection Message
byte SSH_MSG_DISCONNECT
uint32 reason code
string description [RFC2279]
string language tag [RFC1766]
This message causes immediate termination of the connection. All
implementations MUST be able to process this message; they SHOULD be
able to send this message.
The sender MUST NOT send or receive any data after this message, and
the recipient MUST NOT accept any data after receiving this message.
The description field gives a more specific explanation in a human-
readable form. The error code gives the reason in a more machine-
readable format (suitable for localization), and can have the
following values:
#define SSH_DISCONNECT_HOST_NOT_ALLOWED_TO_CONNECT 1
#define SSH_DISCONNECT_PROTOCOL_ERROR 2
#define SSH_DISCONNECT_KEY_EXCHANGE_FAILED 3
#define SSH_DISCONNECT_RESERVED 4
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#define SSH_DISCONNECT_MAC_ERROR 5
#define SSH_DISCONNECT_COMPRESSION_ERROR 6
#define SSH_DISCONNECT_SERVICE_NOT_AVAILABLE 7
#define SSH_DISCONNECT_PROTOCOL_VERSION_NOT_SUPPORTED 8
#define SSH_DISCONNECT_HOST_KEY_NOT_VERIFIABLE 9
#define SSH_DISCONNECT_CONNECTION_LOST 10
#define SSH_DISCONNECT_BY_APPLICATION 11
#define SSH_DISCONNECT_TOO_MANY_CONNECTIONS 12
#define SSH_DISCONNECT_AUTH_CANCELLED_BY_USER 13
#define SSH_DISCONNECT_NO_MORE_AUTH_METHODS_AVAILABLE 14
#define SSH_DISCONNECT_ILLEGAL_USER_NAME 15
If the description string is displayed, control character filtering
discussed in [SSH-ARCH] should be used to avoid attacks by sending
terminal control characters.
9.2 Ignored Data Message
byte SSH_MSG_IGNORE
string data
All implementations MUST understand (and ignore) this message at any
time (after receiving the protocol version). No implementation is
required to send them. This message can be used as an additional
protection measure against advanced traffic analysis techniques.
9.3 Debug Message
byte SSH_MSG_DEBUG
boolean always_display
string message [RFC2279]
string language tag [RFC1766]
All implementations MUST understand this message, but they are
allowed to ignore it. This message is used to pass the other side
information that may help debugging. If always_display is TRUE, the
message SHOULD be displayed. Otherwise, it SHOULD NOT be displayed
unless debugging information has been explicitly requested by the
user.
The message doesn't need to contain a newline. It is, however,
allowed to consist of multiple lines separated by CRLF (Carriage
Return - Line Feed) pairs.
If the message string is displayed, terminal control character
filtering discussed in [SSH-ARCH] should be used to avoid attacks by
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sending terminal control characters.
9.4 Reserved Messages
An implementation MUST respond to all unrecognized messages with an
SSH_MSG_UNIMPLEMENTED message in the order in which the messages were
received. Such messages MUST be otherwise ignored. Later protocol
versions may define other meanings for these message types.
byte SSH_MSG_UNIMPLEMENTED
uint32 packet sequence number of rejected message
10. Summary of Message Numbers
The following message numbers have been defined in this protocol:
#define SSH_MSG_DISCONNECT 1
#define SSH_MSG_IGNORE 2
#define SSH_MSG_UNIMPLEMENTED 3
#define SSH_MSG_DEBUG 4
#define SSH_MSG_SERVICE_REQUEST 5
#define SSH_MSG_SERVICE_ACCEPT 6
#define SSH_MSG_KEXINIT 20
#define SSH_MSG_NEWKEYS 21
/* Numbers 30-49 used for kex packets.
Different kex methods may reuse message numbers in
this range. */
#define SSH_MSG_KEXDH_INIT 30
#define SSH_MSG_KEXDH_REPLY 31
11. Security Considerations
This protocol provides a secure encrypted channel over an insecure
network. It performs server host authentication, key exchange,
encryption, and integrity protection. It also derives a unique
session id that may be used by higher-level protocols.
