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I've been trying to reverse engineer Mega's (mega.co.nz) API calls. And stopped on the step where client needs to decrypt session id with provided RSA key. I can get key data, but I have no idea how to convert this data into actual key.

Here is my Base64 encoded private key:
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From what I understand, it consists of four parts(big integers): p, q, d, u. I know that to get generate private key we need modulus(p*q) and private exponent (d)

So I'm wondering, is there standard private key format for this key, or they just using some internal encoding?

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2 Answers 2

The format is not an official standard, but it is pretty straight forward. There are indeed 4 fields, in order $p$, $q$, $d$ and $u$ (I'm not quite sure what the function of the $u$ is?, it's not the CRT representation that is common in standards).

First decode the base64: it starts with 0x04 0x00 0x8b 0x91 0xff 0x6a 0x8f 0x5c..

Every number is encoded as a big-endian 2 byte number that denotes the number of bits (So here 0x400 = 1024), so we then know the number of bytes (so here 0x80 bytes) that then follows and this is just a bigendian number ($p$). After these bytes we get the second length field (0x400 again) followed by 0x80 bytes (for $q$), a length 0x7fe = 2046 bits so 0x100 bytes that encode $d$ (it makes sense that this should be roughly twice the number of bits of $p$ and $q$, as it is of order $(p-1)(q-1)$) and the last $u$ is again 0x400 = 1024 bits, and after this number we get 0x2d = '-' characters as padding.

Normally these private keys are ASN.1-encoded (e.g. PEM format), and Microsoft has its own format in Windows, etc. Many such encodings are just a decorated version of the length followed by data of that length format, so this is pretty easy to figure out.

Take a bignum library (I use GMP, normally) and try to convert the bytes to numbers and check primality etc etc. to try to verify this.

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without decoding your b64, have a look at the api_setrsa(privk, pubk) function call located in crypto_0.js, thats were it gets encrypted and stored. it is called from keygen_0.js:

api_setrsa([rsa_p,rsa_q,rsa_d,rsa_u],[rsa_pq,rsa_e]);

good look.

by the way: it is never a good idea to post private keys.

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