Question

I’m trying to port the following php functionality over to perl:

public function loadKey($mod, $exp, $type = 'public')
{
    $rsa = new Crypt_RSA();
    $rsa->signatureMode = CRYPT_RSA_SIGNATURE_PKCS1;
    $rsa->setHash('sha256');
    $rsa->modulus = new Math_BigInteger(Magicsig::base64_url_decode($mod), 256);
    $rsa->k = strlen($rsa->modulus->toBytes());
    $rsa->exponent = new Math_BigInteger(Magicsig::base64_url_decode($exp), 256);

    // snip...
}

I need to convert a string in the form (“RSA.$mod.$exp.$private_exp”):

RSA.mVgY8RN6URBTstndvmUUPb4UZTdwvwmddSKE5z_jvKUEK6yk1u3rrC9yN8k6FilGj9K0eeUPe2hf4Pj-5CmHww==.AQAB.Lgy_yL3hsLBngkFdDw1Jy9TmSRMiH6yihYetQ8jy-jZXdsZXd8V5ub3kuBHHk4M39i3TduIkcrjcsiWQb77D8Q==

…to a Crypt::RSA object. I’ve split out the components so I have $mod, $exp, and $private_exp, but the perl Crypt::RSA API doesn’t seem to have a way to explicitly set.

Answer 1

Worked out on IRC, documenting it here for the rest of the world: it’s completely undocumented but Crypt::RSA::Key does have methods called n, e, and d that correspond to the modulus, the public exponent, and the private exponent. Modulo bugs in the check function (which is supposed to work if p and q are unavailable but n is, but actually doesn’t), it’s possible to create a working key with those methods.

We solved the problem together by creating a subclass of Crypt::RSA::Key::Private with a factory method that decodes the base64 encoding (using MIME::Base64::URLSafe) and the additional binary encoding (using Math::BigInt->from_hex and unpack "H*") and then sets those three private members, and the Crypt::RSA modules were able to accept it as a Key.