It is expected that this protocol will sometimes be used without
insisting on reliable association between the server host key and the
server host name. Such use is inherently insecure, but may be
necessary in non-security critical environments, and still provides
protection against passive attacks. However, implementors of
protocols running on top of this protocol should keep this
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possibility in mind.
This protocol is designed to be used over a reliable transport. If
transmission errors or message manipulation occur, the connection is
closed. The connection SHOULD be re-established if this occurs.
Denial of service attacks of this type ("wire cutter") are almost
impossible to avoid.
The protocol was not designed to eliminate covert channels. For
example, the padding, SSH_MSG_IGNORE messages, and several other
places in the protocol can be used to pass covert information, and
the recipient has no reliable way to verify whether such information
is being sent.
Nearly all ciphers specified in this document are used in cipher
block chaining (CBC) mode. It's been known for some time that CBC
modes will reveal information about the plaintext if two ciphertext
blocks encrypted under the same key are equal; this is one of the
reasons this document strongly recommends rekeying at least once per
gigabyte of data, to reduce the chance that a "birthday paradox"
collision will appear.
Recent research has uncovered a new attack on CBC mode which, under
certain conditions, allows a chosen plaintext attacker aware of the
IV for a forthcoming message to have some chance to artificially
induce a system into generating ciphertext collisions, allowing the
attacker's guesses at likely prior plaintexts to be confirmed.
Any protocol which uses CBC in a way which allows advance knowledge
of a message's IV (e.g., by using the last block of the preceding
message as the IV) might be vulnerable to this attack.
Preliminary analysis of this attack as applied to the SSH protocol
suggests that the protocol as implemented today is actually fairly
resistant to this attack. While estimates vary, on average, an
attacker would need tens or hundreds of millions of opportunities to
inject chosen plaintexts to be encrypted with a known IV to confirm
guesses on the value of a few unknown plaintexts.
While this attack involves less work than a brute-force attack on the
underlying cipher (and is thus a matter of some concern), it is also
likely to be significantly more difficult than attacks on other parts
of a system using the SSH protocol, and so is unlikely to be an
immediate risk to real-world systems. Due to this document's
recommendation that rekeying occur once an hour, an attacker also has
a limited amount of time to complete any particular attack.
Nevertheless, work is underway to specify, in a separate document or
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documents, additional cipher modes for the SSH protocol to address
this vulnerability. Implementors should be prepared to add new
algorithms to their implementations as this work progresses.
12. Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification can
be obtained from the IETF Secretariat.
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
13. Additional Information
The current document editor is: Darren.Moffat%Sun.COM@localhost. Comments on
this internet draft should be sent to the IETF SECSH working group,
details at: http://ietf.org/html.charters/secsh-charter.html
References
[FIPS-186] Federal Information Processing Standards Publication,
., "FIPS PUB 186, Digital Signature Standard", May
1994.
[Orm96] Orman, H., "The Okaley Key Determination Protcol
version1, TR97-92", 1996.
[RFC2459] Housley, R., Ford, W., Polk, W. and D. Solo,
"Internet X.509 Public Key Infrastructure Certificate
and CRL Profile", RFC 2459, January 1999.
[RFC1034] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, Nov 1987.
[RFC1766] Alvestrand, H., "Tags for the Identification of
Ylonen, et. al. Expires March 21, 2003 [Page 25]
Internet-Draft SSH Transport Layer Protocol September 2002
Languages", RFC 1766, March 1995.
[RFC1950] Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data
Format Specification version 3.3", RFC 1950, May
1996.
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format
Specification version 1.3", RFC 1951, May 1996.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC
2144, May 1997.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H. and R.
Thayer, "OpenPGP Message Format", RFC 2440, November
1998.
[RFC2693] Ellison, C., Frantz, B., Lampson, B., Rivest, R.,
Thomas, B. and T. Ylonen, "SPKI Certificate Theory",
RFC 2693, September 1999.
