Is it Possible to migrate from PEAR Crypt to PHPSecLib RSA? - php

Question

We migrated to PHPSecLib a while ago, but did not migrate our legacy data that was encrypted by the old PEAR\Crypt_RSA library. We've reached a point where we need to migrate that data into PHPSecLib's RSA format. While investigating this, I came across this old forum thread. I attempted to apply the suggestion in the response, but could not get it to successfully decrypt our data. It's not erroring out or anything, it just appears to still be encrypted or encoded. We're running PHPSecLib 2.0.6 currently, and I suspect the instructions were for 1.x.
Here's a roughed out version of my adapted decryption flow (based off the forum thread):
$rsaDecryptor = new RSA();
// The Private Key is encrypted based off a password
$mc = mcrypt_module_open(MCRYPT_RIJNDAEL_128, '', MCRYPT_MODE_ECB, '');
$iv = mcrypt_create_iv(mcrypt_enc_get_iv_size($mc), MCRYPT_DEV_URANDOM);
$keySize = mcrypt_enc_get_key_size($mc);
$key = substr($rsaDecryptor->password, 0, $keySize);
mcrypt_generic_init($mc, $key, $iv);
$privateKey = mdecrypt_generic($mc, base64_decode($privateKey));
mcrypt_generic_deinit($mc);
mcrypt_module_close($mc);
list($privateKeyModulus, $privateKeyExponent) = unserialize(base64_decode($privateKey));
$privateKeyExponent = new BigInteger(strrev($privateKeyExponent), 256);
$privateKeyModulus = new BigInteger(strrev($privateKeyModulus), 256);
$rsaDecryptor->modulus = $privateKeyModulus;
$rsaDecryptor->exponent = $privateKeyExponent;
$rsaDecryptor->publicExponent = $privateKeyExponent;
$rsaDecryptor->k = strlen($this->decRSA->modulus->toBytes());
// ciphertext is the raw encrypted string created by PEAR\Crypt_RSA
$value = base64_decode($ciphertext);
$value = new BigInteger($value, 256);
$value = $rsaDecryptor->_exponentiate($value)->toBytes();
$value = substr($value, 1);

Answer 1

Bugs In PEAR's Crypt_RSA
So I was playing around with this. There's a bug in PEAR's Crypt_RSA (latest version) that might prevent this from working at all. The following code demonstrates:
$key_pair = new Crypt_RSA_KeyPair(1024);
$privkey = $key_pair->getPrivateKey();
$pubkey = $key_pair->getPublicKey();
$a = $privkey->toString();
$b = $pubkey->toString();
echo $a == $b ? 'same' : 'different';
You'd expect $a and $b to be different, wouldn't you? Well they're not. This is because RSA/KeyPair.php does this:
$this->_public_key = &$obj;
...
$this->_private_key = &$obj;
If you remove the ampersands it works correctly but they're in the code by default.
It looks like this is an unresolved issue as of https://pear.php.net/bugs/bug.php?id=15900
Maybe it behaves differently on PHP4 but I have no idea.
Decrypting Data
Assuming the above bug isn't an issue for you then the following worked for me (using phpseclib 2.0):
function loadKey($key) // for keys genereated with $key->toString() vs $key->toPEMString()
{
if (!($key = base64_decode($key))) {
return false;
}
if (!($key = unserialize($key))) {
return false;
}
list($modulus, $exponent) = $key;
$modulus = new BigInteger(strrev($modulus), 256);
$exponent = new BigInteger(strrev($exponent), 256);
$rsa = new RSA();
$rsa->loadKey(compact('modulus', 'exponent'));
return $rsa;
}
function decrypt($key, $ciphertext)
{
if (!($ciphertext = base64_decode($ciphertext))) {
return false;
}
$key->setEncryptionMode(RSA::ENCRYPTION_NONE);
$ciphertext = strrev($ciphertext);
$plaintext = $key->decrypt($ciphertext);
$plaintext = strrev($plaintext);
$plaintext = substr($plaintext, 0, strpos($plaintext, "\0"));
return $plaintext[strlen($plaintext) - 1] == "\1" ?
substr($plaintext, 0, -1) : false;
}
$key = loadKey($private_key);
$plaintext = decrypt($key, $ciphertext);
echo $plaintext;
Private Keys Generated with toPEMString()
With PEAR's Crypt_RSA you can generate private keys an alternative way:
$key_pair->toPEMString();
This method works without code changes. If you used this approach to generate your private keys the private key starts off with -----BEGIN RSA PRIVATE KEY-----. If this is the case then you don't need to use the loadKey function I wrote. You can do this instead:
$key = new RSA();
$key->loadKey('...');
$plaintext = decrypt($key, $ciphertext);
echo $plaintext;