CODEWITHSUNDEEP
encryptionrsapublic-key-encryption
Jurriaan
Jurriaan

Reputation: 41

RSA private key format

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?

Upvotes: 3

Views: 1565

Answers (1)

Uli K&#246;hler
Uli K&#246;hler

Reputation: 13750

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.

Upvotes: 1

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