On Saturday, August 16, 2003 09:16:33 -0700 "Paul Hoffman / VPNC" <paul.hoffman%vpnc.org@localhost> wrote:
Greetings again. I'm trying to figure out the key exchange in SSH from
reading secsh-transport-16 and have hit a roadblock. Section 5.2 has you
doing hashes over strings that include "session_id", but that is never
defined in the document.
The text in the first paragraph of 5.2 says:
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 sounds like session_id and H are the same thing, but that makes no
sense when looking at the contents of the hashed strings. Why would you
include the contents of "H" twice?
"H" is the exchange hash resulting from the key exchange just completed, and is used to tie the encryption keys to the key exchange, and thus to the host authentication. It changes each time key exchange is performed (that is, during every rekey operation)
"session_id" is the exchange hash resulting from the _first_ key exchange on a given session. For that first exchange, it is the same as "H", but unlike "H", the "session_id" does not change during a rekey. It is used to tie authentication operations to a particular session.
I believe the first paragraph of section 5.2 spells this out, but it might be a little less clear than it could be about the distinction.
-- Jeffrey T. Hutzelman (N3NHS) <jhutz+%cmu.edu@localhost> Sr. Research Systems Programmer School of Computer Science - Research Computing Facility Carnegie Mellon University - Pittsburgh, PA