AES Encryption Golang and Python

Question

I am working on a fun side project for myself. A golang server and a python client. I want data transmitted to be encrypted but cant seem to get the two encryption schemes working together. I am a novice when it comes to encryption so please explain like you are talking to a toddler.

Here is my golang encryption functions:

import (
    "crypto/aes"
    "crypto/cipher"
    "crypto/rand"
    "errors"
    "io"
    "fmt"
)
func Encrypt(key, text []byte) (ciphertext []byte, err error) {
    var block cipher.Block
    if block, err = aes.NewCipher(key); err != nil {
        return nil, err
    }
    ciphertext = make([]byte, aes.BlockSize+len(string(text)))
    iv := ciphertext[:aes.BlockSize]
    fmt.Println(aes.BlockSize)
    if _, err = io.ReadFull(rand.Reader, iv); err != nil {
        return
    }
    cfb := cipher.NewCFBEncrypter(block, iv)
    cfb.XORKeyStream(ciphertext[aes.BlockSize:], text)
    return
}

func Decrypt(key, ciphertext []byte) (plaintext []byte, err error) {
    var block cipher.Block
    if block, err = aes.NewCipher(key); err != nil {
        return
    }
    if len(ciphertext) < aes.BlockSize {
        err = errors.New("ciphertext too short")
        return
    }
    iv := ciphertext[:aes.BlockSize]
    ciphertext = ciphertext[aes.BlockSize:]
    cfb := cipher.NewCFBDecrypter(block, iv)
    cfb.XORKeyStream(ciphertext, ciphertext)
    plaintext = ciphertext
    return
}

and here is my Python implementation:

 class AESCipher:
    def __init__( self, key ):
        self.key = key
        print "INIT KEY" + hexlify(self.key)
    def encrypt( self, raw ):
        print "RAW STRING: " + hexlify(raw)
        iv = Random.new().read( AES.block_size )
        cipher = AES.new( self.key, AES.MODE_CFB, iv )
        r = ( iv + cipher.encrypt( raw ) )
        print "ECRYPTED STRING: " + hexlify(r)
        return r

    def decrypt( self, enc ):
        enc = (enc)
        iv = enc[:16]
        cipher = AES.new(self.key, AES.MODE_CFB, iv)
        x=(cipher.decrypt( enc ))
        print "DECRYPTED STRING: " + hexlify(x)
        return x

i cant quite figure out the output of my python functions either. My Go routines are working perfectly. But i want to be able to encrypt in Go an decrypt in python and vice versa.

Sample Output from Python:

INIT KEY61736466617364666173646661736466
RAW STRING: 54657374206d657373616765
ECRYPTED STRING: dfee33dd40c32fbaf9aac73ac4e0a5a9fc7bd2947d29005dd8d8e21a
dfee33dd40c32fbaf9aac73ac4e0a5a9fc7bd2947d29005dd8d8e21a
DECRYPTED STRING: 77d899b990d2d3172a3229b1b69c6f2554657374206d657373616765
77d899b990d2d3172a3229b1b69c6f2554657374206d657373616765
wØ™¹�ÒÓ*2)±¶œo%Test message

As you can see the message is decrypted but ends up at the end of the string?

EDIT: Sample output decrypting from GO. If i try and decrypt with GO the below (generated with Python)

ECRYPTED STRING: (Test Message) 7af474bc4c8ea9579d83a3353f83a0c2844f8efb019c82618ea0b478

I get

Decrypted Payload: 54 4E 57 9B 90 F8 D6 CD 12 59 0B B1
Decrypted Payload: TNW�����Y�

The strange part is the first character is always correct

here are both full projects:

Github

Answer 1

You forgot to slice off the IV during decryption in Python. Change

x=(cipher.decrypt( enc ))

to

x = cipher.decrypt( enc[16:] )

or to

x = cipher.decrypt( enc )[16:]

Answer 2

Python uses 8-bit segments while Go uses 128-bit segments so the reason the first character works but the following ones don't is because each segment depends on the previous and thus a different segment size breaks the chain.

I made these URL safe (non-padded base64 encoding) encrypt/decrypt functions for Python to optionally encrypt the same way Go does (when you specify block_segments=True).

def decrypt(key, value, block_segments=False):
    # The base64 library fails if value is Unicode. Luckily, base64 is ASCII-safe.
    value = str(value)
    # We add back the padding ("=") here so that the decode won't fail.
    value = base64.b64decode(value + '=' * (4 - len(value) % 4), '-_')
    iv, value = value[:AES.block_size], value[AES.block_size:]
    if block_segments:
        # Python uses 8-bit segments by default for legacy reasons. In order to support
        # languages that encrypt using 128-bit segments, without having to use data with
        # a length divisible by 16, we need to pad and truncate the values.
        remainder = len(value) % 16
        padded_value = value + '\0' * (16 - remainder) if remainder else value
        cipher = AES.new(key, AES.MODE_CFB, iv, segment_size=128)
        # Return the decrypted string with the padding removed.
        return cipher.decrypt(padded_value)[:len(value)]
    return AES.new(key, AES.MODE_CFB, iv).decrypt(value)


def encrypt(key, value, block_segments=False):
    iv = Random.new().read(AES.block_size)
    if block_segments:
        # See comment in decrypt for information.
        remainder = len(value) % 16
        padded_value = value + '\0' * (16 - remainder) if remainder else value
        cipher = AES.new(key, AES.MODE_CFB, iv, segment_size=128)
        value = cipher.encrypt(padded_value)[:len(value)]
    else:
        value = AES.new(key, AES.MODE_CFB, iv).encrypt(value)
    # The returned value has its padding stripped to avoid query string issues.
    return base64.b64encode(iv + value, '-_').rstrip('=')

Note that for secure message passing you want additional security features, such as a nonce to prevent against replay attacks.

Here are the Go equivalent functions:

func Decrypt(key []byte, encrypted string) ([]byte, error) {
    ciphertext, err := base64.RawURLEncoding.DecodeString(encrypted)
    if err != nil {
        return nil, err
    }
    block, err := aes.NewCipher(key)
    if err != nil {
        return nil, err
    }
    if len(ciphertext) < aes.BlockSize {
        return nil, errors.New("ciphertext too short")
    }
    iv := ciphertext[:aes.BlockSize]
    ciphertext = ciphertext[aes.BlockSize:]
    cfb := cipher.NewCFBDecrypter(block, iv)
    cfb.XORKeyStream(ciphertext, ciphertext)
    return ciphertext, nil
}

func Encrypt(key, data []byte) (string, error) {
    block, err := aes.NewCipher(key)
    if err != nil {
        return "", err
    }
    ciphertext := make([]byte, aes.BlockSize+len(data))
    iv := ciphertext[:aes.BlockSize]
    if _, err := io.ReadFull(rand.Reader, iv); err != nil {
        return "", err
    }
    stream := cipher.NewCFBEncrypter(block, iv)
    stream.XORKeyStream(ciphertext[aes.BlockSize:], data)
    return base64.RawURLEncoding.EncodeToString(ciphertext), nil
}