-
Notifications
You must be signed in to change notification settings - Fork 8
/
encrypt-curve25519.go
312 lines (279 loc) · 9.52 KB
/
encrypt-curve25519.go
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
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
package peer
import (
"crypto/aes"
"crypto/ecdh"
"crypto/ed25519"
"crypto/subtle"
"github.com/aperturerobotics/bifrost/util/extra25519"
"github.com/aperturerobotics/util/scrub"
"github.com/klauspost/compress/s2"
b58 "github.com/mr-tron/base58/base58"
"github.com/pkg/errors"
"github.com/zeebo/blake3"
"golang.org/x/crypto/chacha20poly1305"
)
// EncryptToEd25519 encrypts to a ed25519 key using curve25519.
//
// t is the target ed25519 public key.
//
// mix pub key into seed: blake3(context + msgSrc + tPubKey)
// generate the one-time use message priv key (ed25519) from seed
// convert the target public key to a curve25519 point
// convert the message private key to a curve25519 scalar
// generate the nonce with blake3(context + msgPubKeyEd25519 + msgPubKeyCurve25519)[:24]
// xor the nonce with blake3(msgPubKeyEd25519 + msgPubKeyCurve25519)[24:] (8 bytes long)
// generate msgPubKey aes256 key: blake3(context + tPubKey + msgNonce[:4])
// generate key for chacha20poly1305 with ecdh(msgPrivKeyCurve25519, tPubKeyCurve25519)
//
// ciphertext: msgNonce[:4] + aes256(msgPubKey) + chacha20poly1305(s2(message))
//
// context and destination public key must be the same when decrypting
// context should be globally unique, and application-specific.
// A good format for ctx strings is: [application] [commit timestamp] [purpose]
// e.g., "example.com 2019-12-25 16:18:03 session tokens v1"
// the purpose of these requirements is to ensure that an attacker cannot trick two different applications into using the same context string.
func EncryptToEd25519(
tPubKey ed25519.PublicKey,
context string,
msgSrc []byte,
) ([]byte, error) {
if len(tPubKey) != 32 {
return nil, errors.Errorf("unexpected ed25519 public key len: %d", len(tPubKey))
}
// mix pub key and msg src into 32-byte seed: blake3(context + msgSrc + tPubKey)
seedHasher := blake3.NewDeriveKey("bifrost/peer encrypt curve25519 " + context)
_, err := seedHasher.Write(msgSrc)
if err != nil {
return nil, err
}
_, err = seedHasher.Write(tPubKey)
if err != nil {
return nil, err
}
msgSeed := seedHasher.Sum(nil)
seedHasher.Reset()
// generate the message priv key (ed25519) from seed
msgPrivKey := ed25519.NewKeyFromSeed(msgSeed[:])
scrub.Scrub(msgSeed[:])
msgPubKey := msgPrivKey.Public().(ed25519.PublicKey)
defer scrub.Scrub(msgPubKey)
// See: https://words.filippo.io/using-ed25519-keys-for-encryption/
msgPrivKeyCurve25519 := extra25519.PrivateKeyToCurve25519(msgPrivKey)
scrub.Scrub(msgPrivKey)
msgPrivKeyEcdh, err := ecdh.X25519().NewPrivateKey(msgPrivKeyCurve25519[:32])
scrub.Scrub(msgPrivKeyCurve25519)
if err != nil {
return nil, err
}
// generate the nonce
msgNonceHasher := blake3.NewDeriveKey("bifrost/peer encrypt curve25519 nonce " + context)
_, err = msgNonceHasher.Write(msgPubKey)
if err != nil {
return nil, err
}
msgPubKeyHash := msgNonceHasher.Sum(nil)
// use the first 24 bytes of msg pubkey hash as 24-byte nonce
msgNonce := msgPubKeyHash[:chacha20poly1305.NonceSizeX]
