Simplify SPAKE2 implementation.

Currently P224EncryptedKeyExchange uses two pieces of secret: password 
and session-specific key. They are combined and used together as one 
password. It is not really needed and the calling code can do
it when neccessary.

BUG=105214


Review URL: http://codereview.chromium.org/8903001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@114189 0039d316-1c4b-4281-b951-d872f2087c98

3 files changed, 34 insertions, 61 deletions

diff --git a/crypto/p224_spake.cc b/crypto/p224_spake.cc
index 3d83e25..ec0c416 100644
--- a/crypto/p224_spake.cc
+++ b/crypto/p224_spake.cc
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// This code implements SPAKE2, a varient of EKE:
+// This code implements SPAKE2, a variant of EKE:
 //  http://www.di.ens.fr/~pointche/pub.php?reference=AbPo04
 
 #include <crypto/p224_spake.h>
@@ -96,9 +96,7 @@ const crypto::p224::Point kN = {
 namespace crypto {
 
 P224EncryptedKeyExchange::P224EncryptedKeyExchange(
-    PeerType peer_type,
-    const base::StringPiece& password,
-    const base::StringPiece& session)
+    PeerType peer_type, const base::StringPiece& password)
     : state_(kStateInitial),
       is_server_(peer_type == kPeerTypeServer) {
   // x_ is a random scalar.
@@ -108,26 +106,9 @@ P224EncryptedKeyExchange::P224EncryptedKeyExchange(
   p224::Point X;
   p224::ScalarBaseMult(x_, &X);
 
-  // The "password" in the SPAKE2 protocol is
-  // SHA256(P(password) + P(session)) where P is function that prepends a
-  // uint32, big-endian length prefix.
-  uint8 password_length[4], session_length[4];
-  password_length[0] = password.size() >> 24;
-  password_length[1] = password.size() >> 16;
-  password_length[2] = password.size() >> 8;
-  password_length[3] = password.size();
-  session_length[0] = session.size() >> 24;
-  session_length[1] = session.size() >> 16;
-  session_length[2] = session.size() >> 8;
-  session_length[3] = session.size();
-  SHA256HashString(std::string(reinterpret_cast<const char *>(password_length),
-                               sizeof(password_length)) +
-                   password.as_string() +
-                   std::string(reinterpret_cast<const char *>(session_length),
-                               sizeof(session_length)) +
-                   session.as_string(),
-                   pw_,
-                   sizeof(pw_));
+  // Calculate |password| hash to get SPAKE password value.
+  SHA256HashString(std::string(password.data(), password.length()),
+                   pw_, sizeof(pw_));
 
   // The client masks the Diffie-Hellman value, X, by adding M**pw and the
   // server uses N**pw.
@@ -200,7 +181,7 @@ P224EncryptedKeyExchange::Result P224EncryptedKeyExchange::ProcessMessage(
   p224::ScalarMult(Y, x_, &k);
 
   // If everything worked out, then K is the same for both parties.
-  std::string k_str = k.ToString();
+  key_ = k.ToString();
 
   std::string client_masked_dh, server_masked_dh;
   if (is_server_) {
@@ -214,9 +195,9 @@ P224EncryptedKeyExchange::Result P224EncryptedKeyExchange::ProcessMessage(
   // Now we calculate the hashes that each side will use to prove to the other
   // that they derived the correct value for K.
   uint8 client_hash[kSHA256Length], server_hash[kSHA256Length];
-  CalculateHash(kPeerTypeClient, client_masked_dh, server_masked_dh, k_str,
+  CalculateHash(kPeerTypeClient, client_masked_dh, server_masked_dh, key_,
                 client_hash);
-  CalculateHash(kPeerTypeServer, client_masked_dh, server_masked_dh, k_str,
+  CalculateHash(kPeerTypeServer, client_masked_dh, server_masked_dh, key_,
                 server_hash);
 
   const uint8* my_hash = is_server_ ? server_hash : client_hash;
@@ -256,4 +237,9 @@ const std::string& P224EncryptedKeyExchange::error() const {
   return error_;
 }
 
+const std::string& P224EncryptedKeyExchange::GetKey() {
+  DCHECK_EQ(state_, kStateDone);
+  return key_;
+}
+
 }  // namespace crypto
diff --git a/crypto/p224_spake.h b/crypto/p224_spake.h
index 0441efb..01507c9 100644
--- a/crypto/p224_spake.h
+++ b/crypto/p224_spake.h
@@ -12,12 +12,10 @@
 
 namespace crypto {
 
-// P224EncryptedKeyExchange provides a means to authenticate an
-// encrypted transport using a low-entropy, shared secret.
-//
-// You need a value derived from the master secret of the connection in order
-// to bind the authentication to the encrypted channel. It's the |session|
-// argument to the constructor and can be of any length.
+// P224EncryptedKeyExchange implements SPAKE2, a variant of Encrypted
+// Key Exchange. It allows two parties that have a secret common
+// password to establish a common secure key by exchanging messages
+// over unsecure channel without disclosing the password.
 //
 // The password can be low entropy as authenticating with an attacker only
 // gives the attacker a one-shot password oracle. No other information about
@@ -51,13 +49,11 @@ class CRYPTO_EXPORT P224EncryptedKeyExchange {
   };
 
