aboutsummaryrefslogtreecommitdiff
path: root/internal/stupidgcm/stupidgcm.go
blob: 6773a3591c9bdf3edf1130c228d03a47f2dd4332 (plain)
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
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
// Package stupidgcm is a thin wrapper for OpenSSL's GCM encryption and
// decryption functions. It only support 32-byte keys and 16-bit IVs.
package stupidgcm

// #include <openssl/evp.h>
// #cgo pkg-config: libcrypto
import "C"

import (
	"fmt"
	"log"
	"unsafe"
)

const (
	keyLen = 32
	ivLen  = 16
	tagLen = 16
)

// stupidGCM implements the cipher.AEAD interface
type stupidGCM struct {
	key []byte
}

func New(key []byte) stupidGCM {
	if len(key) != keyLen {
		log.Panicf("Only %d-byte keys are supported", keyLen)
	}
	return stupidGCM{key: key}
}

func (g stupidGCM) NonceSize() int {
	return ivLen
}

func (g stupidGCM) Overhead() int {
	return tagLen
}

// Seal - encrypt "in" using "iv" and "authData" and append the result to "dst"
func (g stupidGCM) Seal(dst, iv, in, authData []byte) []byte {
	if len(iv) != ivLen {
		log.Panicf("Only %d-byte IVs are supported", ivLen)
	}
	if len(in) == 0 {
		log.Panic("Zero-length input data is not supported")
	}
	buf := make([]byte, len(in)+tagLen)

	// https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption#Authenticated_Encryption_using_GCM_mode

	// Create scratch space "context"
	ctx := C.EVP_CIPHER_CTX_new()
	if ctx == nil {
		log.Panic("EVP_CIPHER_CTX_new failed")
	}

	// Set cipher to AES-256
	if C.EVP_EncryptInit_ex(ctx, C.EVP_aes_256_gcm(), nil, nil, nil) != 1 {
		log.Panic("EVP_EncryptInit_ex I failed")
	}

	// Use 16-byte IV
	if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_GCM_SET_IVLEN, ivLen, nil) != 1 {
		log.Panic("EVP_CIPHER_CTX_ctrl EVP_CTRL_GCM_SET_IVLEN failed")
	}

	// Set key and IV
	if C.EVP_EncryptInit_ex(ctx, nil, nil, (*C.uchar)(&g.key[0]), (*C.uchar)(&iv[0])) != 1 {
		log.Panic("EVP_EncryptInit_ex II failed")
	}

	// Provide authentication data
	var resultLen C.int
	if C.EVP_EncryptUpdate(ctx, nil, &resultLen, (*C.uchar)(&authData[0]), C.int(len(authData))) != 1 {
		log.Panic("EVP_EncryptUpdate authData failed")
	}
	if int(resultLen) != len(authData) {
		log.Panicf("Unexpected length %d", resultLen)
	}

	// Encrypt "in" into "buf"
	if C.EVP_EncryptUpdate(ctx, (*C.uchar)(&buf[0]), &resultLen, (*C.uchar)(&in[0]), C.int(len(in))) != 1 {
		log.Panic("EVP_EncryptUpdate failed")
	}
	if int(resultLen) != len(in) {
		log.Panicf("Unexpected length %d", resultLen)
	}

	// Finalise encryption
	// Because GCM is a stream encryption, this will not write out any data.
	dummy := make([]byte, 16)
	if C.EVP_EncryptFinal_ex(ctx, (*C.uchar)(&dummy[0]), &resultLen) != 1 {
		log.Panic("EVP_EncryptFinal_ex failed")
	}
	if resultLen != 0 {
		log.Panicf("Unexpected length %d", resultLen)
	}

	// Get GMAC tag and append it to the ciphertext in "buf"
	if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_GCM_GET_TAG, tagLen, (unsafe.Pointer)(&buf[len(in)])) != 1 {
		log.Panic("EVP_CIPHER_CTX_ctrl EVP_CTRL_GCM_GET_TAG failed")
	}

	// Free scratch space
	C.EVP_CIPHER_CTX_free(ctx)

	return append(dst, buf...)
}

// Open - decrypt "in" using "iv" and "authData" and append the result to "dst"
func (g stupidGCM) Open(dst, iv, in, authData []byte) ([]byte, error) {
	if len(iv) != ivLen {
		log.Panicf("Only %d-byte IVs are supported", ivLen)
	}
	if len(in) <= tagLen {
		log.Panic("Input data too short")
	}
	buf := make([]byte, len(in)-tagLen)
	ciphertext := in[:len(in)-tagLen]
	tag := in[len(in)-tagLen:]

	// https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption#Authenticated_Encryption_using_GCM_mode

	// Create scratch space "context"
	ctx := C.EVP_CIPHER_CTX_new()
	if ctx == nil {
		log.Panic("EVP_CIPHER_CTX_new failed")
	}

	// Set cipher to AES-256
	if C.EVP_DecryptInit_ex(ctx, C.EVP_aes_256_gcm(), nil, nil, nil) != 1 {
		log.Panic("EVP_DecryptInit_ex I failed")
	}

	// Use 16-byte IV
	if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_GCM_SET_IVLEN, ivLen, nil) != 1 {
		log.Panic("EVP_CIPHER_CTX_ctrl EVP_CTRL_GCM_SET_IVLEN failed")
	}

	// Set key and IV
	if C.EVP_DecryptInit_ex(ctx, nil, nil, (*C.uchar)(&g.key[0]), (*C.uchar)(&iv[0])) != 1 {
		log.Panic("EVP_DecryptInit_ex II failed")
	}

	// Provide authentication data
	var resultLen C.int
	if C.EVP_DecryptUpdate(ctx, nil, &resultLen, (*C.uchar)(&authData[0]), C.int(len(authData))) != 1 {
		log.Panic("EVP_EncryptUpdate authData failed")
	}
	if int(resultLen) != len(authData) {
		log.Panicf("Unexpected length %d", resultLen)
	}

	// Decrypt "ciphertext" into "buf"
	if C.EVP_DecryptUpdate(ctx, (*C.uchar)(&buf[0]), &resultLen, (*C.uchar)(&ciphertext[0]), C.int(len(ciphertext))) != 1 {
		log.Panic("EVP_DecryptUpdate failed")
	}
	if int(resultLen) != len(ciphertext) {
		log.Panicf("Unexpected length %d", resultLen)
	}

	// Set expected GMAC tag
	if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_GCM_SET_TAG, tagLen, (unsafe.Pointer)(&tag[0])) != 1 {
		log.Panic("EVP_CIPHER_CTX_ctrl failed")
	}

	// Check GMAC
	dummy := make([]byte, 16)
	res := C.EVP_DecryptFinal_ex(ctx, (*C.uchar)(&dummy[0]), &resultLen)
	if resultLen != 0 {
		log.Panicf("Unexpected length %d", resultLen)
	}

	// Free scratch space
	C.EVP_CIPHER_CTX_free(ctx)

	if res != 1 {
		return nil, fmt.Errorf("stupidgcm: message authentication failed")
	}

	return append(dst, buf...), nil
}