Problem - Kevin Tafuro

Often, you’ll want to generate a set of keys, such as an encryption key and a MAC
key. (See Recipe 4.11 for a complete discussion of key derivation.)
Key exchange with Diffie-Hellman isn’t secure unless you have some
secure way of authenticating the other end. Generally, you should digitally
sign messages in this protocol with DSA or RSA, and be sure that
both sides securely authenticate the signature—for example, through a
public key infrastructure.
Once a key or keys are established, the two parties try to communicate. If both sides
are using message integrity checks, they’ll quickly know whether or not the exchange
was successful (if it’s not, nothing will validate on decryption).
If you don’t want to use an existing API, here’s an example of generating a random
secret and computing the value to send to the other party (we use the OpenSSLarbitrary
precision math library):
#include
typedef struct {
BIGNUM *n;
BIGNUM *g; /* use a BIGNUM even though g is usually small. */
BIGNUM *private_value;
BIGNUM *public_value;
} DH_CTX;
/* This function assumes that all BIGNUMs are already allocated, and that n and g
* have already been chosen and properly initialized. After this function
* completes successfully, use BN_bn2bin( ) on ctx->public_value to get a binary
* representation you can send over a network. See Recipe 7.4 for more info on
* BNbinary conversions.
*/
int DH_generate_keys(DH_CTX *ctx) {
BN_CTX *tmp_ctx;
if (!(tmp_ctx = BN_CTX_new( ))) return 0;
if (!BN_rand_range(ctx->private_value, ctx->n)) {
BN_CTX_free(tmp_ctx);
return 0;
}
if (!BN_mod_exp(ctx->public_value, ctx->g, ctx->private_value, ctx->n, tmp_ctx)) {
BN_CTX_free(tmp_ctx);
return 0;
}
BN_CTX_free(tmp_ctx);
return 1;
}
When one side receives the Diffie-Hellman message from the other, it can compute
the shared secret from the DH_CTX object and the message as follows:
BIGNUM *DH_compute_secret(DH_CTX *ctx, BIGNUM *received) {
BIGNUM *secret;
Using Basic Diffie-Hellman Key Agreement | 435
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.