Question

I have a working test of a hardware device that uses RSA encryption, in Python using M2Crypto. Now I need to test a similar device that uses 3DES encryption. But I can’t figure out how to use M2Crypto to do triple DES encryption.

I know it should be possible from this chart. But unfortunately the documentation of M2Crypto I’ve found is sketchy. (The homepage at http://chandlerproject.org/ seems to be gone, along with Chandler.)

I’ve searched for 3DES and “OpenSSL API” and found some hard to grok C code for decrypting which makes it look like I need to use M2Crypto.EVP.Cipher. But I haven’t found any examples of using it for DES. The closest I’ve found is this blog post on using it for AES encryption. It looks like I just need to figure out the correct arguments to M2Crypto.EVP.Cipher.__init__(). I’ll keep digging, but I thought it worth posting this question.

Answer 1

The following code worked for me:

with open(keyfile, 'rb') as f:
    key = f.read()
encrypt = 1
cipher = Cipher(alg='des_ede3_ecb', key=key, op=encrypt, iv='\0'*16)
ciphertext = cipher.update(plaintext)
ciphertext += cipher.final()

Note the keyfile is a 24-byte (binary) file with parity set as sometimes required for DES.

Note also that the iv argument is (I believe) ignored when using ‘des_ede3_ecb’, but I couldn’t pass None.)