RSA private key format

Question

I was trying to find a way to replicate the session-id decoding of mega.co.nz in Ruby using OpenSSL. But, unfortunately, I'm not a cryptography expert.

The problem is I don't understand/recognise the private key format

This is what their code to decompose the private key looks like ( https://eu.static.mega.co.nz/crypto.js ):

// decompose private key
for (var i = 0; i < 4; i++)
{
    var l = ((privk.charCodeAt(0)*256+privk.charCodeAt(1)+7)>>3)+2;

    rsa_privk[i] = mpi2b(privk.substr(0,l));
    if (typeof rsa_privk[i] == 'number') break;
    privk = privk.substr(l);
}

privk itself is 656 bytes long (include eight dashes padding at the end).

After the 'decomposition' they use the decomposed parts to decrypt the session-id ( https://eu.static.mega.co.nz/rsa.js ):

// Compute m**d mod p*q for RSA private key operations.

function RSAdecrypt(m (encrypted session-id), d (rsa_privk[2]), p (rsa_privk[0]), q (rsa_privk[1]), u (rsa_privk[3]))

How to convert this key so OpenSSL knows how to use it?

Answer 1

OpenSSL supports different key formats, including PEM/X.509 and PKCS8 .

The ruby standard library includes an OpenSSL binding .

Using the method provided in this post you can create the key from the exponent and then use for example

key.to_pem()

to convert it to an X.509-formatted string.