Authenticating Communications Using Digital Signatures - Implementing SSL/TLS Using Cryptography and PKI - page 207

copy_huge( &q, &params->q );
divide( &u1, &q, NULL ); // u1 is “v” now

// Check to see if v & s match
match = !compare( &u1, &signature->r );

free_huge( &w );
free_huge( &z );
free_huge( &u1 );
free_huge( &u2 );
return match;
}

As with the signing algorithm, I've added comments so that you can match
what the code is doing with the algorithm. Notice that this doesn't use u1 and
u2 exactly as they're shown in the algorithm, instead putting u1 and u2 into z
and w because you don't need them again, and then using u1 and u2 to hold
the mod_pow values later on.
Also notice how:

( ( g u1 y u2 ) % p ) % q
is put together. You don't want to compute g u1 , then compute y u2 and then mul-
tiply them by each other to fi nally fi gure out “mod p” of the whole mess. You
want to be able to use your mod_pow algorithm to keep the memory constraints
manageable. So instead, factor the v computation out into
v

v

( ( ( g u1 % p ) * ( y u2 % p ) ) % p ) % q

by the distributivity of the modulus operator. Now you can use mod_pow to
compute (g u1 ) % p and ( y u2 % p), multiply these together, which results in at
most 2 p bits, and then apply the modulus operation twice.
You can put together a main routine to test this but, like RSA's e , d , and n , the
DSA parameters g , p , q , x , and y must be specifi cally related and you haven't yet
seen how. So just hardcode a sample set in the routine in Listing 4-34 to show
how it can be called.

Listing 4-34: “dsa.c” test main routine

#ifdef TEST_DSA
int main( int argc, char *argv[] )
{
unsigned char priv[] = {
0x53, 0x61, 0xae, 0x4f, 0x6f, 0x25, 0x98, 0xde, 0xc4, 0xbf, 0x0b, 0xbe, 0x09,
0x5f, 0xdf, 0x90, 0x2f, 0x4c, 0x8e, 0x09 };
unsigned char pub[] = {
0x1b, 0x91, 0x4c, 0xa9, 0x73, 0xdc, 0x06, 0x0d, 0x21, 0xc6, 0xff, 0xab, 0xf6,
(Continued)