Challenge 4: Detect single-character XOR

Probelm

Detect single-character XOR
One of the 60-character strings in this file has been encrypted by single-character XOR.

Find it.

Solution

Using our code from challenge 3 we can solve this problem. We just need to loop through the file and find the string that has the highest score. The string with the highest score is the most likely to be the correct string.

Decoded

Code

package challenge4

import (
  "fmt"
  "os"
  "strings"

  c1 "cryptopals/Set1/challenge1"
)

/*
XORed against a single string, find the character
1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736
*/

func charFreq(char byte) float32 {
  wm := map[byte]float32{
    byte('E'): 12.02,
    byte('T'): 9.10,
    byte('A'): 8.12,
    byte('O'): 7.68,
    byte('I'): 7.31,
    byte('N'): 6.95,
    byte('S'): 6.28,
    byte('R'): 6.02,
    byte('H'): 5.92,
    byte('D'): 4.32,
    byte('L'): 3.98,
    byte('U'): 2.88,
    byte('C'): 2.71,
    byte('M'): 2.61,
    byte('F'): 2.30,
    byte('Y'): 2.11,
    byte('W'): 2.09,
    byte('G'): 2.03,
    byte('P'): 1.82,
    byte('B'): 1.49,
    byte('V'): 1.11,
    byte('K'): 0.69,
    byte('X'): 0.17,
    byte('Q'): 0.11,
    byte('J'): 0.10,
    byte('Z'): 0.07,
    byte(' '): 8,
    byte('a'): 8.167,
    byte('b'): 01.492,
    byte('c'): 02.782,
    byte('d'): 04.253,
    byte('e'): 12.702,
    byte('f'): 02.228,
    byte('g'): 02.015,
    byte('h'): 06.094,
    byte('i'): 06.966,
    byte('j'): 00.153,
    byte('k'): 00.772,
    byte('l'): 04.025,
    byte('m'): 02.406,
    byte('n'): 06.749,
    byte('o'): 07.507,
    byte('p'): 01.929,
    byte('q'): 00.095,
    byte('r'): 05.987,
    byte('s'): 06.327,
    byte('t'): 9.056,
    byte('u'): 02.758,
    byte('v'): 00.978,
    byte('w'): 02.360,
    byte('x'): 00.150,
    byte('y'): 01.974,
    byte('z'): 00.074,
  }
  return wm[char]
}

func Challenge4() {
  answer, err := DecodeFromFile("./Set1/challenge4/4.txt")
  if err != nil {
    fmt.Printf("Error Encountered: %s", err)
    return
  }
  fmt.Printf("Answer:%s", answer)
}

func DecodeFromFile(file string) ([]byte, error) {
  f, err := os.ReadFile(file)
  if err != nil {
    return nil, err
  }
  lines := strings.Split(string(f), "\n")
  var answer []byte
  var score float32
  for i, line := range lines {
    ans, tempScore, err := SingleXorCipher([]byte(line))
    if err != nil {
      return nil, err
    }
    fmt.Printf("Checking line %d, Score: %f\n", i, tempScore)
    if tempScore > score {
      answer = ans
      score = tempScore
    }
  }
  return answer, nil
}

func SingleXorCipher(codedMessage []byte) ([]byte, float32, error) {
  raw := c1.DecodeHex(codedMessage)
  var answer []byte
  var score float32
  for i := 0; i < 256; i++ {
    tempAns := make([]byte, len(raw))
    var tempSc float32
    for j := 0; j < len(raw); j++ {
      c := raw[j] ^ byte(i)
      tempSc += charFreq(c)
      tempAns[j] = c
    }
    if tempSc > score {
      answer = tempAns
      score = tempSc
    }
    tempSc = 0
  }
  return answer, score, nil
}