[SCHNEIER] Schneier, B., "Applied Cryptography Second Edition:
protocols algorithms and source in code in C", 1996.
[TWOFISH] Schneier, B., "The Twofish Encryptions Algorithm: A
128-Bit Block Cipher, 1st Edition", March 1999.
[SSH-ARCH] Ylonen, T., "SSH Protocol Architecture", I-D draft-
ietf-architecture-13.txt, September 2002.
[SSH-TRANS] Ylonen, T., "SSH Transport Layer Protocol", I-D
draft-ietf-transport-15.txt, September 2002.
[SSH-USERAUTH] Ylonen, T., "SSH Authentication Protocol", I-D draft-
ietf-userauth-16.txt, September 2002.
[SSH-CONNECT] Ylonen, T., "SSH Connection Protocol", I-D draft-
ietf-connect-16.txt, September 2002.
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Authors' Addresses
Tatu Ylonen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: ylo%ssh.com@localhost
Tero Kivinen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: kivinen%ssh.com@localhost
Markku-Juhani O. Saarinen
University of Jyvaskyla
Timo J. Rinne
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: tri%ssh.com@localhost
Sami Lehtinen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: sjl%ssh.com@localhost
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Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Ylonen, et. al. Expires March 21, 2003 [Page 28]
Network Working Group T. Ylonen
Internet-Draft T. Kivinen
Expires: March 21, 2003 SSH Communications Security Corp
M. Saarinen
University of Jyvaskyla
T. Rinne
S. Lehtinen
SSH Communications Security Corp
September 20, 2002
SSH Authentication Protocol
draft-ietf-secsh-userauth-16.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on March 21, 2003.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
SSH is a protocol for secure remote login and other secure network
services over an insecure network. This document describes the SSH
authentication protocol framework and public key, password, and host-
based client authentication methods. Additional authentication
methods are described in separate documents. The SSH authentication
protocol runs on top of the SSH transport layer protocol and provides
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a single authenticated tunnel for the SSH connection protocol.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The Authentication Protocol Framework . . . . . . . . . . . . 3
2.1 Authentication Requests . . . . . . . . . . . . . . . . . . . 4
2.2 Responses to Authentication Requests . . . . . . . . . . . . . 4
2.3 The "none" Authentication Request . . . . . . . . . . . . . . 6
2.4 Completion of User Authentication . . . . . . . . . . . . . . 6
2.5 Banner Message . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Authentication Protocol Message Numbers . . . . . . . . . . . 7
4. Public Key Authentication Method: publickey . . . . . . . . . 7
5. Password Authentication Method: password . . . . . . . . . . . 9
6. Host-Based Authentication: hostbased . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. Intellectual Property . . . . . . . . . . . . . . . . . . . . 13
9. Additional Information . . . . . . . . . . . . . . . . . . . . 13
References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
The SSH authentication protocol is a general-purpose user
authentication protocol. It is intended to be run over the SSH
transport layer protocol [SSH-TRANS]. This protocol assumes that the
underlying protocols provide integrity and confidentiality
protection.
This document should be read only after reading the SSH architecture
document [SSH-ARCH]. This document freely uses terminology and
notation from the architecture document without reference or further
explanation.
The service name for this protocol is "ssh-userauth".
When this protocol starts, it receives the session identifier from
the lower-level protocol (this is the exchange hash H from the first
key exchange). The session identifier uniquely identifies this
session and is suitable for signing in order to prove ownership of a
private key. This protocol also needs to know whether the lower-
level protocol provides confidentiality protection.
2. The Authentication Protocol Framework
The server drives the authentication by telling the client which
authentication methods can be used to continue the exchange at any
given time. The client has the freedom to try the methods listed by
the server in any order. This gives the server complete control over
the authentication process if desired, but also gives enough
flexibility for the client to use the methods it supports or that are
most convenient for the user, when multiple methods are offered by
the server.
