1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
|
// Copyright 2024 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package aesctrhmac
import (
"bytes"
"encoding/binary"
"errors"
"fmt"
"github.com/tink-crypto/tink-go/v2/insecuresecretdataaccess"
"github.com/tink-crypto/tink-go/v2/internal/aead"
"github.com/tink-crypto/tink-go/v2/internal/mac/hmac"
"github.com/tink-crypto/tink-go/v2/key"
"github.com/tink-crypto/tink-go/v2/tink"
)
type fullAEAD struct {
aesCTR *aead.AESCTR
hmac *hmac.HMAC
ivSize int
tagSize int
prefix []byte
variant Variant
}
var _ tink.AEAD = (*fullAEAD)(nil)
func newAEAD(key *Key) (tink.AEAD, error) {
tagSize := key.parameters.TagSizeInBytes()
ivSize := key.parameters.IVSizeInBytes()
aesCTR, err := aead.NewAESCTR(key.AESKeyBytes().Data(insecuresecretdataaccess.Token{}), ivSize)
if err != nil {
return nil, err
}
hmac, err := hmac.New(key.parameters.HashType().String(), key.HMACKeyBytes().Data(insecuresecretdataaccess.Token{}), uint32(tagSize))
if err != nil {
return nil, err
}
return &fullAEAD{
aesCTR: aesCTR,
hmac: hmac,
ivSize: ivSize,
tagSize: tagSize,
prefix: key.OutputPrefix(),
variant: key.parameters.Variant(),
}, nil
}
func aadSizeInBits(associatedData []byte) []byte {
n := uint64(len(associatedData)) * 8
buf := make([]byte, 8)
binary.BigEndian.PutUint64(buf, n)
return buf
}
// Encrypt encrypts plaintext with associatedData.
//
// The plaintext is encrypted with an AES-CTR, then HMAC is computed over
// (associatedData || ciphertext || n) where n is associatedData's length
// in bits represented as a 64-bit big endian unsigned integer. The final
// ciphertext format is (IND-CPA ciphertext || mac).
func (a *fullAEAD) Encrypt(plaintext, associatedData []byte) ([]byte, error) {
ctSize := len(a.prefix) + a.ivSize + len(plaintext)
ciphertext := make([]byte, ctSize, ctSize+a.tagSize)
if n := copy(ciphertext, a.prefix); n != len(a.prefix) {
return nil, fmt.Errorf("aesctrhmac: failed to copy prefix")
}
ctNoPrefix, err := a.aesCTR.Encrypt(ciphertext[len(a.prefix):], plaintext)
if err != nil {
return nil, err
}
tag, err := a.hmac.ComputeMAC(associatedData, ctNoPrefix, aadSizeInBits(associatedData))
if err != nil {
return nil, err
}
if len(tag) != a.tagSize {
return nil, errors.New("aesctrhmac: invalid tag size")
}
return append(ciphertext, tag...), nil
}
// Decrypt decrypts ciphertext with associatedData.
func (a *fullAEAD) Decrypt(ciphertext, associatedData []byte) ([]byte, error) {
prefixSize := len(a.prefix)
if len(ciphertext) < prefixSize+a.ivSize+a.tagSize {
return nil, fmt.Errorf("aesctrhmac: ciphertext with size %d is too short", len(ciphertext))
}
prefix := ciphertext[:prefixSize]
if !bytes.Equal(prefix, a.prefix) {
return nil, fmt.Errorf("aesctrhmac: ciphertext prefix does not match: got %x, want %x", prefix, a.prefix)
}
payload := ciphertext[prefixSize : len(ciphertext)-a.tagSize]
tag := ciphertext[len(ciphertext)-a.tagSize:]
if err := a.hmac.VerifyMAC(tag, associatedData, payload, aadSizeInBits(associatedData)); err != nil {
return nil, fmt.Errorf("aesctrhmac: %v", err)
}
return a.aesCTR.Decrypt(nil, payload)
}
// primitiveConstructor creates a [tink.AEAD] from a [key.Key].
//
// The key must be of type [aesctrhmac.Key].
func primitiveConstructor(k key.Key) (any, error) {
that, ok := k.(*Key)
if !ok {
return nil, fmt.Errorf("invalid key type: got %T, want *aesctrhmac.Key", k)
}
return newAEAD(that)
}
|
