function derive($secret, $prefix)
{
if (!$secret || !$prefix) {
WindowsLiveLogin::fatal("Error: derive: secret or prefix is null.");
}
$keyLen = 16;
$key = $prefix . $secret;
if (function_exists('mhash')) {
$key = mhash(MHASH_SHA256, $key);
} else {
$key = hash("sha256", $key, true);
}
if (!$key || strlen($key) < $keyLen) {
WindowsLiveLogin::debug("Error: derive: Unable to derive key.");
return;
}
return substr($key, 0, $keyLen);
}