/**
* Function: getKeyHash
*
* Utility function to retrieve the encryption key used by the
* encryption algorithm based on the master key, the sub-key, and some
* known, random seed. The hash returned should be used as the binary key
* for the encryption algorithm.
*
* Parameters:
* seed - (string) third-level seed for the encryption key. This will
* likely an IV or salt value
* len - (int) length of the desired hash
*/
function getKeyHash($seed, $len = 32)
{
$hash = Crypto::hash($this->getMasterKey() . md5($this->getSubKey()), $seed);
return substr($hash, 0, $len);
}