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Submission: draft-ietf-secsh-publickey-subsystem-03.txt
Hi,
I'd be grateful if you could post the attached submission, which is an
update of draft-galb-secsh-publickey-subsystem-02.txt (and is now a
secsh WG working item, hence the name change from galb -> ietf).
--
Jon Bright
Silicon Circus Ltd.
http://www.siliconcircus.com
Secure Shell Working Group J. Galbraith
Internet-Draft J. Van Dyke
Expires: March 15, 2004 B. McClure
VanDyke Software
J. Bright
Silicon Circus
September 15, 2003
Secure Shell Public-Key Subsystem
draft-ietf-secsh-publickey-subsystem-03.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 15, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
SECSH defines an authentication mechanism that is based on public
keys, but does not define any mechanism for key distribution. No
common key management solution exists in current implementations.
This document describes a protocol that can be used to configure
public keys in an implementation-independent fashion, allowing client
software to take on the burden of this configuration.
This protocol is intended to be used from the Secure Shell Connection
Protocol [4] as a subsystem, as described in Section ``Starting a
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Shell or a Command''. The subsystem name used with this protocol is
"publickey".
The public-key subsystem provides a server-independent mechanism for
clients to add public keys, remove public keys, and list the current
public keys known by the server. Rights to manage public keys are
specific and limited to the authenticated user.
A public key may also be associated with various restrictions,
including a mandatory command or subsystem.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Public-Key Subsystem Overview . . . . . . . . . . . . . . . 4
2.1 Opening the Public-Key Subsystem . . . . . . . . . . . . . . 4
2.2 Requests . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3 Responses . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3.1 The Status Response . . . . . . . . . . . . . . . . . . . . 5
3. Public-Key Subsystem Operations . . . . . . . . . . . . . . 7
3.1 Version Packet . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Adding a public key . . . . . . . . . . . . . . . . . . . . 7
3.3 Removing a public key . . . . . . . . . . . . . . . . . . . 10
3.4 Listing public keys . . . . . . . . . . . . . . . . . . . . 10
3.5 Listing server capabilities . . . . . . . . . . . . . . . . 10
4. Security Considerations . . . . . . . . . . . . . . . . . . 12
Normative References . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . 15
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1. Introduction
SECSH is a protocol for secure remote login and other secure network
services over an insecure network. SECSH defines an authentication
mechanism that is based on public keys, but does not define any
mechanism for key distribution. Common practice is to authenticate
once with password authentication and transfer the public key to the
server. However, to date no two implementations use the same
mechanism to configure a public key for use.
This document describes a subsystem that can be used to configure
public keys in an implementation-independent fashion. This approach
allows client software to take on the burden of this configuration.
The public-key subsystem protocol is designed for extreme simplicity
in implementation. It is not intended as a PKIX replacement.
The Secure Shell Public-Key subsystem has been designed to run on top
of the SECSH transport layer [2] and user authentication protocols
[3]. It provides a simple mechanism for the client to manage public
keys on the server.
This document should be read only after reading the SECSH
architecture [1] and SECSH connection [4] documents.
This protocol requires that the user be able to authenticate in some
fashion before it can be used. If password authentication is used,
servers SHOULD provide a configuration option to disable the use of
password authentication after the first public key is added.
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2. Public-Key Subsystem Overview
The public-key subsystem provides a server-independent mechanism for
clients to add public keys, remove public keys, and list the current
public keys known by the server. The subsystem name is "publickey".
The public keys added, removed, and listed using this protocol are
specific and limited to those of the authenticated user.
The operations to add, remove and list the authenticated user's
public keys are performed as request packets sent to the server. The
server sends response packets that indicate success or failure as
well as provide specific response data.
The format of public-key blobs are detailed in the SSH Transport
Protocol document [2].
2.1 Opening the Public-Key Subsystem
The public-key subsystem is opened when the clients sends a
SSH_MSG_CHANNEL_REQUEST over an existing session.
The details of how a session is opened are described in the SSH
Connection Protocol document [4] in the section "Opening a Session".
To open the public-key subsystem, the client sends:
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient channel
string "subsystem"
boolean want reply
string "publickey"
Client implementations SHOULD reject this request; it is normally
only sent by the client.
If want reply is TRUE, the server MUST respond with
SSH_MSG_CHANNEL_SUCCESS if the public-key subsystem was successfully
started or SSH_MSG_CHANNEL_FAILURE if the server failed to start or
does not support the public-key subsystem.
The server SHOULD respond with SSH_MSG_CHANNEL_FAILURE if the user
authenticated with a restricted public key that does not allow access
to the publickey subsystem.
It is RECOMMENDED that clients request and check the reply for this
request.
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2.2 Requests
All public-key subsystem requests are sent in the following form:
uint32 length
string request-name
... request specific data follows
The length field describes the length of the request-name field and
the request-specific data, but not of the length field itself. The
client MUST receive acknowledgement of each request prior to sending
a new request.
All requests described in Section 3 are a description of the
'request-name' and 'data' portion of the packet.
