Challenge 8: Detect AES in ECB mode

Probelm

Detect AES in ECB mode
In this file are a bunch of hex-encoded ciphertexts.

One of them has been encrypted with ECB.

Detect it.

Remember that the problem with ECB is that it is stateless and deterministic; the same 16 byte plaintext block will always produce the same 16 byte ciphertext.

Solution

• Hex decode the file line by line
• Write a function to detect if the ciphertext is encrypted with ECB
• create a block size of 16 bytes for 128 bit AES
• insert the block into slice and check if the block is already in the slice
• If it does, it means that the same block has appeared before, which suggests that ECB mode might have been used. If a duplicate block is found, the function returns true, indicating that the input line is likely encrypted using ECB.

Decoded

Code

package challenge8

import (
  "fmt"
  "os"
  "strings"

  c1 "cryptopals/Set1/challenge1"
)

func Challenge8() {
  file, err := os.ReadFile("./Set1/challenge8/8.txt")
  if err != nil {
    fmt.Printf("%s", err)
    return
  }
  lines := strings.Split(string(file), "\n")
  aess := make(map[int]bool)
  for i, line := range lines {
    bitByte := []byte(line)
    decoded := c1.DecodeHex(bitByte)
    status := IsAesEcb(decoded)
    fmt.Printf("Line. %d: %t\n", i, status)
    if status == true {
      aess[i] = status
    }
  }
  for i, v := range aess {
    fmt.Printf("\nAES Ciphers Detected. \nLine. %d: %t", i, v)
  }
}

func IsAesEcb(line []byte) bool {
  size := 16
  iteration := len(line) / size
  blocks := [][]byte{}
  for i := 0; i < iteration; i++ {
    start := i * size
    end := (i + 1) * size
    // block size increases by 16 everytime
    block := line[start:end]
    for _, v := range blocks {
      if BytesEqual(v, block) {
        return true
      }
    }
    blocks = append(blocks, block)
  }
  return false
}

func BytesEqual(s1, s2 []byte) bool {
  if len(s1) != len(s2) {
    return false
  }
  for i := 0; i < len(s1); i++ {
    if s1[i] != s2[i] {
      return false
    }
  }
  return true
}