1 // Copyright 2016 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
4 5 // Package chacha20poly1305 implements the ChaCha20-Poly1305 AEAD and its
6 // extended nonce variant XChaCha20-Poly1305, as specified in RFC 8439 and
7 // draft-irtf-cfrg-xchacha-01.
8 package chacha20poly1305
9 10 import (
11 "crypto/cipher"
12 "errors"
13 )
14 15 const (
16 // KeySize is the size of the key used by this AEAD, in bytes.
17 KeySize = 32
18 19 // NonceSize is the size of the nonce used with the standard variant of this
20 // AEAD, in bytes.
21 //
22 // Note that this is too short to be safely generated at random if the same
23 // key is reused more than 2³² times.
24 NonceSize = 12
25 26 // NonceSizeX is the size of the nonce used with the XChaCha20-Poly1305
27 // variant of this AEAD, in bytes.
28 NonceSizeX = 24
29 30 // Overhead is the size of the Poly1305 authentication tag, and the
31 // difference between a ciphertext length and its plaintext.
32 Overhead = 16
33 )
34 35 type chacha20poly1305 struct {
36 key [KeySize]byte
37 }
38 39 // New returns a ChaCha20-Poly1305 AEAD that uses the given 256-bit key.
40 func New(key []byte) (cipher.AEAD, error) {
41 if len(key) != KeySize {
42 return nil, errors.New("chacha20poly1305: bad key length")
43 }
44 ret := &chacha20poly1305{}
45 copy(ret.key[:], key)
46 return ret, nil
47 }
48 49 func (c *chacha20poly1305) NonceSize() int {
50 return NonceSize
51 }
52 53 func (c *chacha20poly1305) Overhead() int {
54 return Overhead
55 }
56 57 func (c *chacha20poly1305) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
58 if len(nonce) != NonceSize {
59 panic("chacha20poly1305: bad nonce length passed to Seal")
60 }
61 62 if uint64(len(plaintext)) > (1<<38)-64 {
63 panic("chacha20poly1305: plaintext too large")
64 }
65 66 return c.seal(dst, nonce, plaintext, additionalData)
67 }
68 69 var errOpen = errors.New("chacha20poly1305: message authentication failed")
70 71 func (c *chacha20poly1305) Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
72 if len(nonce) != NonceSize {
73 panic("chacha20poly1305: bad nonce length passed to Open")
74 }
75 if len(ciphertext) < 16 {
76 return nil, errOpen
77 }
78 if uint64(len(ciphertext)) > (1<<38)-48 {
79 panic("chacha20poly1305: ciphertext too large")
80 }
81 82 return c.open(dst, nonce, ciphertext, additionalData)
83 }
84 85 // sliceForAppend takes a slice and a requested number of bytes. It returns a
86 // slice with the contents of the given slice followed by that many bytes and a
87 // second slice that aliases into it and contains only the extra bytes. If the
88 // original slice has sufficient capacity then no allocation is performed.
89 func sliceForAppend(in []byte, n int) (head, tail []byte) {
90 if total := len(in) + n; cap(in) >= total {
91 head = in[:total]
92 } else {
93 head = []byte{:total}
94 copy(head, in)
95 }
96 tail = head[len(in):]
97 return
98 }
99