code: func NewCipher(nonce, key []byte) (cipher.Stream, error) {
  	return chacha.NewCipher(nonce, key, 20)
  }
docs: // NewCipher returns a new cipher.Stream implementing a ChaCha20 version.
  // The nonce must be unique for one key for all time.
  // 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.
  // If the nonce is neither 64, 96 nor 192 bits long, a non-nil error is returned.