2.3 Responses
All public-key subsystem responses are sent in the following form:
uint32 length
string response-name
... response specific data follows
2.3.1 The Status Response
A request is acknowledged by sending a status packet. If there is
data in response to the request, the status packet is sent after all
data has been sent.
string "status"
uint32 status code
string description [RFC-2279]
string language tag [RFC-1766]
A status message MUST be sent for any unrecognized packets and the
request SHOULD NOT close the subsystem.
2.3.1.1 Status Codes
The status code gives the status in a more machine-readable format
(suitable for localization), and can have the following values:
SSH_PUBLICKEY_SUCCESS 0
SSH_PUBLICKEY_ACCESS_DENIED 1
SSH_PUBLICKEY_STORAGE_EXCEEDED 2
SSH_PUBLICKEY_VERSION_NOT_SUPPORTED 3
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SSH_PUBLICKEY_KEY_NOT_FOUND 4
SSH_PUBLICKEY_KEY_NOT_SUPPORTED 5
SSH_PUBLICKEY_KEY_ALREADY_PRESENT 6
SSH_PUBLICKEY_GENERAL_FAILURE 7
SSH_PUBLICKEY_REQUEST_NOT_SUPPORTED 8
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3. Public-Key Subsystem Operations
The public-key subsystem currently defines four operations: add,
remove, list, and command.
3.1 Version Packet
Both sides MUST start by sending a version packet that indicates the
version of the protocol they are using.
string "version"
uint32 protocol-version-number
The version of the protocol described by this document is version 2.
Both sides send the highest version that they implement. The lower of
the version numbers is the version of the protocol to use. If either
side can't support the lower version, it should close the subsystem
and notify the other side by sending an SSH_MSG_CHANNEL_CLOSE
message. Before closing the subsystem, a status message with the
status SSH_PUBLICKEY_VERSION_NOT_SUPPORTED SHOULD be sent.
Both sides MUST wait to receive this version before continuing.
3.2 Adding a public key
If the client wishes to add a public key, the client sends:
string "add"
string public-key algorithm name
string public-key blob
boolean overwrite
uint32 attribute-count
string attrib-name
string attrib-value
bool mandatory
repeated attribute-count times
The server MUST attempt to store the public key for the user in the
appropriate location so the public key can be used for subsequent
public-key authentications. If the overwrite field is false and the
specified key already exists, the server MUST return
SSH_PUBLICKEY_KEY_ALREADY_PRESENT. If the server returns this, the
client SHOULD provide an option to the user to overwrite the key. If
the overwrite field is true and the specified key already exists but
cannot be overwritten, the server MUST return
SSH_PUBLICKEY_ACCESS_DENIED
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Attribute names are defined following the same scheme laid out for
algorithm names in [1]. If the server does not implement a mandatory
attribute, it MUST fail the add. For the purposes of a mandatory
attribute, storage of the attribute is not sufficient, but requires
that the server understand and implement the intent of the attribute.
The following attributes are currently defined:
"comment"
The value of the comment attribute contains user-specified text about
the public key. The server SHOULD make every effort to preserve this
value and return it with the key during any subsequent list
operation. The server MUST NOT attempt to interpret or act upon the
content of the comment field in any way. The comment attribute must
be specified in UTF-8 format [6].
The comment field is useful so the user can identify the key without
resorting to comparing its fingerprint. This attribute SHOULD NOT be
mandatory.
"comment-language"
If this attribute is specified, it MUST immediately follow a
"comment" attribute and specifies the language for that attribute
[5]. The client MAY specify more than comment if it additionally
specifies a different language for each of those comments. The
server SHOULD attempt to store each comment, together with that
comment's lanuage attribute. This attribute SHOULD NOT be mandatory.
"command-override"
"command-override" specifies a command to be executed when this key
is in use. The command should be executed by the server when it
receives an "exec" or "shell" request from the client, in place of
the command or shell which would otherwise have been executed as a
result of that request. If the command string is empty, both "exec"
and "shell" requests should be denied. If no "command-override"
attribute is specified, all "exec" and "shell" requests should be
permitted (as long as they satisfy other security or authorisation
checks the server may perform). This attribute SHOULD be mandatory.
"subsystem"
"subsystem" specifies a comma-separated list of subsystems that may
be started (using a "subsystem" request) when this key is in use.
This attribute SHOULD be mandatory. If the value is empty, no
subsystems may be started.
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"x11"
"x11" specifies that X11 forwarding may not be performed when this
key is in use. The attribute-value field SHOULD be empty for this
attribute. This attribute SHOULD be mandatory.
"shell"
"shell" specifies that session channel "shell" requests should be
denied when this key is in use. The attribute-value field SHOULD be
empty for this attribute. This attribute SHOULD be mandatory.
"exec"
"exec" specifies that session channel "exec" requests should be
denied when this key is in use. The attribute-value field SHOULD be
empty for this attribute. This attribute SHOULD be mandatory.
