AES-128 ECB Mode in Go

Since I’ve become a security engineer, I’ve been working my way through the Cryptopals challenges. I’m going to cover challenges 7 and 8 in this article.

What is AES-128 ECB mode?

AES is an encryption standard that applies a series of transformations to plaintext to produce encrypted ciphertext based on a given key. If you’re curious about the actual AES encryption process, you can read my write up here!

ECB stands for Electronic Code Book. In ECB mode, the plaintext is broken into blocks of a given size (128 bits in this case), and the encryption algorithm is run on each block of plaintext individually. The weakness of this encryption mode is that it’s possible to see patterns in the ciphertext. Two identical blocks of plaintext will result in two identical blocks of ciphertext. Like every cryptography blog does, I’ll use this image of the Linux penguin to illustrate the problem with ECB.

Original

Encrypted Using ECB

After encrypting Tux using ECB, the color is distorted, but we can still see a very obvious penguin.

Implementing ECB mode in Go

In challenge 7, we’re given a file that has been encrypted using AES-128 ECB mode with the key YELLOW SUBMARINE and then base64 encoded. Go has built in AES support, but because ECB mode is not secure, it is not supported by default, so we will have to go through and decrypt the ciphertext block by block instead of passing in the whole ciphertext at once.

import "crypto/aes"
func DecryptEcb(encryptedBytes, key []byte) ([]byte, error) {

   cipher, err := aes.NewCipher(key)
   if err != nil {
      return nil, err
   }
   plainText := make([]byte, len(encryptedBytes))
   for i, j:= 0, 16; i< len(encryptedBytes); i, j = i+16, j+16 {
      cipher.Decrypt(plainText[i:j], encryptedBytes[i:j])
   }
   return plainText, nil
}

(NOTE: We should probably do some checks to make sure we are not going out of bounds of our byte slice, but I’ll leave those details to you.)

The only thing left is to read in the file and decode the base64, which we’ve done in a few examples already, so I’ll leave those to you.

Detecting ECB

Challenge 8 gives us a file with a bunch of encrypted text thats been hex encoded. One of the lines has been encrypted using ECB, and we need to figure out which one it is. This is actually pretty easy to do. We know that two identical blocks of plaintext will produce two identical blocks of ciphertext. To break it down, if our plaintext was This is a test!!This is a test!!we would break that down to two blocks of 16 bytes:

This is a test!!   
This is a test!! 

Those blocks would encrypt to (hex encoded for visibility):

25D073E4679A75C4A32ADE82CA7D8B44
25D073E4679A75C4A32ADE82CA7D8B44

So, to see if our ciphertext was encrypted in ECB mode, we just need to check if any blocks are repeating. If they are, then we can be pretty sure that ECB mode was used.

1. Read in file
2. For each line
   A. Hex decode the line
   B. Break into 16 byte chunks
   C. If any chunks match, return the line number
   D. If no matches, continue to next line
   E. Return line number of result

import (
    "bufio"
    "bytes"
    "encoding/hex"
    "io/ioutil"
)
func DetectECB(fileName string) (int, error) {
  var input [][]byte
  f, err := ioutil.ReadFile(fileName)
  if err != nil {
      return -1, err
  }
scanner := bufio.NewScanner(bytes.NewReader(f))
  for scanner.Scan() {
      hdl := make([]byte, hex.DecodedLen(len(scanner.Bytes())))
      _, err := hex.Decode(hdl, scanner.Bytes())
      if err != nil {
            return -1, err
      }
input = append(input, hdl)
  }
  res := Detect(input)
  return res, nil
}
func Detect (input [][]byte) int {
    for i, ln := range input {
        chunks := make([][]byte, 0)
        for j := 0; j < len(ln); j += 16 {
            batch := ln[j:min(j+15, len(ln))]
            for _, c := range chunks {
                if bytes.Equal(c, batch) {
                    return i+1
                }
           }
          chunks = append(chunks, batch)
       }
   } 
   return 0
}
func min(a, b int) int {
    if a <= b {
        return a
    }
    return b
}

We’re done!

As per usual, if you want to check out the complete code along with tests and a nifty command line interface, you can check out the project on my github! These challenges bring us to the end of Set 1 of Cryptopals!