Authentication methods are identified by their name, as defined in
[SSH-ARCH]. The "none" method is reserved, and MUST NOT be listed as
supported. However, it MAY be sent by the client. The server MUST
always reject this request, unless the client is to be allowed in
without any authentication, in which case the server MUST accept this
request. The main purpose of sending this request is to get the list
of supported methods from the server.
The server SHOULD have a timeout for authentication, and disconnect
if the authentication has not been accepted within the timeout
period. The RECOMMENDED timeout period is 10 minutes. Additionally,
the implementation SHOULD limit the number of failed authentication
attempts a client may perform in a single session (the RECOMMENDED
limit is 20 attempts). If the threshold is exceeded, the server
SHOULD disconnect.
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2.1 Authentication Requests
All authentication requests MUST use the following message format.
Only the first few fields are defined; the remaining fields depend on
the authentication method.
byte SSH_MSG_USERAUTH_REQUEST
string user name (in ISO-10646 UTF-8 encoding [RFC2279])
string service name (in US-ASCII)
string method name (US-ASCII)
The rest of the packet is method-specific.
The user name and service are repeated in every new authentication
attempt, and MAY change. The server implementation MUST carefully
check them in every message, and MUST flush any accumulated
authentication states if they change. If it is unable to flush some
authentication state, it MUST disconnect if the user or service name
changes.
The service name specifies the service to start after authentication.
There may be several different authenticated services provided. If
the requested service is not available, the server MAY disconnect
immediately or at any later time. Sending a proper disconnect
message is RECOMMENDED. In any case, if the service does not exist,
authentication MUST NOT be accepted.
If the requested user does not exist, the server MAY disconnect, or
MAY send a bogus list of acceptable authentication methods, but never
accept any. This makes it possible for the server to avoid
disclosing information on which accounts exist. In any case, if the
user does not exist, the authentication request MUST NOT be accepted.
While there is usually little point for clients to send requests that
the server does not list as acceptable, sending such requests is not
an error, and the server SHOULD simply reject requests that it does
not recognize.
An authentication request MAY result in a further exchange of
messages. All such messages depend on the authentication method
used, and the client MAY at any time continue with a new
SSH_MSG_USERAUTH_REQUEST message, in which case the server MUST
abandon the previous authentication attempt and continue with the new
one.
2.2 Responses to Authentication Requests
If the server rejects the authentication request, it MUST respond
with the following:
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byte SSH_MSG_USERAUTH_FAILURE
string authentications that can continue
boolean partial success
"Authentications that can continue" is a comma-separated list of
authentication method names that may productively continue the
authentication dialog.
It is RECOMMENDED that servers only include those methods in the list
that are actually useful. However, it is not illegal to include
methods that cannot be used to authenticate the user.
Already successfully completed authentications SHOULD NOT be included
in the list, unless they really should be performed again for some
reason.
"Partial success" MUST be TRUE if the authentication request to which
this is a response was successful. It MUST be FALSE if the request
was not successfully processed.
When the server accepts authentication, it MUST respond with the
following:
byte SSH_MSG_USERAUTH_SUCCESS
Note that this is not sent after each step in a multi-method
authentication sequence, but only when the authentication is
complete.
The client MAY send several authentication requests without waiting
for responses from previous requests. The server MUST process each
request completely and acknowledge any failed requests with a
SSH_MSG_USERAUTH_FAILURE message before processing the next request.
A request that results in further exchange of messages will be
aborted by a second request. It is not possible to send a second
request without waiting for a response from the server, if the first
request will result in further exchange of messages. No
SSH_MSG_USERAUTH_FAILURE message will be sent for the aborted method.
SSH_MSG_USERAUTH_SUCCESS MUST be sent only once. When
SSH_MSG_USERAUTH_SUCCESS has been sent, any further authentication
requests received after that SHOULD be silently ignored.
Any non-authentication messages sent by the client after the request
that resulted in SSH_MSG_USERAUTH_SUCCESS being sent MUST be passed
to the service being run on top of this protocol. Such messages can
be identified by their message numbers (see Section Message Numbers
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(Section 3)).
2.3 The "none" Authentication Request
A client may request a list of authentication methods that may
continue by using the "none" authentication method.
