Authenticating Communications Using Digital Signatures - Implementing SSL/TLS Using Cryptography and PKI - page 182

also a 64-iteration round that's applied to a - h where they shift positions each
round and whichever input is at the head is subject to a complex computation.
The code should be more or less self-explanatory; even if you can't see why
this works, you should be more than convinced that the output is a random
permutation of the input, which is what you want from a hash function. This
is shown in Listing 4-17.
Listing 4-17: “sha.c” SHA-256 block operate
void sha256_block_operate( const unsigned char *block,
unsigned int hash[ 8 ] )
{
unsigned int W[ 64 ];
unsigned int a, b, c, d, e, f, g, h;
unsigned int T1, T2;
int t, i;
/**
* The first 32 bits of the fractional parts of the cube roots
* of the first sixty-four prime numbers.
*/
static const unsigned int k[] =
{
0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1,
0x923f82a4, 0xab1c5ed5, 0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3,
0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174, 0xe49b69c1, 0xefbe4786,
0x0fc19dc6, 0x240ca1cc, 0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7, 0xc6e00bf3, 0xd5a79147,
0x06ca6351, 0x14292967, 0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13,
0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85, 0xa2bfe8a1, 0xa81a664b,
0xc24b8b70, 0xc76c51a3, 0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a,
0x5b9cca4f, 0x682e6ff3, 0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208,
0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2
};
// deal with little-endian-ness
for ( i = 0; i < 8; i++ )
{
hash[ i ] = ntohl( hash[ i ] );
}
for ( t = 0; t < 64; t++ )
{
if ( t <= 15 )
{
W[ t ] = ( block[ ( t * 4 ) ] << 24 ) |
( block[ ( t * 4 ) + 1 ] << 16 ) |