/**
* This is where the actual encrypt/decryption takes place. Since
* encryption and decryption are the same algorithm in DES, we only
* need one function to do both.
*
* @param string $data The string to be encrypted or decrypted
* @return boolean Returns true
*/
protected function des(&$data)
{
$l = array();
$r = array();
// step two: Initial Permutation (IP) of plaintext
$data = $this->ip($data);
// divide the permuted block IP into a left half L0 of 32 bits,
// and a right half R0 of 32 bits
$l[0] = substr($data, 0, 32);
$r[0] = substr($data, 32, 32);
for ($n = 1; $n <= 16; ++$n) {
$l[$n] = $r[$n - 1];
if ($this->operation() == parent::DECRYPT) {
$f = $this->F($r[$n - 1], $this->sub_keys[16 - $n]);
} else {
$f = $this->F($r[$n - 1], $this->sub_keys[$n - 1]);
}
// XOR F with Ln
$r[$n] = $this->xorBin($l[$n - 1], $f);
}
// now we combine L[16] and R[16] back into a 64-bit string, but we reverse
// L[16] and R[16] so that it becomes R[16]L[16]
$data = $r[16] . $l[16];
// now do the final permutation
$data = $this->fp($data);
$data = parent::bin2Str($data);
return true;
}