If no authentication at all is needed for the user, the server MUST
return SSH_MSG_USERAUTH_SUCCESS. Otherwise, the server MUST return
SSH_MSG_USERAUTH_FAILURE and MAY return with it a list of
authentication methods that can continue.
This method MUST NOT be listed as supported by the server.
2.4 Completion of User Authentication
Authentication is complete when the server has responded with
SSH_MSG_USERAUTH_SUCCESS; all authentication related messages
received after sending this message SHOULD be silently ignored.
After sending SSH_MSG_USERAUTH_SUCCESS, the server starts the
requested service.
2.5 Banner Message
In some jurisdictions, sending a warning message before
authentication may be relevant for getting legal protection. Many
UNIX machines, for example, normally display text from `/etc/issue',
or use "tcp wrappers" or similar software to display a banner before
issuing a login prompt.
The SSH server may send a SSH_MSG_USERAUTH_BANNER message at any time
before authentication is successful. This message contains text to
be displayed to the client user before authentication is attempted.
The format is as follows:
byte SSH_MSG_USERAUTH_BANNER
string message (ISO-10646 UTF-8)
string language tag (as defined in [RFC1766])
The client SHOULD by default display the message on the screen.
However, since the message is likely to be sent for every login
attempt, and since some client software will need to open a separate
window for this warning, the client software may allow the user to
explicitly disable the display of banners from the server. The
message may consist of multiple lines.
If the message string is displayed, control character filtering
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discussed in [SSH-ARCH] SHOULD be used to avoid attacks by sending
terminal control characters.
3. Authentication Protocol Message Numbers
All message numbers used by this authentication protocol are in the
range from 50 to 79, which is part of the range reserved for
protocols running on top of the SSH transport layer protocol.
Message numbers of 80 and higher are reserved for protocols running
after this authentication protocol, so receiving one of them before
authentication is complete is an error, to which the server MUST
respond by disconnecting (preferably with a proper disconnect message
sent first to ease troubleshooting).
After successful authentication, such messages are passed to the
higher-level service.
These are the general authentication message codes:
#define SSH_MSG_USERAUTH_REQUEST 50
#define SSH_MSG_USERAUTH_FAILURE 51
#define SSH_MSG_USERAUTH_SUCCESS 52
#define SSH_MSG_USERAUTH_BANNER 53
In addition to the above, there is a range of message numbers
(60..79) reserved for method-specific messages. These messages are
only sent by the server (client sends only SSH_MSG_USERAUTH_REQUEST
messages). Different authentication methods reuse the same message
numbers.
4. Public Key Authentication Method: publickey
The only REQUIRED authentication method is public key authentication.
All implementations MUST support this method; however, not all users
need to have public keys, and most local policies are not likely to
require public key authentication for all users in the near future.
With this method, the possession of a private key serves as
authentication. This method works by sending a signature created
with a private key of the user. The server MUST check that the key
is a valid authenticator for the user, and MUST check that the
signature is valid. If both hold, the authentication request MUST be
accepted; otherwise it MUST be rejected. (Note that the server MAY
require additional authentications after successful authentication.)
Private keys are often stored in an encrypted form at the client
host, and the user must supply a passphrase before the signature can
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be generated. Even if they are not, the signing operation involves
some expensive computation. To avoid unnecessary processing and user
interaction, the following message is provided for querying whether
authentication using the key would be acceptable.
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "publickey"
boolean FALSE
string public key algorithm name
string public key blob
Public key algorithms are defined in the transport layer
specification [SSH-TRANS]. The public key blob may contain
certificates.
Any public key algorithm may be offered for use in authentication.
In particular, the list is not constrained by what was negotiated
during key exchange. If the server does not support some algorithm,
it MUST simply reject the request.
