Someone asked me what a verifier is. I'm answering to the list, as it
might be of interest to other readers.
A verifier is some information that depends on the password in a
one-way fashion, and is used by the server when verifying that a user
knows the password. It's somewhat like the crypted passwords stored in
/etc/passwd or /etc/shadow, but more sophisticated, in that the user
need not send the clear password to the server to have it verified.
In SRP, if the password is p, the verifier is v = g^H(p) (except that
there's also some salting). The user proves to the server that she
knows a discrete logarithm on the verifier, without revealing either p
or H(p) to the server or to eavesdroppers.
However, that is only one half of the SRP authentication. Furthermore,
the server proves to the client that it knows the verifier. I think
this authentication of the host is the most important part of SRP, in
the SSH context.
More information on SRP can be found at http://srp.stanford.edu.
Regards,
/Niels
