@noble/ciphers/src/_arx.ts

Audited & minimal JS implementation of Salsa20, ChaCha and AES

/**
 * Basic utils for ARX (add-rotate-xor) salsa and chacha ciphers.

RFC8439 requires multi-step cipher stream, where
authKey starts with counter: 0, actual msg with counter: 1.

For this, we need a way to re-use nonce / counter:

    const counter = new Uint8Array(4);
    chacha(..., counter, ...); // counter is now 1
    chacha(..., counter, ...); // counter is now 2

This is complicated:

- 32-bit counters are enough, no need for 64-bit: max ArrayBuffer size in JS is 4GB
- Original papers don't allow mutating counters
- Counter overflow is undefined [^1]
- Idea A: allow providing (nonce | counter) instead of just nonce, re-use it
- Caveat: Cannot be re-used through all cases:
- * chacha has (counter | nonce)
- * xchacha has (nonce16 | counter | nonce16)
- Idea B: separate nonce / counter and provide separate API for counter re-use
- Caveat: there are different counter sizes depending on an algorithm.
- salsa & chacha also differ in structures of key & sigma:
  salsa20:      s[0] | k(4) | s[1] | nonce(2) | cnt(2) | s[2] | k(4) | s[3]
  chacha:       s(4) | k(8) | cnt(1) | nonce(3)
  chacha20orig: s(4) | k(8) | cnt(2) | nonce(2)
- Idea C: helper method such as `setSalsaState(key, nonce, sigma, data)`
- Caveat: we can't re-use counter array

xchacha [^2] uses the subkey and remaining 8 byte nonce with ChaCha20 as normal
(prefixed by 4 NUL bytes, since [RFC8439] specifies a 12-byte nonce).

[^1]: https://mailarchive.ietf.org/arch/msg/cfrg/gsOnTJzcbgG6OqD8Sc0GO5aR_tU/
[^2]: https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-xchacha#appendix-A.2

 * @module
 */
import {
  type PRG,
  type XorStream,
  abool,
  abytes,
  anumber,
  checkOpts,
  clean,
  copyBytes,
  randomBytes,
  u32,
} from './utils.ts';

// Replaces `TextEncoder`, which is not available in all environments
const encodeStr = (str: string) => Uint8Array.from(str.split(''), (c) => c.charCodeAt(0));
const sigma16 = encodeStr('expand 16-byte k');
const sigma32 = encodeStr('expand 32-byte k');
const sigma16_32 = u32(sigma16);
const sigma32_32 = u32(sigma32);

/** Rotate left. */
export function rotl(a: number, b: number): number {
  return (a << b) | (a >>> (32 - b));
}

/** Ciphers must use u32 for efficiency. */
export type CipherCoreFn = (
  sigma: Uint32Array,
  key: Uint32Array,
  nonce: Uint32Array,
  output: Uint32Array,
  counter: number,
  rounds?: number
) => void;

/** Method which extends key + short nonce into larger nonce / diff key. */
export type ExtendNonceFn = (
  sigma: Uint32Array,
  key: Uint32Array,
  input: Uint32Array,
  output: Uint32Array
) => void;

/** ARX cipher options.
 * * `allowShortKeys` for 16-byte keys
 * * `counterLength` in bytes
 * * `counterRight`: right: `nonce|counter`; left: `counter|nonce`
 * */
export type CipherOpts = {
  allowShortKeys?: boolean; // Original salsa / chacha allow 16-byte keys
  extendNonceFn?: ExtendNonceFn;
  counterLength?: number;
  counterRight?: boolean;
  rounds?: number;
};

// Is byte array aligned to 4 byte offset (u32)?
function isAligned32(b: Uint8Array) {
  return b.byteOffset % 4 === 0;
}

// Salsa and Chacha block length is always 512-bit
const BLOCK_LEN = 64;
const BLOCK_LEN32 = 16;

