Problem - Kevin Tafuro

Discussion
Remember, authentication requires a trusted third party or a secure
channel for exchange of public DSA keys. If you’d prefer a passwordbased
protocol that can achieve all the same properties you would get
from Diffie-Hellman and DSA, see the discussion of PAX in Recipe 8.15.
Given a client initiating a connection with a server, the station-to-station protocol is
as follows:
1. The client generates a random Diffie-Hellman secret x and the corresponding
public value A.
2. The client sends A to the server.
3. The server generates a random Diffie-Hellman secret y and the corresponding
public value B.
4. The server computes the Diffie-Hellman shared secret.
5. The server signs a string consisting of the public values A and B with the server’s
private DSA key.
6. The server sends B and the signature to the client.
7. The client computes the shared secret.
8. The client validates the signature, failing if it isn’t valid.
9. The client signs A concatenated with B using its private DSA key, and it encrypts
the result using the shared secret (the secret can be postprocessed first, as long as
both sides do the same processing).
10. The client sends the encrypted signature to the server.
11. The server decrypts the signature and validates it.
The station-to-station protocol works only if your Diffie-Hellman keys are always
one-time values. If you need a protocol that doesn’t expose the private values of each
party, use Recipe 8.16. That basic protocol can be adapted from RSA to Diffie-Hellman
with DSA if you so desire.
Unless you allow for anonymous connection establishment, the client needs to identify
itself as part of this protocol. The client can send its public key (or a digital certificate
containing the public key) at Step 2. The server should already have a record of
the client based on its public key, or else it should fail. Alternatively, you can drop
the client validation steps (9–11) and use a traditional login mechanism after the
encrypted link is established.
Using Diffie-Hellman and DSA Together | 437
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.