A Usable, Secure Communications Protocol: Client-Side TLS - Implementing SSL/TLS Using Cryptography and PKI - page 322

{
fprintf( stderr, “Error, server wants compression.\n” );
return NULL;
}
// TODO - abort if there's more data here than in the spec (per section
// 7.4.1.2, forward compatibility note)
// TODO - abort if version < 3.1 with “protocol_version” alert error
// 28 random bytes, but the preceding four bytes are the reported GMT unix
// time
memcpy( ( void * ) parameters->server_random, &hello.random.gmt_unix_time, 4
);
memcpy( ( void * ) ( parameters->server_random + 4 ),
( void * ) hello.random.random_bytes, 28 );
return read_pos;
}
Note that if the server asked for compression, this function returns null
because this implementation doesn't support compression. This is recognized
by the calling routine and is used to generate an alert. Here the terseness of the
TLS alert protocol shows. If the server asked for compression, it just gets back a
nondescript illegal parameter but receives no indication of which parameter was
illegal. It certainly would be more robust if you were allowed to tell it which
parameter you were complaining about. This is generally not a problem for users
of TLS software — if you get an illegal parameter while using, say, a browser,
that means that the programmer of the browser did something wrong — but is a
hassle when developing/testing TLS software like the library developed in this
topic. When developing, therefore, it's best to test against a client or server with
its debug levels set to maximum so that if you do get back an illegal parameter
(or any other nondescript alert message), you can go look at the server logs to
see what you actually did wrong.
This routine stores the server random, of course, because it is needed later on
in the master secret computation. Primarily, though, it sets the values pending_
send_parameters and pending_recv_parameters with the selected suite. Expand
the defi nition of ProtectionParameters to keep track of this in Listing 6-24.
Listing 6-24: “tls.h” ProtectionParameters with cipher suite
typedef struct
{
unsigned char *MAC_secret;
unsigned char *key;
unsigned char *IV;
CipherSuiteIdentifier suite;
}
ProtectionParameters;