The server MUST respond to this message with either
SSH_MSG_USERAUTH_FAILURE or with the following:
byte SSH_MSG_USERAUTH_PK_OK
string public key algorithm name from the request
string public key blob from the request
To perform actual authentication, the client MAY then send a
signature generated using the private key. The client MAY send the
signature directly without first verifying whether the key is
acceptable. The signature is sent using the following packet:
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "publickey"
boolean TRUE
string public key algorithm name
string public key to be used for authentication
string signature
Signature is a signature by the corresponding private key over the
following data, in the following order:
string session identifier
byte SSH_MSG_USERAUTH_REQUEST
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string user name
string service
string "publickey"
boolean TRUE
string public key algorithm name
string public key to be used for authentication
When the server receives this message, it MUST check whether the
supplied key is acceptable for authentication, and if so, it MUST
check whether the signature is correct.
If both checks succeed, this method is successful. Note that the
server may require additional authentications. The server MUST
respond with SSH_MSG_USERAUTH_SUCCESS (if no more authentications are
needed), or SSH_MSG_USERAUTH_FAILURE (if the request failed, or more
authentications are needed).
The following method-specific message numbers are used by the
publickey authentication method.
/* Key-based */
#define SSH_MSG_USERAUTH_PK_OK 60
5. Password Authentication Method: password
Password authentication uses the following packets. Note that a
server MAY request the user to change the password. All
implementations SHOULD support password authentication.
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "password"
boolean FALSE
string plaintext password (ISO-10646 UTF-8)
Note that the password is encoded in ISO-10646 UTF-8. It is up to
the server how it interprets the password and validates it against
the password database. However, if the client reads the password in
some other encoding (e.g., ISO 8859-1 (ISO Latin1)), it MUST convert
the password to ISO-10646 UTF-8 before transmitting, and the server
MUST convert the password to the encoding used on that system for
passwords.
Note that even though the cleartext password is transmitted in the
packet, the entire packet is encrypted by the transport layer. Both
the server and the client should check whether the underlying
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transport layer provides confidentiality (i.e., if encryption is
being used). If no confidentiality is provided (none cipher),
password authentication SHOULD be disabled. If there is no
confidentiality or no MAC, password change SHOULD be disabled.
Normally, the server responds to this message with success or
failure. However, if the password has expired the server SHOULD
indicate this by responding with SSH_MSG_USERAUTH_PASSWD_CHANGEREQ.
In anycase the server MUST NOT allow an expired password to be used
for authentication.
byte SSH_MSG_USERAUTH_PASSWD_CHANGEREQ
string prompt (ISO-10646 UTF-8)
string language tag (as defined in [RFC1766])
In this case, the client MAY continue with a different authentication
method, or request a new password from the user and retry password
authentication using the following message. The client MAY also send
this message instead of the normal password authentication request
without the server asking for it.
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "password"
boolean TRUE
string plaintext old password (ISO-10646 UTF-8)
string plaintext new password (ISO-10646 UTF-8)
The server must reply to request message with
SSH_MSG_USERAUTH_SUCCESS, SSH_MSG_USERAUTH_FAILURE, or another
SSH_MSG_USERAUTH_PASSWD_CHANGEREQ. The meaning of these is as
follows:
SSH_MSG_USERAUTH_SUCCESS The password has been changed, and
authentication has been successfully completed.
SSH_MSG_USERAUTH_FAILURE with partial success The password has
been changed, but more authentications are needed.
SSH_MSG_USERAUTH_FAILURE without partial success The password has
not been changed. Either password changing was not supported, or
the old password was bad. Note that if the server has already
sent SSH_MSG_USERAUTH_PASSWD_CHANGEREQ, we know that it supports
changing the password.
SSH_MSG_USERAUTH_CHANGEREQ The password was not changed because
the new password was not acceptable (e.g. too easy to guess).
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The following method-specific message numbers are used by the
password authentication method.
#define SSH_MSG_USERAUTH_PASSWD_CHANGEREQ 60
6. Host-Based Authentication: hostbased
Some sites wish to allow authentication based on the host where the
user is coming from, and the user name on the remote host. While
this form of authentication is not suitable for high-security sites,
it can be very convenient in many environments. This form of
authentication is OPTIONAL. When used, special care SHOULD be taken
to prevent a regular user from obtaining the private host key.
