diffie-hellman ssh keyxchange
- by Chuck
Hi,
I've set out to make a primitive SSH client in C#; you might remember me from posts such as http://stackoverflow.com/questions/2872279/c-primitive-ssh-connection-lowlevel hehe.
Anyway, things are great up until the time when I initiate a DH key exchange.
I've compared the traffic when I establish a ssh connection (from openssh client to openssh server), to the traffic when my client connects to the same openssh server.
OpenSSH client - OpenSSH server (S for server, C for client):
S: SSH-2.0-OpenSSH_5.1p1 Debian-6ubuntu2\r (saying hello)
C: SSH-2.0-OpenSSH_5.2\r (introducing myself)
C: Key Exchange Init (0x14 = 20)
S: Key Exchange Init
C: Diffie-Hellman GEX Request (0x22 = 34) (with DH GEX min, number of bits and max)
S: Diffie-Hellman Key Exchange Reply (with P, G, etc.)
C: Diffie-Hellman GEX Init
S: Diffie-Hellman GEX Reply
My client - OpenSSH server:
S: SSH-2.0-OpenSSH_5.1p1 Debian-6ubuntu2\r (saying hello)
C: SSH-2.0-Some_Name\r (introducing myself)
C: Key Exchange Init (0x14 = 20)
S: Key Exchange Init
C: Diffie-Hellman GEX Request (0x22 = 34) (with DH GEX min, number of bits and max)
and then a bogus TCP packet as reply (probably the server connection has been terminated after/upon GEX Request.
I have yet to use AES128 (which I think is the encryption chosen, but I'm not sure how to verify this...), and I'm still sending in a non-compressed format, looking to get the P, G etc. values to make the DH calculations.
So where I'm stranded is:
RFC 4419 page 3 http://www.ietf.org/rfc/rfc4419.txt
I've send SSH_MSG_KEY_DH_GEX_REQUEST, but the server does not respond SSH_MSG_KEX_DH_GEX_GROUP.
Can anyone give me a little advice on what I'm not understanding here? Does the server not understand my GEX request (due to it expecting encryption, or?)?
Any help is very much appreciated, thanks :)
