code: func XORKeyStream(dst, src, nonce, key []byte) {
  	chacha.XORKeyStream(dst, src, nonce, key, 20)
  }
docs: // XORKeyStream crypts bytes from src to dst using the given nonce and key.
  // The length of the nonce determinds the version of ChaCha20:
  // - 8 bytes:  ChaCha20 with a 64 bit nonce and a 2^64 * 64 byte period.
  // - 12 bytes: ChaCha20 as defined in RFC 7539 and a 2^32 * 64 byte period.
  // - 24 bytes: XChaCha20 with a 192 bit nonce and a 2^64 * 64 byte period.
  // Src and dst may be the same slice but otherwise should not overlap.
  // If len(dst) < len(src) this function panics.
  // If the nonce is neither 64, 96 nor 192 bits long, this function panics.