set-1-challenge-3.md

Single-byte XOR cipher

The challenge

The hex encoded string:

1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736

... has been XOR'd against a single character. Find the key, decrypt the message.

You can do this by hand. But don't: write code to do it for you.

How? Devise some method for "scoring" a piece of English plaintext. Character frequency is a good metric. Evaluate each output and choose the one with the best score.

The solution

NodeJS:

const encodedString = "1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736"
const rankText = text => {
  const letterFrequenciesEnglish={'a': 0.0651738,'b': 0.0124248,'c': 0.0217339,'d': 0.0349835,'e': 0.1041442,'f': 0.0197881,'g': 0.0158610,'h': 0.0492888,'i': 0.0558094,'j': 0.0009033,'k': 0.0050529,'l': 0.0331490,'m': 0.0202124,'n': 0.0564513,'o': 0.0596302,'p': 0.0137645,'q': 0.0008606,'r': 0.0497563,'s': 0.0515760,'t': 0.0729357,'u': 0.0225134,'v': 0.0082903,'w': 0.0171272,'x': 0.0013692,'y': 0.0145984,'z': 0.0007836,' ': 0.1918182}
  const chars = text
    .split("")
    .map(char => char.toLowerCase())

  let score = 0;
  for (let letter of chars) {
    score += letterFrequenciesEnglish[letter] || -1
  }
  return score
}

const generateDecodedText = (input, key) => Buffer.from(input, "hex").map(e => e^key).toString()

let results = []
for (let i = 0; i < 255; i++) {
  let decodedText = generateDecodedText(encodedString, i.toString(16))
  results.push([decodedText,rankText(decodedText)])
}
console.log(results.sort((a,b) => b[1] - a[1])[0][0])