   // peer_type: the type of the local authentication party.
-  // password: a, possibly low-entropy, mutually known password.
-  // session: a value securely derived from the connection's master secret.
-  //     Both parties to the authentication must pass the same value. For the
-  //     case of a TLS connection, see RFC 5705.
+  // password: secret session password. Both parties to the
+  //     authentication must pass the same value. For the case of a
+  //     TLS connection, see RFC 5705.
   P224EncryptedKeyExchange(PeerType peer_type,
-                           const base::StringPiece& password,
-                           const base::StringPiece& session);
+                           const base::StringPiece& password);
 
   // GetMessage returns a byte string which must be passed to the other party
   // in the authentication.
@@ -71,6 +67,10 @@ class CRYPTO_EXPORT P224EncryptedKeyExchange {
   // return a human readable error message.
   const std::string& error() const;
 
+  // The key established as result of the key exchange. Must be called
+  // at then end after ProcessMessage() returns kResultSuccess.
+  const std::string& GetKey();
+
  private:
   // The authentication state machine is very simple and each party proceeds
   // through each of these states, in order.
@@ -106,6 +106,8 @@ class CRYPTO_EXPORT P224EncryptedKeyExchange {
   // expected_authenticator_ is used to store the hash value expected from the
   // other party.
   uint8 expected_authenticator_[kSHA256Length];
+
+  std::string key_;
 };
 
 }  // namespace crypto
diff --git a/crypto/p224_spake_unittest.cc b/crypto/p224_spake_unittest.cc
index e45ec82..bcd448c 100644
--- a/crypto/p224_spake_unittest.cc
+++ b/crypto/p224_spake_unittest.cc
@@ -43,37 +43,24 @@ bool RunExchange(P224EncryptedKeyExchange* client,
 }
 
 static const char kPassword[] = "foo";
-static const char kSession[] = "bar";
 
 TEST(MutualAuth, CorrectAuth) {
   P224EncryptedKeyExchange client(
-      P224EncryptedKeyExchange::kPeerTypeClient,
-      kPassword, kSession);
+      P224EncryptedKeyExchange::kPeerTypeClient, kPassword);
   P224EncryptedKeyExchange server(
-      P224EncryptedKeyExchange::kPeerTypeServer,
-      kPassword, kSession);
+      P224EncryptedKeyExchange::kPeerTypeServer, kPassword);
 
   EXPECT_TRUE(RunExchange(&client, &server));
+  EXPECT_EQ(client.GetKey(), server.GetKey());
 }
 
 TEST(MutualAuth, IncorrectPassword) {
   P224EncryptedKeyExchange client(
       P224EncryptedKeyExchange::kPeerTypeClient,
-      kPassword, kSession);
-  P224EncryptedKeyExchange server(
-      P224EncryptedKeyExchange::kPeerTypeServer,
-      "wrongpassword", kSession);
-
-  EXPECT_FALSE(RunExchange(&client, &server));
-}
-
-TEST(MutualAuth, IncorrectSession) {
-  P224EncryptedKeyExchange client(
-      P224EncryptedKeyExchange::kPeerTypeClient,
-      kPassword, kSession);
+      kPassword);
   P224EncryptedKeyExchange server(
       P224EncryptedKeyExchange::kPeerTypeServer,
-      kPassword, "wrongsession");
+      "wrongpassword");
 
   EXPECT_FALSE(RunExchange(&client, &server));
 }
@@ -83,11 +70,9 @@ TEST(MutualAuth, Fuzz) {
 
   for (unsigned i = 0; i < kIterations; i++) {
     P224EncryptedKeyExchange client(
-        P224EncryptedKeyExchange::kPeerTypeClient,
-        kPassword, kSession);
+        P224EncryptedKeyExchange::kPeerTypeClient, kPassword);
     P224EncryptedKeyExchange server(
-        P224EncryptedKeyExchange::kPeerTypeServer,
-        kPassword, kSession);
+        P224EncryptedKeyExchange::kPeerTypeServer, kPassword);
 
     // We'll only be testing small values of i, but we don't want that to bias
     // the test coverage. So we disperse the value of i by multiplying by the