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. 4 5 #ifndef CRYPTO_P224_SPAKE_H_ 6 #define CRYPTO_P224_SPAKE_H_ 7 8 #include <stdint.h> 9 10 #include <string> 11 12 #include "base/gtest_prod_util.h" 13 #include "base/strings/string_piece.h" 14 #include "crypto/p224.h" 15 #include "crypto/sha2.h" 16 17 namespace crypto { 18 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. 23 // 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.) 28 // 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. 37 // 38 // In each exchange, each peer always sends a message. 39 class CRYPTO_EXPORT P224EncryptedKeyExchange { 40 public: 41 enum Result { 42 kResultPending, 43 kResultFailed, 44 kResultSuccess, 45 }; 46 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. 50 enum PeerType { 51 kPeerTypeClient, 52 kPeerTypeServer, 53 }; 54 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); 60 61 // GetNextMessage returns a byte string which must be passed to the other 62 // party in the authentication. 63 const std::string& GetNextMessage(); 64 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); 68 69 // In the event that ProcessMessage() returns kResultFailed, error will 70 // return a human readable error message. 71 const std::string& error() const; 72 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; 76 77 // The key established as result of the key exchange. Can be called after 78 // the first ProcessMessage() 79 const std::string& GetUnverifiedKey() const; 80 81 private: 82 // The authentication state machine is very simple and each party proceeds 83 // through each of these states, in order. 84 enum State { 85 kStateInitial, 86 kStateRecvDH, 87 kStateSendHash, 88 kStateRecvHash, 89 kStateDone, 90 }; 91 92 FRIEND_TEST_ALL_PREFIXES(MutualAuth, ExpectedValues); 93 94 void Init(); 95 96 // Sets internal random scalar. Should be used by tests only. 97 void SetXForTesting(const std::string& x); 98 99 State state_; 100 const bool is_server_; 101 // next_message_ contains a value for GetNextMessage() to return. 102 std::string next_message_; 103 std::string error_; 104 105 // 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); 112 113 // x_ is the secret Diffie-Hellman exponent (see paper referenced in .cc 114 // file). 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 120 // other party. 121 uint8_t expected_authenticator_[kSHA256Length]; 122 123 std::string key_; 124 }; 125 126 } // namespace crypto 127 128 #endif // CRYPTO_P224_SPAKE_H_ 129