/**
* HMAC-Based One-Time Password Algorithm
* @see RFC 4226
* @param $key shared secret, treated as binary
* @param $counter 8-byte counter
* [@param $digits = 6] Length of the output code
* [@param $algorithm = 'sha1'] HMAC algorithm - sha1, sha256, and sha512 permitted
* @return string n-character numeric code
*/
function HOTP(Secret $key, int $counter, int $digits = 6, string $algorithm = 'sha1') : string
{
if (!in_array($algorithm, ['sha1', 'sha256', 'sha512'])) {
throw new OutOfRangeException('Unexpected algorithm');
}
if ($digits < 6 || $digits > 8) {
// "Implementations MUST extract a 6-digit code at a minimum and
// possibly 7 and 8-digit code."
throw new LengthException('RFC4226 requires a 6 to 8-digit output');
}
if (strlen($key->reveal()) < 128 / 8) {
throw new LengthException('Key must be at least 128 bits long (160+ recommended)');
}
$counter = pack('J', $counter);
// Convert to 8-byte string
$hash = hash_hmac($algorithm, $counter, $key->reveal(), true);
// Determine the offset: get the last nibble of the hash output
$offset = ord(substr($hash, -1)) & 0xf;
// Index into the hash output by $offset bytes, take four bytes
$dbc1 = unpack('N', substr($hash, $offset, 4))[1];
// Mask out the high bit (per the spec, avoids signed/unsigned issues)
$dbc2 = $dbc1 & 0x7fffffff;
// Use the last $digits by using modulo 10^digits
$code = (string) ($dbc2 % pow(10, $digits));
// Finally, prepend zeroes to match the string length
return str_pad($code, $digits, '0', \STR_PAD_LEFT);
}
private function sign(Secret $key)
{
$alg = $this->headers['alg'];
// DEFAULT?
$payload = self::b64encode($this->headers) . '.' . self::b64encode($this->claims);
switch ($alg) {
case Algorithm::NONE:
$data = '';
break;
case Algorithm::HMAC_SHA_256:
$data = hash_hmac('SHA256', $payload, $key->reveal(), true);
break;
case Algorithm::HMAC_SHA_384:
$data = hash_hmac('SHA384', $payload, $key->reveal(), true);
break;
case Algorithm::HMAC_SHA_512:
$data = hash_hmac('SHA512', $payload, $key->reveal(), true);
break;
default:
throw new Exception("Unsupported algorithm");
// use openssl_sign and friends to do the signing
}
return rtrim(strtr(base64_encode($data), '+/', '-_'), '=');
}