// new Uint32Array([2**32])   // => Uint32Array(1) [ 0 ]
// new Uint32Array([2**32-1]) // => Uint32Array(1) [ 4294967295 ]
const MAX_COUNTER = 2 ** 32 - 1;

const U32_EMPTY = Uint32Array.of();
function runCipher(
  core: CipherCoreFn,
  sigma: Uint32Array,
  key: Uint32Array,
  nonce: Uint32Array,
  data: Uint8Array,
  output: Uint8Array,
  counter: number,
  rounds: number
): void {
  const len = data.length;
  const block = new Uint8Array(BLOCK_LEN);
  const b32 = u32(block);
  // Make sure that buffers aligned to 4 bytes
  const isAligned = isAligned32(data) && isAligned32(output);
  const d32 = isAligned ? u32(data) : U32_EMPTY;
  const o32 = isAligned ? u32(output) : U32_EMPTY;
  for (let pos = 0; pos < len; counter++) {
    core(sigma, key, nonce, b32, counter, rounds);
    if (counter >= MAX_COUNTER) throw new Error('arx: counter overflow');
    const take = Math.min(BLOCK_LEN, len - pos);
    // aligned to 4 bytes
    if (isAligned && take === BLOCK_LEN) {
      const pos32 = pos / 4;
      if (pos % 4 !== 0) throw new Error('arx: invalid block position');
      for (let j = 0, posj: number; j < BLOCK_LEN32; j++) {
        posj = pos32 + j;
        o32[posj] = d32[posj] ^ b32[j];
      }
      pos += BLOCK_LEN;
      continue;
    }
    for (let j = 0, posj; j < take; j++) {
      posj = pos + j;
      output[posj] = data[posj] ^ block[j];
    }
    pos += take;
  }
}

/** Creates ARX-like (ChaCha, Salsa) cipher stream from core function. */
export function createCipher(core: CipherCoreFn, opts: CipherOpts): XorStream {
  const { allowShortKeys, extendNonceFn, counterLength, counterRight, rounds } = checkOpts(
    { allowShortKeys: false, counterLength: 8, counterRight: false, rounds: 20 },
    opts
  );
  if (typeof core !== 'function') throw new Error('core must be a function');
  anumber(counterLength);
  anumber(rounds);
  abool(counterRight);
  abool(allowShortKeys);
  return (
    key: Uint8Array,
    nonce: Uint8Array,
    data: Uint8Array,
    output?: Uint8Array,
    counter = 0
  ): Uint8Array => {
    abytes(key, undefined, 'key');
    abytes(nonce, undefined, 'nonce');
    abytes(data, undefined, 'data');
    const len = data.length;
    if (output === undefined) output = new Uint8Array(len);
    abytes(output, undefined, 'output');
    anumber(counter);
    if (counter < 0 || counter >= MAX_COUNTER) throw new Error('arx: counter overflow');
    if (output.length < len)
      throw new Error(`arx: output (${output.length}) is shorter than data (${len})`);
    const toClean = [];

    // Key & sigma
    // key=16 -> sigma16, k=key|key
    // key=32 -> sigma32, k=key
    let l = key.length;
    let k: Uint8Array;
    let sigma: Uint32Array;
    if (l === 32) {
      toClean.push((k = copyBytes(key)));
      sigma = sigma32_32;
    } else if (l === 16 && allowShortKeys) {
      k = new Uint8Array(32);
      k.set(key);
      k.set(key, 16);
      sigma = sigma16_32;
      toClean.push(k);
    } else {
      abytes(key, 32, 'arx key');
      throw new Error('invalid key size');
      // throw new Error(`"arx key" expected Uint8Array of length 32, got length=${l}`);
    }

    // Nonce
    // salsa20:      8   (8-byte counter)
    // chacha20orig: 8   (8-byte counter)
    // chacha20:     12  (4-byte counter)
    // xsalsa20:     24  (16 -> hsalsa,  8 -> old nonce)
    // xchacha20:    24  (16 -> hchacha, 8 -> old nonce)
    // Align nonce to 4 bytes
    if (!isAligned32(nonce)) toClean.push((nonce = copyBytes(nonce)));

    const k32 = u32(k);
    // hsalsa & hchacha: handle extended nonce
    if (extendNonceFn) {
      if (nonce.length !== 24) throw new Error(`arx: extended nonce must be 24 bytes`);
      extendNonceFn(sigma, k32, u32(nonce.subarray(0, 16)), k32);
      nonce = nonce.subarray(16);
    }