// xor the remaining 8 bytes of the hash with the nonce
xorHash := msgPubKeyHash[chacha20poly1305.NonceSizeX:]
for i := range msgNonce {
msgNonce[i] ^= xorHash[(i+2)%len(xorHash)]
}
// convert public key to curve25519 montgomery point
tPubKeyCurve25519, valid := extra25519.PublicKeyToCurve25519(tPubKey)
if !valid {
return nil, ErrInvalidEd25519PubKeyForCurve25519
}
tPubKeyEcdh, err := ecdh.X25519().NewPublicKey(tPubKeyCurve25519[:])
scrub.Scrub(tPubKeyCurve25519[:])
if err != nil {
return nil, err
}
// compress the message
msg := s2.EncodeBetter(nil, msgSrc)
// build the seed for the message prefix: blake3(context, tPubKey+msgNonce[:4])
seedHasher = blake3.NewDeriveKey("bifrost/peer encrypt curve25519 prefix " + context)
_, err = seedHasher.Write(tPubKey[:])
if err != nil {
return nil, err
}
_, err = seedHasher.Write(msgNonce[:4])
if err != nil {
return nil, err
}
aes256Seed := seedHasher.Sum(nil)
// encrypt the message prefix with aes256
// allocate enough space for the ciphertext as well
prefix := make([]byte, 4+32, len(msgPubKey)+chacha20poly1305.Overhead+len(msg)+8)
copy(prefix[:4], msgNonce[:4])
copy(prefix[4:], msgPubKey[:])
prefixCipher, err := aes.NewCipher(aes256Seed[:32])
if err != nil {
scrub.Scrub(aes256Seed)
return nil, err
}
prefixCipher.Encrypt(prefix[4:], prefix[4:])
scrub.Scrub(aes256Seed)
// sharedSecret is 32 bytes with ecdh(msgPrivKey, tPubKey)
sharedSecret, err := msgPrivKeyEcdh.ECDH(tPubKeyEcdh)
if err != nil {
return nil, err
}
// encrypt with chacha20poly1305 cipher
// prepend the 32-byte curve25519 one-time message key
// re-use the prefix buf
cipher, err := chacha20poly1305.NewX(sharedSecret)
scrub.Scrub(sharedSecret)
if err != nil {
return nil, err
}
return cipher.Seal(
prefix,
msgNonce,
msg,
msgPubKey[:],
), nil
}
// DecryptWithEd25519 decrypts with a ed25519 key using curve25519.
//
// tPrivKey is the target (destination) private key.
//
// derive aes256 key: blake3(context + tPubKey + ciphertext[:4])
// decrypt msgPubKey with aes256 from ciphertext[4:][:32]
// convert the message public key to a curve25519 point
// convert the target private key to a curve25519 scalar
// derive key for chacha20poly1305 with ecdh(privKeyCurve25519, msgPubKeyCurve25519)
// derive nonce with blake3(context, msgPubKey)[:24]
// xor the nonce with blake3(context, msgPubKey)[24:] (8 bytes long)
//
// ciphertext: msgNonce[:4] + aes256(msgPubKey) + chacha20poly1305(s2(message))
//
// context and destination key must be the same as when encrypting
func DecryptWithEd25519(
tPrivKey ed25519.PrivateKey,
context string,
ciphertext []byte,
) ([]byte, error) {
if len(tPrivKey) != 64 {
return nil, errors.Errorf("unexpected ed25519 private key len: %d", len(tPrivKey))
}
if len(ciphertext) < 34 {
return nil, ErrShortMessage
}
// initialize tPubKey with ed25519 public key
tPubKey := tPrivKey.Public().(ed25519.PublicKey)
defer scrub.Scrub(tPubKey[:])
// build the seed for the aes256 key
// ciphertext[:4] is msgNonce[:4]
seedHasher := blake3.NewDeriveKey("bifrost/peer encrypt curve25519 prefix " + context)
_, err := seedHasher.Write(tPubKey)
if err != nil {
return nil, err
}
_, err = seedHasher.Write(ciphertext[:4])
if err != nil {
return nil, err
}
aes256Seed := seedHasher.Sum(nil)
