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 fips140Enforced() {
42 return nil, errors.New("chacha20poly1305: use of ChaCha20Poly1305 is not allowed in FIPS 140-only mode")
43 }
44 if len(key) != KeySize {
45 return nil, errors.New("chacha20poly1305: bad key length")
46 }
47 ret := new(chacha20poly1305)
48 copy(ret.key[:], key)
49 return ret, nil
50 }
51 52 func (c *chacha20poly1305) NonceSize() int {
53 return NonceSize
54 }
55 56 func (c *chacha20poly1305) Overhead() int {
57 return Overhead
58 }
59 60 func (c *chacha20poly1305) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
61 if len(nonce) != NonceSize {
62 panic("chacha20poly1305: bad nonce length passed to Seal")
63 }
64 65 if uint64(len(plaintext)) > (1<<38)-64 {
66 panic("chacha20poly1305: plaintext too large")
67 }
68 69 return c.seal(dst, nonce, plaintext, additionalData)
70 }
71 72 var errOpen = errors.New("chacha20poly1305: message authentication failed")
73 74 func (c *chacha20poly1305) Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
75 if len(nonce) != NonceSize {
76 panic("chacha20poly1305: bad nonce length passed to Open")
77 }
78 if len(ciphertext) < 16 {
79 return nil, errOpen
80 }
81 if uint64(len(ciphertext)) > (1<<38)-48 {
82 panic("chacha20poly1305: ciphertext too large")
83 }
84 85 return c.open(dst, nonce, ciphertext, additionalData)
86 }
87 88 // sliceForAppend takes a slice and a requested number of bytes. It returns a
89 // slice with the contents of the given slice followed by that many bytes and a
90 // second slice that aliases into it and contains only the extra bytes. If the
91 // original slice has sufficient capacity then no allocation is performed.
92 func sliceForAppend(in []byte, n int) (head, tail []byte) {
93 if total := len(in) + n; cap(in) >= total {
94 head = in[:total]
95 } else {
96 head = make([]byte, total)
97 copy(head, in)
98 }
99 tail = head[len(in):]
100 return
101 }
102