A Usable, Secure Communications Protocol: Client-Side TLS - Implementing SSL/TLS Using Cryptography and PKI - page 349

6. With the handshake digest defi ned and updated, the client can send its
finished message, in Listing 6-51.

Listing 6-51: “tls.c” tls_connect with client fi nished message

if ( !( send_change_cipher_spec( connection, parameters ) ) )
{
perror( “Unable to send client change cipher spec” );
return 4;
}

// This message will be encrypted using the newly negotiated keys
if ( !( send_finished( connection, parameters ) ) )
{
perror( “Unable to send client finished” );
return 5;
}

7. send_finished itself is straightforward, as shown in Listing 6-52.

Listing 6-52: “tls.c” send_fi nished

static int send_finished( int connection,
TLSParameters *parameters )
{
unsigned char verify_data[ VERIFY_DATA_LEN ];
compute_verify_data( “client finished”, parameters, verify_data );
send_handshake_message( connection, finished, verify_data, VERIFY_DATA_LEN,
parameters );

return 1;
}

8. Of course, as you can likely guess, the challenge is in the computation of
verify data . This is shown in Listing 6-53.

Listing 6-53: “tls.c” compute_verify_data

/**
* 7.4.9:
* verify_data = PRF( master_secret, “client finished”, MD5(handshake_messages)
* + SHA-1(handshake_messages)) [0..11]
*
* master_secret = PRF( pre_master_secret, “master secret”, ClientHello.random +
* ServerHello.random );
* always 48 bytes in length.
*/
#define VERIFY_DATA_LEN 12

static void compute_verify_data( const char *finished_label,
TLSParameters *parameters,

(Continued)