Protecting Against Eavesdroppers with Symmetric Cryptography - Implementing SSL/TLS Using Cryptography and PKI - page 69

Listing 2-34: “aes.c” sub_bytes

static void sub_bytes( unsigned char state[ ][ 4 ] )
{
int r, c;

for ( r = 0; r < 4; r++ )
{
for ( c = 0; c < 4; c++ )
{
state[ r ][ c ] = sbox[ ( state[ r ][ c ] & 0xF0 ) >> 4 ]
[ state[ r ][ c ] & 0x0F ];
}
}
}

Row shifting is a rotation applied to each row. The fi rst row is rotated zero
places, the second one place, the third two, and the fourth three. Graphically,
this can be viewed as shown in Figure 2-13.

0 4 8 C 0 4 8 C

1 5 9 D D 1 5 9

2 6 A E A E 2 6

3 7 B F 7 B F 3

Figure 2-13: AES row shift

In code, this is shown in Listing 2-35.

Listing 2-35: “aes.c” shift_rows

static void shift_rows( unsigned char state[ ][ 4 ] )
{
int tmp;

tmp = state[ 1 ][ 0 ];
state[ 1 ][ 0 ] = state[ 1 ][ 1 ];
state[ 1 ][ 1 ] = state[ 1 ][ 2 ];
state[ 1 ][ 2 ] = state[ 1 ][ 3 ];
state[ 1 ][ 3 ] = tmp;

tmp = state[ 2 ][ 0 ];
state[ 2 ][ 0 ] = state[ 2 ][ 2 ];

(Continued)