Golang AES ECB Encryption

Trying to emulate an algorithm in Go that is basically AES ECB Mode encryption.

Here's what I have so far

func Decrypt(data []byte) []byte {
	cipher, err := aes.NewCipher([]byte(KEY))
	if err == nil {
		cipher.Decrypt(data, PKCS5Pad(data))
		return data
	}
	return nil
}

I also have a PKCS5Padding algorithm, which is tested and working, which pads the data first. I cant find any information on how to switch the encryption mode in the Go AES package (it's definitely not in the docs).

I have this code in another language, which is how I know this algorithm isn't working quite correctly.

EDIT: Here is the method as I have interpreted from on the issue page

func AESECB(ciphertext []byte) []byte {
    cipher, _ := aes.NewCipher([]byte(KEY))
    fmt.Println("AESing the data")
    bs := 16
    if len(ciphertext)%bs != 0     {
	    panic("Need a multiple of the blocksize")
    }

    plaintext := make([]byte, len(ciphertext))
    for len(plaintext) > 0 {
	    cipher.Decrypt(plaintext, ciphertext)
	    plaintext = plaintext[bs:]
	    ciphertext = ciphertext[bs:]
    }
    return plaintext
}

This is actually not returning any data, maybe I screwed something up when changing it from encripting to decripting

Electronic codebook ("ECB") is a very straightforward mode of operation. The data to be encrypted is divided into byte blocks, all having the same size. For each block, a cipher is applied, in this case AES, generating the encrypted block.

The code snippet below decrypts AES-128 data in ECB (note that the block size is 16 bytes):

package main

import (
	"crypto/aes"
)

func DecryptAes128Ecb(data, key []byte) []byte {
	cipher, _ := aes.NewCipher([]byte(key))
	decrypted := make([]byte, len(data))
	size := 16

	for bs, be := 0, size; bs < len(data); bs, be = bs+size, be+size {
		cipher.Decrypt(decrypted[bs:be], data[bs:be])
	}

	return decrypted
}

As mentioned by @OneOfOne, ECB is insecure and very easy to detect, as repeated blocks will always encrypt to the same encrypted blocks. This Crypto SE answer gives a very good explanation why.

> Why? We left ECB out intentionally: it's insecure, and if needed it's
trivial to implement.

https://github.com/golang/go/issues/5597

I used your code so I feel the need to show you how I fixed it.

I am doing the cryptopals challenges for this problem in Go.

I'll walk you through the mistake since the code is mostly correct.

for len(plaintext) > 0 {
    cipher.Decrypt(plaintext, ciphertext)
    plaintext = plaintext[bs:]
    ciphertext = ciphertext[bs:]
}

The loop does decrypt the data but does not put it anywhere. It simply shifts the two arrays along producing no output.

i := 0
plaintext := make([]byte, len(ciphertext))
finalplaintext := make([]byte, len(ciphertext))
for len(ciphertext) > 0 {
	cipher.Decrypt(plaintext, ciphertext)
	ciphertext = ciphertext[bs:]
	decryptedBlock := plaintext[:bs]
	for index, element := range decryptedBlock {
		finalplaintext[(i*bs)+index] = element
	}
	i++
	plaintext = plaintext[bs:]
} 
return finalplaintext[:len(finalplaintext)-5]

What this new improvement does is store the decrypted data into a new []byte called finalplaintext. If you return that you get the data.

It's important to do it this way since the Decrypt function only works one block size at a time.

I return a slice because I suspect it's padded. I am new to cryptography and Go so anyone feel free to correct/revise this.

Ideally you want to implement the crypto/cipher#BlockMode interface. Since an official one doesn't exist, I used crypto/cipher#NewCBCEncrypter as a starting point:

package ecb
import "crypto/cipher"

type ecbEncrypter struct { cipher.Block }

func newECBEncrypter(b cipher.Block) cipher.BlockMode {
   return ecbEncrypter{b}
}

func (x ecbEncrypter) BlockSize() int {
   return x.Block.BlockSize()
}

func (x ecbEncrypter) CryptBlocks(dst, src []byte) {
   size := x.BlockSize()
   if len(src) % size != 0 {
      panic("crypto/cipher: input not full blocks")
   }
   if len(dst) < len(src) {
      panic("crypto/cipher: output smaller than input")
   }
   for len(src) > 0 {
      x.Encrypt(dst, src)
      src, dst = src[size:], dst[size:]
   }
}

I was confused by a couple of things.

First i needed a aes-256 version of the above algorithm, but apparently the aes.Blocksize (which is 16) won't change when the given key has length 32. So it is enough to give a key of length 32 to make the algorithm aes-256

Second, the decrypted value still contains padding and the padding value changes depending on the length of the encrypted string. E.g. when there are 5 padding characters the padding character itself will be 5.

Here is my function which returns a string:

func DecryptAes256Ecb(hexString string, key string) string {
  data, _ := hex.DecodeString(hexString)

  cipher, _ := aes.NewCipher([]byte(key))

  decrypted := make([]byte, len(data))
  size := 16

  for bs, be := 0, size; bs < len(data); bs, be = bs+size, be+size {
	cipher.Decrypt(decrypted[bs:be], data[bs:be])
  }

  // remove the padding. The last character in the byte array is the number of padding chars
  paddingSize := int(decrypted[len(decrypted)-1])
  return string(decrypted[0 : len(decrypted)-paddingSize])
}