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
|
// Package ratchet provides the methods necessary to establish a new double
// ratchet session.
package ratchet
import (
"encoding/base64"
"encoding/binary"
"go.mau.fi/libsignal/ecc"
"go.mau.fi/libsignal/kdf"
"go.mau.fi/libsignal/keys/chain"
"go.mau.fi/libsignal/keys/root"
"go.mau.fi/libsignal/keys/session"
)
var b64 = base64.StdEncoding.EncodeToString
func genDiscontinuity() [32]byte {
var discontinuity [32]byte
for i := range discontinuity {
discontinuity[i] = 0xFF
}
return discontinuity
}
// CalculateSenderSession calculates the key agreement for a recipient. This
// should be used when we are trying to send a message to someone for the
// first time.
func CalculateSenderSession(parameters *SenderParameters) (*session.KeyPair, error) {
var secret [32]byte
var publicKey [32]byte
var privateKey [32]byte
masterSecret := []byte{} // Create a master shared secret that is 5 different 32-byte values
discontinuity := genDiscontinuity()
masterSecret = append(masterSecret, discontinuity[:]...)
// Calculate the agreement using their signed prekey and our identity key.
publicKey = parameters.TheirSignedPreKey().PublicKey()
privateKey = parameters.OurIdentityKey().PrivateKey().Serialize()
secret = kdf.CalculateSharedSecret(
publicKey,
privateKey,
)
masterSecret = append(masterSecret, secret[:]...)
// Calculate the agreement using their identity key and our base key.
publicKey = parameters.TheirIdentityKey().PublicKey().PublicKey()
privateKey = parameters.OurBaseKey().PrivateKey().Serialize()
secret = kdf.CalculateSharedSecret(
publicKey,
privateKey,
)
masterSecret = append(masterSecret, secret[:]...)
// Calculate the agreement using their signed prekey and our base key.
publicKey = parameters.TheirSignedPreKey().PublicKey()
privateKey = parameters.OurBaseKey().PrivateKey().Serialize()
secret = kdf.CalculateSharedSecret(
publicKey,
privateKey,
)
masterSecret = append(masterSecret, secret[:]...)
// If they have a one-time prekey, use it to calculate the shared secret with their
// one time key and our base key.
if parameters.TheirOneTimePreKey() != nil {
publicKey = parameters.TheirOneTimePreKey().PublicKey()
privateKey = parameters.OurBaseKey().PrivateKey().Serialize()
secret = kdf.CalculateSharedSecret(
publicKey,
privateKey,
)
masterSecret = append(masterSecret, secret[:]...)
}
// Derive the root and chain keys based on the master secret.
derivedKeysBytes, err := kdf.DeriveSecrets(masterSecret, nil, []byte("WhisperText"), root.DerivedSecretsSize)
if err != nil {
return nil, err
}
derivedKeys := session.NewDerivedSecrets(derivedKeysBytes)
chainKey := chain.NewKey(kdf.DeriveSecrets, derivedKeys.ChainKey(), 0)
rootKey := root.NewKey(kdf.DeriveSecrets, derivedKeys.RootKey())
// Add the root and chain keys to a structure that will hold both keys.
sessionKeys := session.NewKeyPair(rootKey, chainKey)
return sessionKeys, nil
}
// CalculateReceiverSession calculates the key agreement for a sender. This should
// be used when we are receiving a message from someone for the first time.
func CalculateReceiverSession(parameters *ReceiverParameters) (*session.KeyPair, error) {
var secret [32]byte
var publicKey [32]byte
var privateKey [32]byte
masterSecret := []byte{} // Create a master shared secret that is 5 different 32-byte values
discontinuity := genDiscontinuity()
masterSecret = append(masterSecret, discontinuity[:]...)
// Calculate the agreement using their identity key and our signed pre key.
publicKey = parameters.TheirIdentityKey().PublicKey().PublicKey()
privateKey = parameters.OurSignedPreKey().PrivateKey().Serialize()
secret = kdf.CalculateSharedSecret(
publicKey,
privateKey,
)
masterSecret = append(masterSecret, secret[:]...)
// Calculate the agreement using their base key and our identity key.
publicKey = parameters.TheirBaseKey().PublicKey()
privateKey = parameters.OurIdentityKeyPair().PrivateKey().Serialize()
secret = kdf.CalculateSharedSecret(
publicKey,
privateKey,
)
masterSecret = append(masterSecret, secret[:]...)
// Calculate the agreement using their base key and our signed prekey.
publicKey = parameters.TheirBaseKey().PublicKey()
privateKey = parameters.OurSignedPreKey().PrivateKey().Serialize()
secret = kdf.CalculateSharedSecret(
publicKey,
privateKey,
)
masterSecret = append(masterSecret, secret[:]...)
// If we had a one-time prekey, use it to calculate the shared secret with our
// one time key and their base key.
if parameters.OurOneTimePreKey() != nil {
publicKey = parameters.TheirBaseKey().PublicKey()
privateKey = parameters.OurOneTimePreKey().PrivateKey().Serialize()
secret = kdf.CalculateSharedSecret(
publicKey,
privateKey,
)
masterSecret = append(masterSecret, secret[:]...)
}
// Derive the root and chain keys based on the master secret.
derivedKeysBytes, err := kdf.DeriveSecrets(masterSecret, nil, []byte("WhisperText"), root.DerivedSecretsSize)
if err != nil {
return nil, err
}
derivedKeys := session.NewDerivedSecrets(derivedKeysBytes)
chainKey := chain.NewKey(kdf.DeriveSecrets, derivedKeys.ChainKey(), 0)
rootKey := root.NewKey(kdf.DeriveSecrets, derivedKeys.RootKey())
// Add the root and chain keys to a structure that will hold both keys.
sessionKeys := session.NewKeyPair(rootKey, chainKey)
return sessionKeys, nil
}
// CalculateSymmetricSession calculates the key agreement between two users. This
// works by both clients exchanging KeyExchange messages to first establish a session.
// This is useful for establishing a session if both users are online.
func CalculateSymmetricSession(parameters *SymmetricParameters) (*session.KeyPair, error) {
// Compare the base public keys so we can deterministically know whether we should
// be setting up a sender or receiver session. If our key converted to an integer is
// less than the other user's, act as a sender.
if isSender(parameters.OurBaseKey.PublicKey(), parameters.TheirBaseKey) {
senderParameters := &SenderParameters{
ourBaseKey: parameters.OurBaseKey,
ourIdentityKeyPair: parameters.OurIdentityKeyPair,
theirRatchetKey: parameters.TheirRatchetKey,
theirIdentityKey: parameters.TheirIdentityKey,
theirSignedPreKey: parameters.TheirBaseKey,
}
return CalculateSenderSession(senderParameters)
}
// If our base public key was larger than the other user's, act as a receiver.
receiverParameters := &ReceiverParameters{
ourIdentityKeyPair: parameters.OurIdentityKeyPair,
ourRatchetKey: parameters.OurRatchetKey,
ourSignedPreKey: parameters.OurBaseKey,
theirBaseKey: parameters.TheirBaseKey,
theirIdentityKey: parameters.TheirIdentityKey,
}
return CalculateReceiverSession(receiverParameters)
}
// isSender is a private method for determining if a symmetric session should
// be calculated as the sender or receiver. It does so by converting the given
// keys into integers and comparing the size of those integers.
func isSender(ourKey, theirKey ecc.ECPublicKeyable) bool {
ourKeyInt := binary.BigEndian.Uint32(ourKey.Serialize())
theirKeyInt := binary.BigEndian.Uint32(theirKey.Serialize())
return ourKeyInt < theirKeyInt
}
|