"agent"
"agent" specifies that session channel "auth-agent-req" requests
should be denied when this key is in use. The attribute-value field
SHOULD be empty for this attribute. This attribute SHOULD be
mandatory.
"env"
"env" specifies that session channel "env" requests should be denied
when this key is in use. The attribute-value field SHOULD be empty
for this attribute. This attribute SHOULD be mandatory.
"from"
"from" specifies a comma-separated list of hosts from which the key
may be used. If a host not in this list attempts to use this key for
authorisation purposes, the authorisation attempt MUST be denied.
The server SHOULD make a log entry regarding this.
"port-forward"
"port-forward" specifies that no "direct-tcpip" requests should be
accepted, except to those hosts specified in the comma-separated list
supplied as a value to this attribute. If the value of this
attribute is empty, all "direct-tcpip" requests should be refused
when using this key. This attribute SHOULD be mandatory.
"reverse-forward"
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"reverse-forward" specifies that no "tcpip-forward" requests should
be accepted, accept for the port numbers in the comma-separated list
supplied as a value to this attribute. If the value of this
attribute is empty, all "tcpip-forward" requests should be refused
when using this key. This attribute SHOULD be mandatory.
In addition to the attributes specified by the client, the server MAY
provide a method for administrators to compulsorily enforce certain
attributes.
3.3 Removing a public key
If the client wishes to remove a public key, the client sends:
string "remove"
string public-key algorithm name
string public-key blob
The server MUST attempt to remove the public key for the user from
the appropriate location, so that the public key cannot be used for
subsequent authentications.
3.4 Listing public keys
If the client wishes to list the known public keys, the client sends:
string "list"
The server will respond with zero or more of the following responses:
string "publickey"
string public-key algorithm name
string public-key blob
uint32 attribute-count
string attrib-name
string attrib-value
repeated attribute-count times
Following the last "publickey" response, a status packet MUST be
sent.
An implementation MAY choose not to support this request.
3.5 Listing server capabilities
If the client wishes to know which key attributes the server
supports, it sends:
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string "listattributes"
The server will respond with zero or more of the following responses:
string "attribute"
string attribute name
boolean compulsory
The "compulsory" field indicates whether this attribute will be
compulsorily applied to any added keys (irrespective of whether the
attribute has been specified by the client) due to administrative
settings on the server. If the server does not support
administrative settings of this nature, it MUST return false in the
compulsory field.
Following the last "attribute" response, a status packet MUST be
sent.
An implementation MAY choose not to support this request.
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4. Security Considerations
This protocol assumes that it is run over a secure channel and that
the endpoints of the channel have been authenticated. Thus, this
protocol assumes that it is externally protected from network-level
attacks.
This protocol provides a mechanism that allows client authentication
data to be uploaded and manipulated. It is the responsibility of the
server implementation to enforce any access controls that may be
required to limit the access allowed for any particular user (the
user being authenticated externally to this protocol, typically using
the SSH User Authentication Protocol [3]). In particular, it is
possible for users to overwrite an existing key on the server with
this protocol, whilst at the same time specifying fewer restrictions
for the new key than were previously present. Servers should take
care that when doing this, clients are not able to override presets
from the server's administrator.
This protocol requires the client to assume that the server will
correctly implement and observe attributes applied to keys.
Implementation errors in the server could cause clients to authorise
keys for access they were not intended to have, or to apply fewer
restrictions than were intended.
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Normative References
[1] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S.
Lehtinen, "SSH Protocol Architecture",
draft-ietf-secsh-architecture-13 (work in progress), January
2002.
[2] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S.
Lehtinen, "SSH Transport Layer Protocol",
draft-ietf-secsh-transport-15 (work in progress), March 2002.
[3] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S.
Lehtinen, "SSH Authentication Protocol",
draft-ietf-secsh-userauth-16 (work in progress), February 2002.
[4] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S.
Lehtinen, "SSH Connection Protocol", draft-ietf-secsh-connect-16
(work in progress), January 2002.
[5] Alvestrand, H., "Tags for the Identification of Languages", RFC
1766, March 1995.
[6] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.
Authors' Addresses
Joseph Galbraith
VanDyke Software
4848 Tramway Ridge Blvd
Suite 101
Albuquerque, NM 87111
US
Phone: +1 505 332 5700
EMail: galb-list%vandyke.com@localhost
Jeff P. Van Dyke
VanDyke Software
4848 Tramway Ridge Blvd
Suite 101
Albuquerque, NM 87111
US
Phone: +1 505 332 5700
EMail: jpv%vandyke.com@localhost
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Brent McClure
VanDyke Software
4848 Tramway Ridge Blvd
Suite 101
Albuquerque, NM 87111
US
Phone: +1 505 332 5700
EMail: bdm%vandyke.com@localhost
Jon Bright
Silicon Circus
24 Jubilee Road
Chichester, West Sussex PO19 7XB
UK
Phone: +49 172 524 0521
EMail: jon%siliconcircus.com@localhost
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
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Galbraith, et al. Expires March 15, 2004 [Page 16]
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