The client requests this form of authentication by sending the
following message. It is similar to the UNIX "rhosts" and
"hosts.equiv" styles of authentication, except that the identity of
the client host is checked more rigorously.
This method works by having the client send a signature created with
the private key of the client host, which the server checks with that
host's public key. Once the client host's identity is established,
authorization (but no further authentication) is performed based on
the user names on the server and the client, and the client host
name.
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "hostbased"
string public key algorithm for host key
string public host key and certificates for client host
string client host name (FQDN; US-ASCII)
string user name on the client host (ISO-10646 UTF-8)
string signature
Public key algorithm names for use in "public key algorithm for host
key" are defined in the transport layer specification. The "public
host key for client host" may include certificates.
Signature is a signature with the private host key of the following
data, in this order:
string session identifier
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
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string "hostbased"
string public key algorithm for host key
string public host key and certificates for client host
string client host name (FQDN; US-ASCII)
string user name on the client host(ISO-10646 UTF-8)
The server MUST verify that the host key actually belongs to the
client host named in the message, that the given user on that host is
allowed to log in, and that the signature is a valid signature on the
appropriate value by the given host key. The server MAY ignore the
client user name, if it wants to authenticate only the client host.
It is RECOMMENDED that whenever possible, the server perform
additional checks to verify that the network address obtained from
the (untrusted) network matches the given client host name. This
makes exploiting compromised host keys more difficult. Note that
this may require special handling for connections coming through a
firewall.
7. Security Considerations
The purpose of this protocol is to perform client user
authentication. It assumed that this runs over a secure transport
layer protocol, which has already authenticated the server machine,
established an encrypted communications channel, and computed a
unique session identifier for this session. The transport layer
provides forward secrecy for password authentication and other
methods that rely on secret data.
The server may go into a "sleep" period after repeated unsuccessful
authentications to make key search harder.
If the transport layer does not provide encryption, authentication
methods that rely on secret data SHOULD be disabled. If it does not
provide MAC protection, requests to change authentication data (e.g.
password change) SHOULD be disabled to avoid an attacker from
modifying the ciphertext without being noticed, rendering the new
authentication data unusable (denial of service).
Several authentication methods with different security
characteristics are allowed. It is up to the server's local policy
to decide which methods (or combinations of methods) it is willing to
accept for each user. Authentication is no stronger than the weakest
combination allowed.
Special care should be taken when designing debug messages. These
messages may reveal surprising amounts of information about the host
if not properly designed. Debug messages can be disabled (during
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user authentication phase) if high security is required.
8. Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification can
be obtained from the IETF Secretariat.
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
9. Additional Information
The current document editor is: Darren.Moffat%Sun.COM@localhost. Comments on
this internet draft should be sent to the IETF SECSH working group,
details at: http://ietf.org/html.charters/secsh-charter.html
References
[RFC1766] Alvestrand, H., "Tags for the Identification of
Languages", RFC 1766, March 1995.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[SSH-ARCH] Ylonen, T., "SSH Protocol Architecture", I-D draft-
ietf-architecture-13.txt, September 2002.
[SSH-TRANS] Ylonen, T., "SSH Transport Layer Protocol", I-D
draft-ietf-transport-15.txt, September 2002.
[SSH-USERAUTH] Ylonen, T., "SSH Authentication Protocol", I-D draft-
ietf-userauth-16.txt, September 2002.
[SSH-CONNECT] Ylonen, T., "SSH Connection Protocol", I-D draft-
ietf-connect-16.txt, September 2002.
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Authors' Addresses
Tatu Ylonen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: ylo%ssh.com@localhost
Tero Kivinen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: kivinen%ssh.com@localhost
Markku-Juhani O. Saarinen
University of Jyvaskyla
Timo J. Rinne
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: tri%ssh.com@localhost
Sami Lehtinen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: sjl%ssh.com@localhost
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Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Ylonen, et. al. Expires March 21, 2003 [Page 15]
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