Different Results in Go and Pycrypto when using AES-CFB

Question

I am adding a go application to an already existing python codebase. I\'ve been having trouble dealing with encryption between the languages. This is using go 1.2.1 and Pyth

Answer 1

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
}