1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
5 // This code implements SPAKE2, a variant of EKE:
6 // http://www.di.ens.fr/~pointche/pub.php?reference=AbPo04
8 #include "crypto/p224_spake.h"
12 #include "base/logging.h"
13 #include "crypto/p224.h"
14 #include "crypto/random.h"
15 #include "crypto/secure_util.h"
19 // The following two points (M and N in the protocol) are verifiable random
20 // points on the curve and can be generated with the following code:
22 // #include <stdint.h>
24 // #include <string.h>
26 // #include <openssl/ec.h>
27 // #include <openssl/obj_mac.h>
28 // #include <openssl/sha.h>
30 // // Silence a presubmit.
31 // #define PRINTF printf
33 // static const char kSeed1[] = "P224 point generation seed (M)";
34 // static const char kSeed2[] = "P224 point generation seed (N)";
36 // void find_seed(const char* seed) {
38 // uint8_t digest[SHA256_DIGEST_LENGTH];
40 // SHA256_Init(&sha256);
41 // SHA256_Update(&sha256, seed, strlen(seed));
42 // SHA256_Final(digest, &sha256);
45 // EC_GROUP* p224 = EC_GROUP_new_by_curve_name(NID_secp224r1);
46 // EC_POINT* p = EC_POINT_new(p224);
48 // for (unsigned i = 0;; i++) {
50 // BN_bin2bn(digest, 28, &x);
52 // if (EC_POINT_set_compressed_coordinates_GFp(
53 // p224, p, &x, digest[28] & 1, NULL)) {
55 // EC_POINT_get_affine_coordinates_GFp(p224, p, &x, &y, NULL);
56 // char* x_str = BN_bn2hex(&x);
57 // char* y_str = BN_bn2hex(&y);
58 // PRINTF("Found after %u iterations:\n%s\n%s\n", i, x_str, y_str);
59 // OPENSSL_free(x_str);
60 // OPENSSL_free(y_str);
66 // SHA256_Init(&sha256);
67 // SHA256_Update(&sha256, digest, sizeof(digest));
68 // SHA256_Final(digest, &sha256);
74 // EC_GROUP_free(p224);
83 const crypto::p224::Point kM = {
84 {174237515, 77186811, 235213682, 33849492,
85 33188520, 48266885, 177021753, 81038478},
86 {104523827, 245682244, 266509668, 236196369,
87 28372046, 145351378, 198520366, 113345994},
88 {1, 0, 0, 0, 0, 0, 0, 0},
91 const crypto::p224::Point kN = {
92 {136176322, 263523628, 251628795, 229292285,
93 5034302, 185981975, 171998428, 11653062},
94 {197567436, 51226044, 60372156, 175772188,
95 42075930, 8083165, 160827401, 65097570},
96 {1, 0, 0, 0, 0, 0, 0, 0},
99 } // anonymous namespace
103 P224EncryptedKeyExchange::P224EncryptedKeyExchange(PeerType peer_type,
104 base::StringPiece password)
105 : state_(kStateInitial), is_server_(peer_type == kPeerTypeServer) {
106 memset(&x_, 0, sizeof(x_));
107 memset(&expected_authenticator_, 0, sizeof(expected_authenticator_));
109 // x_ is a random scalar.
110 RandBytes(x_, sizeof(x_));
112 // Calculate |password| hash to get SPAKE password value.
113 SHA256HashString(std::string(password.data(), password.length()),
119 void P224EncryptedKeyExchange::Init() {
122 p224::ScalarBaseMult(x_, &X);
124 // The client masks the Diffie-Hellman value, X, by adding M**pw and the
125 // server uses N**pw.
127 p224::ScalarMult(is_server_ ? kN : kM, pw_, &MNpw);
129 // X* = X + (N|M)**pw
131 p224::Add(X, MNpw, &Xstar);
133 next_message_ = Xstar.ToString();
136 const std::string& P224EncryptedKeyExchange::GetNextMessage() {
137 if (state_ == kStateInitial) {
138 state_ = kStateRecvDH;
139 return next_message_;
140 } else if (state_ == kStateSendHash) {
141 state_ = kStateRecvHash;
142 return next_message_;
145 LOG(FATAL) << "P224EncryptedKeyExchange::GetNextMessage called in"
146 " bad state " << state_;
148 return next_message_;
151 P224EncryptedKeyExchange::Result P224EncryptedKeyExchange::ProcessMessage(
152 base::StringPiece message) {
153 if (state_ == kStateRecvHash) {
154 // This is the final state of the protocol: we are reading the peer's
155 // authentication hash and checking that it matches the one that we expect.
