/**
* Exponentiate with or without Chinese Remainder Theorem.
* Operation with primes 'p' and 'q' is appox. 2x faster.
*
* @param \Jose\KeyConverter\RSAKey $key
* @param \Jose\Util\BigInteger $c
*
* @return \Jose\Util\BigInteger
*/
private static function exponentiate(RSAKey $key, BigInteger $c)
{
if ($key->isPublic() || empty($key->getPrimes())) {
return $c->modPow($key->getExponent(), $key->getModulus());
}
$p = $key->getPrimes()[0];
$q = $key->getPrimes()[1];
$dP = $key->getExponents()[0];
$dQ = $key->getExponents()[1];
$qInv = $key->getCoefficient();
$m1 = $c->modPow($dP, $p);
$m2 = $c->modPow($dQ, $q);
$h = $qInv->multiply($m1->subtract($m2)->add($p))->mod($p);
$m = $m2->add($h->multiply($q));
return $m;
}
