Adding TLS 1.2 Support to Your TLS Library - Implementing SSL/TLS Using Cryptography and PKI - page 539

using the curve and the point specifi ed by the server in the server key exchange,
which is shown in Listing 9-43.
Listing 9-43: “tls.c” ecdh_key_exchange
int ecdh_key_exchange( point *server_ecdh_key,
elliptic_curve *curve,
unsigned char *premaster_secret,
unsigned char **key_exchange_message )
{
ecc_key A;
point K;
set_huge( &A.d, 4 ); // TODO this should be random, and larger
set_huge( &A.Q.x, 0 );
set_huge( &A.Q.y, 0 );
copy_huge( &A.Q.x, &curve->G.x );
copy_huge( &A.Q.y, &curve->G.y );
multiply_point( &A.Q, &A.d, &curve->a, &curve->p );
// Response is now A.Q; put that into “key_exchange_message” as an
// explicit point
// XXX x & y must both be the same size for the encoding to work.
*key_exchange_message = ( unsigned char * ) malloc( A.Q.x.size +
A.Q.y.size + 2 );
(*key_exchange_message)[ 0 ] = A.Q.x.size + A.Q.y.size + 1;
(*key_exchange_message)[ 1 ] = 0x04;
memcpy( (*key_exchange_message) + 2, A.Q.x.rep, A.Q.x.size );
memcpy( (*key_exchange_message) + 2 + A.Q.x.size, A.Q.y.rep, A.Q.y.size );
// Now compute the premaster secret from the server's point
set_huge( &K.x, 0 );
set_huge( &K.y, 0 );
copy_huge( &K.x, &server_ecdh_key->x );
copy_huge( &K.y, &server_ecdh_key->y );
multiply_point( &K, &A.d, &curve->a, &curve->p );
// The premaster secret is in K.x
memcpy( premaster_secret, K.x.rep, curve->p.size );
free_huge( &K.x );
free_huge( &K.y );
free_huge( &A.d );
free_huge( &A.Q.x );
free_huge( &A.Q.y );
return A.Q.x.size + A.Q.y.size + 3;
}
The client responsibility in an ECDH key exchange — just as in an integer DH
key exchange — is lighter than the server's; the client must just respond with a