@bpe/one-time-pad/index.js

A more complete One-time pad implementation supporting Strings, Buffers and Files

const crypto = require('crypto');

module.exports = {
  OneTimePad: {
    generatePad(uint8Array) {
      let randomBytes = crypto.randomBytes(uint8Array.length);
      randomBytes.forEach((value, index) => {
        if (value === 0) {
          randomBytes[index] += 1;
        }
      });
      return Uint8Array.from(randomBytes);
    },
    encrypt(padBuffer, plainTextBuffer) {
      let resultBuffer = new Uint8Array(plainTextBuffer.length);
      if (plainTextBuffer.length != padBuffer.length) {
        throw new Error(`Pad of length:${padBuffer.length} must match plainText of length: ${plainTextBuffer.length}`);
      };
      plainTextBuffer.forEach((value, index) => {
        resultBuffer[index] = (value + padBuffer[index]) % 256;
      });
      return resultBuffer;
    },
    decrypt(padBuffer, encryptedBuffer) {
      let resultBuffer = new Uint8Array(encryptedBuffer.length);
      if (encryptedBuffer.length != padBuffer.length) {
        throw new Error(`Pad of length:${padBuffer.length} must match encryptedBuffer of length: ${encryptedBuffer.length}`);
      };
      /*
        byteToEncrypt = 34
        padValue = 227

        Encryption:
        (34 + 227) % 256 = 5

        Decryption:
        5 - 227 = -222
        -222 = 1111 1111 0010 0010 in 2's complement

        ResultBuffer is a unsigned integer array therefore
        the upper 2 bytes are truncated resulting in: 0010 0010 = 34
      */
      encryptedBuffer.forEach((value, index) => {
        // This also works
        // const temp = value - padBuffer[index];
        // resultBuffer[index] = temp - 256 * Math.floor(temp / 256);;

        resultBuffer[index] = value - padBuffer[index];
      });
      return resultBuffer;
    }
  }
}