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// Copyright (c) 2012 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_
#include <base/string_piece.h>
#include <crypto/p224.h>
#include <crypto/sha2.h>
namespace crypto {
// 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
// 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 {
enum Result {
// 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 {
// peer_type: the type of the local authentication party.
// 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);
// 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;
// The key established as result of the key exchange. Must be called
// at then end after ProcessMessage() returns kResultSuccess.
const std::string& GetKey();
// The authentication state machine is very simple and each party proceeds
// through each of these states, in order.
enum State {
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];
std::string key_;
} // namespace crypto
#endif // CRYPTO_P224_SPAKE_H_