CODEWITHSUNDEEP
.netpythonrsapublic-key-encryptionpycrypto
MartinStettner
MartinStettner

Reputation: 29174

RSA encryption in .NET, decryption in python

I'm trying to encrypt a short message using RSA algorithm in C# and decrypt the message using a python skript.

I'd like to use the .NET classes on the C# side and pycrypto on python side. I've managed both sides to use the same key (which was not even trivial since .NET doesn't support the standard PEM/DER format directly). Encryption/decryption works on both sides independently.

I'm using PKCS#1 padding on the C# side (setting the fOAEP parameter of RSACryptoServiceProvider.Encrypt to false), so I'd expect that after decrypting the block in python I should see some kind of clear text (i.e. my "message" together with padding bytes)

But all I see is garbage :(.

Are there any caveats/pitfalls I'm not aware of on either side? I'm kind of out of ideas ...

Thx Martin

Sample code

C# / Encryption

Console.Write("Input string:");
var s = Console.ReadLine();
var b = Encoding.Default.GetBytes(s);
var encrypted = rsa.Encrypt(b, false);
using (var file = new FileStream(filename, FileMode.Create)) {
  file.Write(encrypted, 0, encrypted.Length);
  file.Flush();
  file.Close();
}

Python / Decryption

f = open(filename, "rb")
msg = f.read()
f.close()
decrypted = rsa.decrypt(msg)
print "Decrypted message:"
print_hex(decrypted)

For Key transfer I'm using the ToXmlString() method of RSACryptoServiceProvider. The resulting XML is parsed in python and the pycrypto-RSA object is initialized with

r = Crypto.PublicKey.RSA.construct((modulus, exponent, d, p, q))

where modulus, exponent, d, p and q are the respective fields of the .NET-RSAParameters structure. (as I've mentioned, I can encrypt/decrypt a message using this key in python, also p*q yields modulus, so I think the key import is working correctly ...)

Upvotes: 3

Views: 3405

Answers (1)

MartinStettner
MartinStettner

Reputation: 29174

Ok, it was my fault, I just didn't look at the results: The leading zero (from PKCS#1 padding) is not part of the python result string, which looks (in hex) like:

02 a2 16 4e 51 45 aa 8d 
94 b0 de 64 4d 4c 4c bd 
0b 01 b8 d2 de dc ed 23 
0b 25 c2 11 6c 0a 0b 1f 
4f 19 d0 33 18 db e0 81 
25 33 f6 e3 70 8d 97 d2 
c7 ef 32 ef 27 3c c0 ac 
47 68 c0 5b 7b 6d 0d ba 
44 da cb bf e8 71 75 d3 
2f 9a b1 97 6b 70 4f ff 
98 6f 5a 9a 74 3c 65 94 
eb 57 52 8a 2f 73 1f 14 
7d 76 08 d3 e5 8b 82 b8 
5d ed 2b 75 52 29 b5 22 
af 76 55 bc 5d e9 41 99 
00 4d 61 72 74 69 6e

So, 02 at the beginning points to random padding (somehow I was expecting 0xff padding...). The last 6 bytes (after the zero) are exactly the "Message" I was expecting, but a normal print didn't show them just because of the zero byte...

Upvotes: 2

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