Advanced SSL Topics - Implementing SSL/TLS Using Cryptography and PKI - page 441

To verify an RSA signature over the server key exchange parameters, imple-
ment the verify_signature function in Listing 8-21.
Listing 8-21: “tls.c” verify_signature
static int verify_signature( unsigned char *message,
int message_len,
unsigned char *signature,
int signature_len,
TLSParameters *parameters )
{
unsigned char *decrypted_signature;
int decrypted_signature_length;
digest_ctx md5_digest;
digest_ctx sha1_digest;
new_sha1_digest( &sha1_digest );
update_digest( &sha1_digest, parameters->client_random, RANDOM_LENGTH );
update_digest( &sha1_digest, parameters->server_random, RANDOM_LENGTH );
update_digest( &sha1_digest, message, message_len );
finalize_digest( &sha1_digest );
new_md5_digest( &md5_digest );
update_digest( &md5_digest, parameters->client_random, RANDOM_LENGTH );
update_digest( &md5_digest, parameters->server_random, RANDOM_LENGTH );
update_digest( &md5_digest, message, message_len );
finalize_digest( &md5_digest );
decrypted_signature_length = rsa_decrypt( signature, signature_len,
&decrypted_signature,
&parameters->server_public_key.rsa_public_key );
if ( memcmp( md5_digest.hash, decrypted_signature, MD5_BYTE_SIZE ) ||
memcmp( sha1_digest.hash, decrypted_signature + MD5_BYTE_SIZE,
SHA1_BYTE_SIZE ) )
{
return 0;
}
free( decrypted_signature );
return 1;
}
Each digest is over the two random values and then the parameters; incorpo-
rating the random values this way prevents replay attacks. The whole signature
process is illustrated in Figure 8-5.