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 5 package chacha20poly1305
6 7 import (
8 "crypto/cipher"
9 "errors"
10 11 "golang.org/x/crypto/chacha20"
12 )
13 14 type 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.
24 func NewX(key []byte) (cipher.AEAD, error) {
25 if fips140Enforced() {
26 return nil, errors.New("chacha20poly1305: use of ChaCha20Poly1305 is not allowed in FIPS 140-only mode")
27 }
28 if len(key) != KeySize {
29 return nil, errors.New("chacha20poly1305: bad key length")
30 }
31 ret := new(xchacha20poly1305)
32 copy(ret.key[:], key)
33 return ret, nil
34 }
35 36 func (*xchacha20poly1305) NonceSize() int {
37 return NonceSizeX
38 }
39 40 func (*xchacha20poly1305) Overhead() int {
41 return Overhead
42 }
43 44 func (x *xchacha20poly1305) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
45 if len(nonce) != NonceSizeX {
46 panic("chacha20poly1305: bad nonce length passed to Seal")
47 }
48 49 // XChaCha20-Poly1305 technically supports a 64-bit counter, so there is no
50 // size limit. However, since we reuse the ChaCha20-Poly1305 implementation,
51 // the second half of the counter is not available. This is unlikely to be
52 // an issue because the cipher.AEAD API requires the entire message to be in
53 // memory, and the counter overflows at 256 GB.
54 if uint64(len(plaintext)) > (1<<38)-64 {
55 panic("chacha20poly1305: plaintext too large")
56 }
57 58 c := new(chacha20poly1305)
59 hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
60 copy(c.key[:], hKey)
61 62 // The first 4 bytes of the final nonce are unused counter space.
63 cNonce := make([]byte, NonceSize)
64 copy(cNonce[4:12], nonce[16:24])
65 66 return c.seal(dst, cNonce[:], plaintext, additionalData)
67 }
68 69 func (x *xchacha20poly1305) Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
70 if len(nonce) != NonceSizeX {
71 panic("chacha20poly1305: bad nonce length passed to Open")
72 }
73 if len(ciphertext) < 16 {
74 return nil, errOpen
75 }
76 if uint64(len(ciphertext)) > (1<<38)-48 {
77 panic("chacha20poly1305: ciphertext too large")
78 }
79 80 c := new(chacha20poly1305)
81 hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
82 copy(c.key[:], hKey)
83 84 // The first 4 bytes of the final nonce are unused counter space.
85 cNonce := make([]byte, NonceSize)
86 copy(cNonce[4:12], nonce[16:24])
87 88 return c.open(dst, cNonce[:], ciphertext, additionalData)
89 }
90