<?php
  /**
   * Get a key from a secret.
   * What we do is create two different hashes from the secret, combine them
   * and pick out the number of characters we need.
   * We used the raw binary output of the hash function for maximum
   * bit strength (we have 255 chars to choose from, instead of 16).
   *
   * @param string $secret
   *   The secret to generate a key from.
   *
   * @param integer $ks
   *   The key size.
   *
   * @return binary
   *   Key created from secret.
   */
  private static function getKey($secret, $ks) {
    /* Split the secret into two parts. */
    $secretSplit = floor(strlen($secret));
    $secret1 = substr($secret, 0, $secretSplit);
    $secret2 = substr($secret, $secretSplit);

    /* Hash the two parts seperatly and return the result in raw format. */
    $key1 = hash('sha256', $secret1, true);
    $key2 = hash('sha256', $secret2, true);

    /* Return the part of the key we need. */
    return substr($key2.$key1, 0, $ks);
  }