Naive crypto algorithm

naive-cypto.ts

import { randomBytes } from 'crypto'

const initCircularBuffer = (buffer: Buffer) => {
  let index = 0;
  
  return (): number => {
    if (index >= buffer.length) {
      index = 0;
    }

    const byte = buffer.at(index)
    if (!byte) {
      throw new Error(`Invalid index ${index}. Expected number between 0 and ${buffer.length - 1}`)
    }

    index++;

    return byte;
  }
}

function naiveEncryptOrDecrypt(
  privateKey: Buffer, 
  plainOrCypher: string | Buffer
): Buffer {
  const getNextByte = initCircularBuffer(privateKey);
  const plainOrCypherBuffer = Buffer.from(plainOrCypher);
  const encryptedBuffer = plainOrCypherBuffer
    .map((byte) => byte ^ getNextByte())

  return Buffer.from(encryptedBuffer)
}

const plaintext = process.argv[2] || 'this is an encryption test';
const privateKey = process.argv[3] ? Buffer.from(process.argv[3], 'base64') : randomBytes(16)
const encrypted = naiveEncryptOrDecrypt(privateKey, plaintext)
const decrypted = naiveEncryptOrDecrypt(privateKey, encrypted)

console.log(JSON.stringify({
  plaintext,
  privateKey: Buffer.from(privateKey).toString('base64'),
  encrypted: encrypted.toString('base64'),
  decrypted: decrypted.toString(),
  match: Buffer.from(plaintext).compare(decrypted) === 0
}, null, 2))

Usage

Test it like this:
npx ts-node naive-crypto.ts "text to encrypt"

Optionally, you can provide a base64 encoded private key like this:
npx ts-node naive-crypto.ts "text to encrypt" lxETn/UkphIe09JmSGhR2g==

Based on https://www.crypto101.io/