set-1-challenge-5.md

Implement repeating-key XOR

The challenge

Here is the opening stanza of an important work of the English language:

Burning 'em, if you ain't quick and nimble
I go crazy when I hear a cymbal

Encrypt it, under the key "ICE", using repeating-key XOR.

In repeating-key XOR, you'll sequentially apply each byte of the key; the first byte of plaintext will be XOR'd against I, the next C, the next E, then I again for the 4th byte, and so on.

It should come out to:

0b3637272a2b2e63622c2e69692a23693a2a3c6324202d623d63343c2a26226324272765272
a282b2f20430a652e2c652a3124333a653e2b2027630c692b20283165286326302e27282f

Encrypt a bunch of stuff using your repeating-key XOR function. Encrypt your mail. Encrypt your password file. Your .sig file. Get a feel for it. I promise, we aren't wasting your time with this.

The solution

NodeJS:

const text = "Burning 'em, if you ain't quick and nimble\nI go crazy when I hear a cymbal"
const key = "ICE"

const textBuffer = Buffer.from(text)
const keyBuffer = Buffer.from(key)

const decoded = textBuffer.map((a,i) => a ^ keyBuffer[i%key.length]).toString("hex")
console.log(decoded)