Problem - Kevin Tafuro

BN_CTX *tmp_ctx;
if (!(secret = BN_new( ))) return 0;
if (!(tmp_ctx = BN_CTX_new( ))) {
BN_free(secret);
return 0;
}
if (!BN_mod_exp(secret, received, ctx->private_value, ctx->n, tmp_ctx)) {
BN_CTX_free(tmp_ctx);
BN_free(secret);
return 0;
}
BN_CTX_free(tmp_ctx);
return secret;
}
You can turn the shared secret into a key by converting the BIGNUM object returned by
DH_compute_secret( ) to binary (see Recipe 7.4) and then hashing it with SHA1, as
discussed above.
Traditional Diffie-Hellman is sometimes called ephemeral Diffie-Hellman, because
the algorithm can be seen as generating key pairs for one-time use. There are variants
of Diffie-Hellman that always use the same values for each client. There are
some hidden “gotchas” when doing that, so we don’t particularly recommend it.
However, if you wish to explore it, see RFC 2631 and RFC 2785 for more information.
See Also
• RFC 2631: Diffie-Hellman Key Agreement Method
• RFC 2785: Methods for Avoiding the “Small-Subgroup” Attacks on the Diffie-
Hellman Key Agreement Method for S/MIME
• Recipes 4.11, 7.1, 7.4, 7.5, 7.15, 8.17, 8.18
8.18 Using Diffie-Hellman and DSA Together
Problem
You want to use Diffie-Hellman for key exchange, and you need some secure way to
authenticate the key agreement to protect against a man-in-the-middle attack.
Solution
Use the station-to-station protocol for two-way authentication. A simple modification
provides one-way authentication. For example, the server may not care to
authenticate the client using public key cryptography.
436 | Chapter 8: Authentication and Key Exchange
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.