1// Copyright 2018 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
5package chacha20poly1305
6
7import (
8 "crypto/cipher"
9 "errors"
10
11 "golang.org/x/crypto/chacha20"
12)
13
14type xchacha20poly1305 struct {
15 key [KeySize]byte
16}
17
18// NewX returns a XChaCha20-Poly1305 AEAD that uses the given 256-bit key.
19//
20// XChaCha20-Poly1305 is a ChaCha20-Poly1305 variant that takes a longer nonce,
21// suitable to be generated randomly without risk of collisions. It should be
22// preferred when nonce uniqueness cannot be trivially ensured, or whenever
23// nonces are randomly generated.
24func NewX(key []byte) (cipher.AEAD, error) {
25 if len(key) != KeySize {
26 return nil, errors.New("chacha20poly1305: bad key length")
27 }
28 ret := new(xchacha20poly1305)
29 copy(ret.key[:], key)
30 return ret, nil
31}
32
33func (*xchacha20poly1305) NonceSize() int {
34 return NonceSizeX
35}
36
37func (*xchacha20poly1305) Overhead() int {
38 return Overhead
39}
40
41func (x *xchacha20poly1305) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
42 if len(nonce) != NonceSizeX {
43 panic("chacha20poly1305: bad nonce length passed to Seal")
44 }
45
46 // XChaCha20-Poly1305 technically supports a 64-bit counter, so there is no
47 // size limit. However, since we reuse the ChaCha20-Poly1305 implementation,
48 // the second half of the counter is not available. This is unlikely to be
49 // an issue because the cipher.AEAD API requires the entire message to be in
50 // memory, and the counter overflows at 256 GB.
51 if uint64(len(plaintext)) > (1<<38)-64 {
52 panic("chacha20poly1305: plaintext too large")
53 }
54
55 c := new(chacha20poly1305)
56 hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
57 copy(c.key[:], hKey)
58
59 // The first 4 bytes of the final nonce are unused counter space.
60 cNonce := make([]byte, NonceSize)
61 copy(cNonce[4:12], nonce[16:24])
62
63 return c.seal(dst, cNonce[:], plaintext, additionalData)
64}
65
66func (x *xchacha20poly1305) Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
67 if len(nonce) != NonceSizeX {
68 panic("chacha20poly1305: bad nonce length passed to Open")
69 }
70 if len(ciphertext) < 16 {
71 return nil, errOpen
72 }
73 if uint64(len(ciphertext)) > (1<<38)-48 {
74 panic("chacha20poly1305: ciphertext too large")
75 }
76
77 c := new(chacha20poly1305)
78 hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
79 copy(c.key[:], hKey)
80
81 // The first 4 bytes of the final nonce are unused counter space.
82 cNonce := make([]byte, NonceSize)
83 copy(cNonce[4:12], nonce[16:24])
84
85 return c.open(dst, cNonce[:], ciphertext, additionalData)
86}