net: add low-entropy, shared secret authentication protocol.

crypto/p224_spake.h

+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CRYPTO_P224_SPAKE_H_
+#define CRYPTO_P224_SPAKE_H_
+#pragma once
+
+#include <base/string_piece.h>
+#include <crypto/p224.h>
+#include <crypto/sha2.h>
+
+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.
+//
+// The password can be low entropy as authenticating with an attacker only
+// gives the attacker a one-shot password oracle. No other information about
+// the password is leaked. (However, you must be sure to limit the number of
+// permitted authentication attempts otherwise they get many one-shot oracles.)
+//
+// The protocol requires several RTTs (actually two, but you shouldn't assume
+// that.) To use the object, call GetMessage() and pass that message to the
+// peer. Get a message from the peer and feed it into ProcessMessage. Then
+// examine the return value of ProcessMessage:
+//   kResultPending: Another round is required. Call GetMessage and repeat.
+//   kResultFailed: The authentication has failed. You can get a human readable
+//       error message by calling error().
+//   kResultSuccess: The authentication was successful.
+//
+// In each exchange, each peer always sends a message.
+class CRYPTO_EXPORT P224EncryptedKeyExchange {
+ public:
+  enum Result {
+    kResultPending,
+    kResultFailed,
+    kResultSuccess,
+  };
+
+  // PeerType's values are named client and server due to convention. But
+  // they could be called "A" and "B" as far as the protocol is concerned so
+  // long as the two parties don't both get the same label.
+  enum PeerType {
+    kPeerTypeClient,
+    kPeerTypeServer,
+  };
+
+  // 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.
+  P224EncryptedKeyExchange(PeerType peer_type,
+                           const base::StringPiece& password,
+                           const base::StringPiece& session);
+
+  // GetMessage returns a byte string which must be passed to the other party
+  // in the authentication.
+  const std::string& GetMessage();
+
+  // ProcessMessage processes a message which must have been generated by a
+  // call to GetMessage() by the other party.
+  Result ProcessMessage(const base::StringPiece& message);
+
+  // In the event that ProcessMessage() returns kResultFailed, error will
+  // return a human readable error message.
+  const std::string& error() const;
+
+ private:
+  // The authentication state machine is very simple and each party proceeds
+  // through each of these states, in order.
+  enum State {
+    kStateInitial,
+    kStateRecvDH,
+    kStateSendHash,
+    kStateRecvHash,
+    kStateDone,
+  };
+
+  State state_;
+  const bool is_server_;
+  // next_message_ contains a value for GetMessage() to return.
+  std::string next_message_;
+  std::string error_;
+
+  // CalculateHash computes the verification hash for the given peer and writes
+  // |kSHA256Length| bytes at |out_digest|.
+  void CalculateHash(
+      PeerType peer_type,
+      const std::string& client_masked_dh,
+      const std::string& server_masked_dh,
+      const std::string& k,
+      uint8* out_digest);
+
+  // x_ is the secret Diffie-Hellman exponent (see paper referenced in .cc
+  // file).
+  uint8 x_[p224::kScalarBytes];
+  // pw_ is SHA256(P(password), P(session))[:28] where P() prepends a uint32,
+  // big-endian length prefix (see paper refereneced in .cc file).
+  uint8 pw_[p224::kScalarBytes];
+  // expected_authenticator_ is used to store the hash value expected from the
+  // other party.
+  uint8 expected_authenticator_[kSHA256Length];
+};
+
+}  // namespace crypto
+
+#endif  // CRYPTO_P224_SPAKE_H_