Protecting Against Eavesdroppers with Symmetric Cryptography - Implementing SSL/TLS Using Cryptography and PKI - page 78

unsigned char t[ 4 ];
for ( c = 0; c < 4; c++ )
{
t[ 0 ] = dot( 0x0e, s[ 0 ][ c ] ) ^ dot( 0x0b, s[ 1 ][ c ] ) ^
dot( 0x0d, s[ 2 ][ c ] ) ^ dot( 0x09, s[ 3 ][ c ] );
t[ 1 ] = dot( 0x09, s[ 0 ][ c ] ) ^ dot( 0x0e, s[ 1 ][ c ] ) ^
dot( 0x0b, s[ 2 ][ c ] ) ^ dot( 0x0d, s[ 3 ][ c ] );
t[ 2 ] = dot( 0x0d, s[ 0 ][ c ] ) ^ dot( 0x09, s[ 1 ][ c ] ) ^
dot( 0x0e, s[ 2 ][ c ] ) ^ dot( 0x0b, s[ 3 ][ c ] );
t[ 3 ] = dot( 0x0b, s[ 0 ][ c ] ) ^ dot( 0x0d, s[ 1 ][ c ] ) ^
dot( 0x09, s[ 2 ][ c ] ) ^ dot( 0x0e, s[ 3 ][ c ] );
s[ 0 ][ c ] = t[ 0 ];
s[ 1 ][ c ] = t[ 1 ];
s[ 2 ][ c ] = t[ 2 ];
s[ 3 ][ c ] = t[ 3 ];
}
}
And the AES block decryption operation is shown in Listing 2-42.
Listing 2-42: “aes.c” aes_block_decrypt
static void aes_block_decrypt( const unsigned char *input_block,
unsigned char *output_block,
const unsigned char *key,
int key_size )
{
int r, c;
int round;
int nr;
unsigned char state[ 4 ][ 4 ];
unsigned char w[ 60 ][ 4 ];
for ( r = 0; r < 4; r++ )
{
for ( c = 0; c < 4; c++ )
{
state[ r ][ c ] = input_block[ r + ( 4 * c ) ];
}
}
// rounds = key size in 4-byte words + 6
nr = ( key_size >> 2 ) + 6;
compute_key_schedule( key, key_size, w );
add_round_key( state, &w[ nr * 4 ] );
for ( round = nr; round > 0; round-- )
{
inv_shift_rows( state );
inv_sub_bytes( state );