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// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package chacha20poly1305 implements the ChaCha20-Poly1305 AEAD and its
// extended nonce variant XChaCha20-Poly1305, as specified in RFC 8439 and
// draft-irtf-cfrg-xchacha-01.
package chacha20poly1305 // import ""
import (
const (
// KeySize is the size of the key used by this AEAD, in bytes.
KeySize = 32
// NonceSize is the size of the nonce used with the standard variant of this
// AEAD, in bytes.
// Note that this is too short to be safely generated at random if the same
// key is reused more than 2³² times.
NonceSize = 12
// NonceSizeX is the size of the nonce used with the XChaCha20-Poly1305
// variant of this AEAD, in bytes.
NonceSizeX = 24
// Overhead is the size of the Poly1305 authentication tag, and the
// difference between a ciphertext length and its plaintext.
Overhead = 16
type chacha20poly1305 struct {
key [KeySize]byte
// New returns a ChaCha20-Poly1305 AEAD that uses the given 256-bit key.
func New(key []byte) (cipher.AEAD, error) {
if len(key) != KeySize {
return nil, errors.New("chacha20poly1305: bad key length")
ret := new(chacha20poly1305)
copy(ret.key[:], key)
return ret, nil
func (c *chacha20poly1305) NonceSize() int {
return NonceSize
func (c *chacha20poly1305) Overhead() int {
return Overhead
func (c *chacha20poly1305) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
if len(nonce) != NonceSize {
panic("chacha20poly1305: bad nonce length passed to Seal")
if uint64(len(plaintext)) > (1<<38)-64 {
panic("chacha20poly1305: plaintext too large")
return c.seal(dst, nonce, plaintext, additionalData)
var errOpen = errors.New("chacha20poly1305: message authentication failed")
func (c *chacha20poly1305) Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
if len(nonce) != NonceSize {
panic("chacha20poly1305: bad nonce length passed to Open")
if len(ciphertext) < 16 {
return nil, errOpen
if uint64(len(ciphertext)) > (1<<38)-48 {
panic("chacha20poly1305: ciphertext too large")
return, nonce, ciphertext, additionalData)
// sliceForAppend takes a slice and a requested number of bytes. It returns a
// slice with the contents of the given slice followed by that many bytes and a
// second slice that aliases into it and contains only the extra bytes. If the
// original slice has sufficient capacity then no allocation is performed.
func sliceForAppend(in []byte, n int) (head, tail []byte) {
if total := len(in) + n; cap(in) >= total {
head = in[:total]
} else {
head = make([]byte, total)
copy(head, in)
tail = head[len(in):]