/**
* Function F - To calculate f, we first expand each block Rn-1 from 32 bits to 48 bits.
* This is done by using a selection table that repeats some of the bits in Rn-1. We'll
* call the use of this selection table the function E. Thus E(Rn-1) has a 32 bit input
* block, and a 48 bit output block.
*
* @param array $r 32 bit binary, each bit in an array element
* @param string $k 48 bit binary string
* @return string 48 bit binary string
*/
private function f($r, $k)
{
$bin = parent::xorBin($k, $this->E($r));
// create a 32-bit string from $bits by passing it through the S-Boxes
$bin = $this->s($bin);
// now send permute $bin as defined by table self::$_p
$bin = $this->p($bin);
return $bin;
}