156 if (message.size() != sizeof(expected_authenticator_)) {
157 error_ = "peer's hash had an incorrect size";
158 return kResultFailed;
160 if (!SecureMemEqual(message.data(), expected_authenticator_,
162 error_ = "peer's hash had incorrect value";
163 return kResultFailed;
166 return kResultSuccess;
169 if (state_ != kStateRecvDH) {
170 LOG(FATAL) << "P224EncryptedKeyExchange::ProcessMessage called in"
171 " bad state " << state_;
172 error_ = "internal error";
173 return kResultFailed;
176 // Y* is the other party's masked, Diffie-Hellman value.
178 if (!Ystar.SetFromString(message)) {
179 error_ = "failed to parse peer's masked Diffie-Hellman value";
180 return kResultFailed;
183 // We calculate the mask value: (N|M)**pw
184 p224::Point MNpw, minus_MNpw, Y, k;
185 p224::ScalarMult(is_server_ ? kM : kN, pw_, &MNpw);
186 p224::Negate(MNpw, &minus_MNpw);
188 // Y = Y* - (N|M)**pw
189 p224::Add(Ystar, minus_MNpw, &Y);
192 p224::ScalarMult(Y, x_, &k);
194 // If everything worked out, then K is the same for both parties.
197 std::string client_masked_dh, server_masked_dh;
199 client_masked_dh = message.as_string();
200 server_masked_dh = next_message_;
202 client_masked_dh = next_message_;
203 server_masked_dh = message.as_string();
206 // Now we calculate the hashes that each side will use to prove to the other
207 // that they derived the correct value for K.
208 uint8_t client_hash[kSHA256Length], server_hash[kSHA256Length];
209 CalculateHash(kPeerTypeClient, client_masked_dh, server_masked_dh, key_,
211 CalculateHash(kPeerTypeServer, client_masked_dh, server_masked_dh, key_,
214 const uint8_t* my_hash = is_server_ ? server_hash : client_hash;
215 const uint8_t* their_hash = is_server_ ? client_hash : server_hash;
218 std::string(reinterpret_cast<const char*>(my_hash), kSHA256Length);
219 memcpy(expected_authenticator_, their_hash, kSHA256Length);
220 state_ = kStateSendHash;
221 return kResultPending;
224 void P224EncryptedKeyExchange::CalculateHash(
226 const std::string& client_masked_dh,
227 const std::string& server_masked_dh,
228 const std::string& k,
229 uint8_t* out_digest) {
230 std::string hash_contents;
232 if (peer_type == kPeerTypeServer) {
233 hash_contents = "server";
235 hash_contents = "client";
238 hash_contents += client_masked_dh;
239 hash_contents += server_masked_dh;
241 std::string(reinterpret_cast<const char *>(pw_), sizeof(pw_));
244 SHA256HashString(hash_contents, out_digest, kSHA256Length);
247 const std::string& P224EncryptedKeyExchange::error() const {
251 const std::string& P224EncryptedKeyExchange::GetKey() const {
252 DCHECK_EQ(state_, kStateDone);
253 return GetUnverifiedKey();
256 const std::string& P224EncryptedKeyExchange::GetUnverifiedKey() const {
257 // Key is already final when state is kStateSendHash. Subsequent states are
258 // used only for verification of the key. Some users may combine verification
259 // with sending verifiable data instead of |expected_authenticator_|.
260 DCHECK_GE(state_, kStateSendHash);
264 void P224EncryptedKeyExchange::SetXForTesting(const std::string& x) {
265 memset(&x_, 0, sizeof(x_));
266 memcpy(&x_, x.data(), std::min(x.size(), sizeof(x_)));
270 } // namespace crypto