Golang AES CFB - Mutating IV

Question

I've writing a client-side app in Go that needs to interact with a C program on the server-side. The client does an AES CFB encrypt and the server decrypts. Unfortunately the server-side has a bug with reusing an initialization vector. It tries to do 3 decrypt operations based on:-
key1, iv
key2, iv
key3, iv

Due to this issue the iv is actually modified between decrypt operations. My problem now is how to reproduce this behaviour on the client side using Go.

By inserting a Println into the encrypt function below, I can see the cfb struct which, I think, contains the modified IV for the next block but because it's a stream interface, I'm not sure how to extract it into a byte slice. Any suggestions?

Thanks

package main

import (
  "fmt"
  "encoding/hex"
  "crypto/cipher"
  "crypto/aes"
)

func encrypt_aes_cfb(plain, key, iv []byte) (encrypted []byte) {
  block, err := aes.NewCipher(key)
  if err != nil {
    panic(err)
  }
  encrypted = make([]byte, len(plain))
  stream := cipher.NewCFBEncrypter(block, iv)
  stream.XORKeyStream(encrypted, plain)
  fmt.Println(stream)
  return
}

func main() {
  plain := []byte("Hello world...16Hello world...32")
  key := make([]byte, 32)
  iv := make([]byte, 16)
  enc := encrypt_aes_cfb(plain, key, iv)
  fmt.Println("Key: ", hex.EncodeToString(key))
  fmt.Println("IV:  ", hex.EncodeToString(iv))
  fmt.Println("Enc: ", hex.EncodeToString(enc))
}

Answer 1

Going down the path you're hinting at is a bit ugly, and prone to break when the implementation changes.

You can get the IV from the stream by:

s := reflect.Indirect(reflect.ValueOf(stream))
lastIV := s.FieldByName("next").Bytes()

But, there's an easier way! Concatenate the plain text inputs, so that the stream for the second starts with the IV from the end of the first (and so on).

Playground Example

combined := append(plain, plain2...)
encCombined := encrypt_aes_cfb(combined, key, iv)

enc := encCombined[:len(plain)]
enc2 := encCombined[len(plain):]