// msgPubKey is the 32-byte per-message ed25519 pub key
var msgPubKey [32]byte
copy(msgPubKey[:], ciphertext[4:])
defer scrub.Scrub(msgPubKey[:])
// decrypt the message prefix (msg pub key) with aes256
prefixCipher, err := aes.NewCipher(aes256Seed[:32])
if err == nil {
prefixCipher.Decrypt(msgPubKey[:], msgPubKey[:])
}
scrub.Scrub(aes256Seed)
if err != nil {
return nil, err
}
// build msgPubKeyEcdh
msgPubKeyCurve25519, valid := extra25519.PublicKeyToCurve25519(msgPubKey[:])
if !valid {
return nil, ErrInvalidEd25519PubKeyForCurve25519
}
msgPubKeyEcdh, err := ecdh.X25519().NewPublicKey(msgPubKeyCurve25519[:])
defer scrub.Scrub(msgPubKeyCurve25519[:])
if err != nil {
return nil, err
}
// build tPrivKeyEcdh
tPrivKeyCurve25519 := extra25519.PrivateKeyToCurve25519(tPrivKey)
tPrivKeyEcdh, err := ecdh.X25519().NewPrivateKey(tPrivKeyCurve25519[:32])
scrub.Scrub(tPrivKeyCurve25519[:])
if err != nil {
return nil, err
}
// sharedSecret is 32 bytes with (tPrivKeyCurve25519, msgPubKeyCurve25519)
sharedSecret, err := tPrivKeyEcdh.ECDH(msgPubKeyEcdh)
if err != nil {
scrub.Scrub(sharedSecret)
return nil, err
}
// generate the nonce
msgNonceHasher := blake3.NewDeriveKey("bifrost/peer encrypt curve25519 nonce " + context)
_, err = msgNonceHasher.Write(msgPubKey[:])
if err != nil {
return nil, err
}
msgPubKeyHash := msgNonceHasher.Sum(nil)
// use the first 24 bytes of msg pubkey hash as 24-byte nonce
msgNonce := msgPubKeyHash[:chacha20poly1305.NonceSizeX]
// xor the remaining 8 bytes of the hash with the nonce
xorHash := msgPubKeyHash[chacha20poly1305.NonceSizeX:]
for i := range msgNonce {
msgNonce[i] ^= xorHash[(i+2)%len(xorHash)]
}
// decrypt message with shared secret
cipher, err := chacha20poly1305.NewX(sharedSecret)
scrub.Scrub(sharedSecret)
if err != nil {
return nil, err
}
msgEnc := ciphertext[32+4:]
msgDec, err := cipher.Open(nil, msgNonce, msgEnc, msgPubKey[:])
if err != nil {
return nil, err
}
// decompress message, re-use scrubbed shared secret buffer
msgSrc, err := s2.Decode(sharedSecret, msgDec)
if err != nil {
return nil, err
}
// verify message: re-generate ed25519 private key
// mix pub key into 32-byte seed: blake3(context + msgSrc + tPubKey)
seedHasher = blake3.NewDeriveKey("bifrost/peer encrypt curve25519 " + context)
_, err = seedHasher.Write(msgSrc)
if err != nil {
return nil, err
}
_, err = seedHasher.Write(tPubKey[:])
if err != nil {
return nil, err
}
msgSeed := seedHasher.Sum(nil)
// generate the message priv key (ed25519) from seed
msgEdPrivKey := ed25519.NewKeyFromSeed(msgSeed[:])
scrub.Scrub(msgSeed)
msgEdPubKey := msgEdPrivKey.Public().(ed25519.PublicKey)
scrub.Scrub(msgEdPrivKey)
defer scrub.Scrub(msgEdPubKey)
// convert message pub key to curve25519 msg pub key
expectedMsgPubKeyCurve25519, valid := extra25519.PublicKeyToCurve25519(msgEdPubKey)
scrub.Scrub(msgEdPubKey)
if !valid {
return nil, ErrInvalidEd25519PubKeyForCurve25519
}
defer scrub.Scrub(expectedMsgPubKeyCurve25519[:])
// check that they match
if subtle.ConstantTimeCompare(expectedMsgPubKeyCurve25519, msgPubKeyCurve25519) == 0 {
return nil, errors.Errorf(
"message pubkey %s does not match expected pubkey %s",
b58.Encode(expectedMsgPubKeyCurve25519[:]),
b58.Encode(msgPubKeyCurve25519[:]),
)
}
// done
return msgSrc, nil
}