    // Handle nonce counter
    const nonceNcLen = 16 - counterLength;
    if (nonceNcLen !== nonce.length)
      throw new Error(`arx: nonce must be ${nonceNcLen} or 16 bytes`);

    // Pad counter when nonce is 64 bit
    if (nonceNcLen !== 12) {
      const nc = new Uint8Array(12);
      nc.set(nonce, counterRight ? 0 : 12 - nonce.length);
      nonce = nc;
      toClean.push(nonce);
    }
    const n32 = u32(nonce);
    runCipher(core, sigma, k32, n32, data, output, counter, rounds);
    clean(...toClean);
    return output;
  };
}

/** Internal class which wraps chacha20 or chacha8 to create CSPRNG. */
export class _XorStreamPRG implements PRG {
  readonly blockLen: number;
  readonly keyLen: number;
  readonly nonceLen: number;
  private state: Uint8Array;
  private buf: Uint8Array;
  private key: Uint8Array;
  private nonce: Uint8Array;
  private pos: number;
  private ctr: number;
  private cipher: XorStream;
  constructor(
    cipher: XorStream,
    blockLen: number,
    keyLen: number,
    nonceLen: number,
    seed: Uint8Array
  ) {
    this.cipher = cipher;
    this.blockLen = blockLen;
    this.keyLen = keyLen;
    this.nonceLen = nonceLen;
    this.state = new Uint8Array(this.keyLen + this.nonceLen);
    this.reseed(seed);
    this.ctr = 0;
    this.pos = this.blockLen;
    this.buf = new Uint8Array(this.blockLen);
    this.key = this.state.subarray(0, this.keyLen);
    this.nonce = this.state.subarray(this.keyLen);
  }
  private reseed(seed: Uint8Array) {
    abytes(seed);
    if (!seed || seed.length === 0) throw new Error('entropy required');
    for (let i = 0; i < seed.length; i++) this.state[i % this.state.length] ^= seed[i];
    this.ctr = 0;
    this.pos = this.blockLen;
  }
  addEntropy(seed: Uint8Array): void {
    this.state.set(this.randomBytes(this.state.length));
    this.reseed(seed);
  }
  randomBytes(len: number): Uint8Array {
    anumber(len);
    if (len === 0) return new Uint8Array(0);
    const out = new Uint8Array(len);
    let outPos = 0;
    // Leftovers
    if (this.pos < this.blockLen) {
      const take = Math.min(len, this.blockLen - this.pos);
      out.set(this.buf.subarray(this.pos, this.pos + take), 0);
      this.pos += take;
      outPos += take;
      if (outPos === len) return out; // fast path
    }
    // Full blocks directly to out
    const blocks = Math.floor((len - outPos) / this.blockLen);
    if (blocks > 0) {
      const blockBytes = blocks * this.blockLen;
      const b = out.subarray(outPos, outPos + blockBytes);
      this.cipher(this.key, this.nonce, b, b, this.ctr);
      this.ctr += blocks;
      outPos += blockBytes;
    }
    // Save leftovers
    const left = len - outPos;
    if (left > 0) {
      this.buf.fill(0);
      // NOTE: cipher will handle overflow
      this.cipher(this.key, this.nonce, this.buf, this.buf, this.ctr++);
      out.set(this.buf.subarray(0, left), outPos);
      this.pos = left;
    }
    return out;
  }
  clone(): _XorStreamPRG {
    return new _XorStreamPRG(
      this.cipher,
      this.blockLen,
      this.keyLen,
      this.nonceLen,
      this.randomBytes(this.state.length)
    );
  }
  clean(): void {
    this.pos = 0;
    this.ctr = 0;
    this.buf.fill(0);
    this.state.fill(0);
  }
}

export type XorPRG = (seed?: Uint8Array) => _XorStreamPRG;

export const createPRG = (
  cipher: XorStream,
  blockLen: number,
  keyLen: number,
  nonceLen: number
): XorPRG => {
  return (seed: Uint8Array = randomBytes(32)): _XorStreamPRG =>
    new _XorStreamPRG(cipher, blockLen, keyLen, nonceLen, seed);
};