Different Results in Go and Pycrypto when using AES-CFB

I found that easiest way to deal with this from Python side is to use M2Crypto library.

Final code looks like:

import M2Crypto.EVP

iv = ciphertext[:16]
ciphertext = ciphertext[16:]

cipher = M2Crypto.EVP.Cipher('aes_256_cfb', t, iv, 0)
text = cipher.update(ciphertext)
print text

Works perfect without need to change something in Go.

Research on the current system has revealed that our python system uses CFB8 (8 bit segments). Go does not support this out of the box, but the source code used in the current CFBDecrypter / CFBEncrypter looks like it can be adapted fairly easily.

It seems that the cipher can be made compatible to Go's crypto/cipher if we change segment_size of AES object from the default 8 to AES.block_size*8 (which is 128), like this:

Crypto.Cipher.AES.new(
            key=password, 
            mode=Crypto.Cipher.AES.MODE_CFB, 
            IV=iv,
            segment_size=AES.block_size*8
)

If anyone is looking for Go implementation of CFB mode with segment size = 8 you can use this:

import "crypto/cipher"

// CFB stream with 8 bit segment size
// See http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf
type cfb8 struct {
    b         cipher.Block
    blockSize int
    in        []byte
    out       []byte

    decrypt bool
}

func (x *cfb8) XORKeyStream(dst, src []byte) {
    for i := range src {
        x.b.Encrypt(x.out, x.in)
        copy(x.in[:x.blockSize-1], x.in[1:])
        if x.decrypt {
            x.in[x.blockSize-1] = src[i]
        }
        dst[i] = src[i] ^ x.out[0]
        if !x.decrypt {
            x.in[x.blockSize-1] = dst[i]
        }
    }
}

// NewCFB8Encrypter returns a Stream which encrypts with cipher feedback mode
// (segment size = 8), using the given Block. The iv must be the same length as
// the Block's block size.
func newCFB8Encrypter(block cipher.Block, iv []byte) cipher.Stream {
    return newCFB8(block, iv, false)
}

// NewCFB8Decrypter returns a Stream which decrypts with cipher feedback mode
// (segment size = 8), using the given Block. The iv must be the same length as
// the Block's block size.
func newCFB8Decrypter(block cipher.Block, iv []byte) cipher.Stream {
    return newCFB8(block, iv, true)
}

func newCFB8(block cipher.Block, iv []byte, decrypt bool) cipher.Stream {
    blockSize := block.BlockSize()
    if len(iv) != blockSize {
        // stack trace will indicate whether it was de or encryption
        panic("cipher.newCFB: IV length must equal block size")
    }
    x := &cfb8{
        b:         block,
        blockSize: blockSize,
        out:       make([]byte, blockSize),
        in:        make([]byte, blockSize),
        decrypt:   decrypt,
    }
    copy(x.in, iv)

    return x
}