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 #ifndef CRYPTO_P224_SPAKE_H_
6 #define CRYPTO_P224_SPAKE_H_
12 #include "base/gtest_prod_util.h"
13 #include "base/strings/string_piece.h"
14 #include "crypto/p224.h"
15 #include "crypto/sha2.h"
19 // P224EncryptedKeyExchange implements SPAKE2, a variant of Encrypted
20 // Key Exchange. It allows two parties that have a secret common
21 // password to establish a common secure key by exchanging messages
22 // over an insecure channel without disclosing the password.
24 // The password can be low entropy as authenticating with an attacker only
25 // gives the attacker a one-shot password oracle. No other information about
26 // the password is leaked. (However, you must be sure to limit the number of
27 // permitted authentication attempts otherwise they get many one-shot oracles.)
29 // The protocol requires several RTTs (actually two, but you shouldn't assume
30 // that.) To use the object, call GetNextMessage() and pass that message to the
31 // peer. Get a message from the peer and feed it into ProcessMessage. Then
32 // examine the return value of ProcessMessage:
33 // kResultPending: Another round is required. Call GetNextMessage and repeat.
34 // kResultFailed: The authentication has failed. You can get a human readable
35 // error message by calling error().
36 // kResultSuccess: The authentication was successful.
38 // In each exchange, each peer always sends a message.
39 class CRYPTO_EXPORT P224EncryptedKeyExchange {
47 // PeerType's values are named client and server due to convention. But
48 // they could be called "A" and "B" as far as the protocol is concerned so
49 // long as the two parties don't both get the same label.
55 // peer_type: the type of the local authentication party.
56 // password: secret session password. Both parties to the
57 // authentication must pass the same value. For the case of a
58 // TLS connection, see RFC 5705.
59 P224EncryptedKeyExchange(PeerType peer_type, base::StringPiece password);
61 // GetNextMessage returns a byte string which must be passed to the other
62 // party in the authentication.
63 const std::string& GetNextMessage();
65 // ProcessMessage processes a message which must have been generated by a
66 // call to GetNextMessage() by the other party.
67 Result ProcessMessage(base::StringPiece message);
69 // In the event that ProcessMessage() returns kResultFailed, error will
70 // return a human readable error message.
71 const std::string& error() const;
73 // The key established as result of the key exchange. Must be called
74 // at then end after ProcessMessage() returns kResultSuccess.
75 const std::string& GetKey() const;
77 // The key established as result of the key exchange. Can be called after
78 // the first ProcessMessage()
79 const std::string& GetUnverifiedKey() const;
82 // The authentication state machine is very simple and each party proceeds
83 // through each of these states, in order.
92 FRIEND_TEST_ALL_PREFIXES(MutualAuth, ExpectedValues);
96 // Sets internal random scalar. Should be used by tests only.
97 void SetXForTesting(const std::string& x);
100 const bool is_server_;
101 // next_message_ contains a value for GetNextMessage() to return.
102 std::string next_message_;
105 // CalculateHash computes the verification hash for the given peer and writes
106 // |kSHA256Length| bytes at |out_digest|.
107 void CalculateHash(PeerType peer_type,
108 const std::string& client_masked_dh,
109 const std::string& server_masked_dh,
110 const std::string& k,
111 uint8_t* out_digest);
113 // x_ is the secret Diffie-Hellman exponent (see paper referenced in .cc
115 uint8_t x_[p224::kScalarBytes];
116 // pw_ is SHA256(P(password), P(session))[:28] where P() prepends a uint32_t,
117 // big-endian length prefix (see paper referenced in .cc file).
118 uint8_t pw_[p224::kScalarBytes];
119 // expected_authenticator_ is used to store the hash value expected from the
121 uint8_t expected_authenticator_[kSHA256Length];
126 } // namespace crypto
128 #endif // CRYPTO_P224_SPAKE_H_