Challenge 5: Implement repeating-key XOR

Problem

Implement repeating-key XOR
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.

Solution

In repeating key XOR we take a key and XOR it with the plaintext. The key is repeated until it is the same length as the plaintext. The key is then XOR’d with the plaintext. The result is the ciphertext. The ciphertext can be decrypted by XORing it with the key. The key is repeated until it is the same length as the ciphertext. The key is then XOR’d with the ciphertext. The result is the plaintext.

For, P = payload, C= cipher-text, K=key

P XOR> K = C

P XOR> C = K

Decoded

Code

package challenge5

import (
  "fmt"

  c1 "cryptopals/Set1/challenge1"
)

func Challenge5() {
  plaintext := []byte(`Burning 'em, if you ain't quick and nimble
I go crazy when I hear a cymbal`)

  value := EncryptWithRepeatingKey(plaintext, []byte("ICE"))

  encoded := c1.EncodeHex(value)

  fmt.Printf("%s", encoded)
}

func EncryptWithRepeatingKey(payload, key []byte) []byte {
  cipher := make([]byte, len(payload))
  for i := 0; i < len(payload); i++ {
    cipher[i] = payload[i] ^ key[i%len(key)]
  }
  return cipher
}