Re: DSA authentication under SSH

"J" == J Sommers <jsommers@xxxxxxxxxxxxxx> writes:

J> For this kind of authentication, SSH clients send the basic DSA
J> parameters (p, q, y and g) to the server, plus the signature (r
and
J> s). My question is, what is the message being signed?

J> If I understand it correctly, both client and server must have
J> access to the message - the client, to create the signature, and
J> the server, to verify the signature. However, it is not at all
J> clear to me what message is being used during the authentication
J> process. Can anyone help?

This question is best answered by reading RFC 4252. From section 7:

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 name
string "publickey"
boolean TRUE
string public key algorithm name
string public key to be used for authentication
string signature

The value of '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
string user name
string service name
string "publickey"
boolean TRUE
string public key algorithm name
string public key to be used for authentication

Of course, you need to read the rest to understand what all these
things
are. In short, the key element here is the "session identifier." This
is
a value produced by the key exchange, with two important properties: it
is
different for every SSH connection, and its value cannot be forced by
either side. This makes it effective in countering man-in-the-middle
attacks. The encapsulating SSH transport protocol does this as well
during server authentication.

--
Richard Silverman
res@xxxxxxxx

.

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