WindowsLiveLogin::derive

function derive($secret, $prefix)
{
	if (!$secret || !$prefix) {
		$this->fatal("Error: derive: secret or prefix is null.");
	}

	$keyLen = 16;
	$key = $prefix . $secret;

	$key = hash("sha256", $key, true);

	if (!$key || (strlen($key) < $keyLen)) {
		$this->debug("Error: derive: Unable to derive key.");
		return;
	}

	return substr($key, 0, $keyLen);
}