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 <crypto/p224.h> 9 #include <crypto/sha2.h> 10 #include <stdint.h> 11 12 #include "base/gtest_prod_util.h" 13 #include "base/strings/string_piece.h" 14 15 namespace crypto { 16 17 // P224EncryptedKeyExchange implements SPAKE2, a variant of Encrypted 18 // Key Exchange. It allows two parties that have a secret common 19 // password to establish a common secure key by exchanging messages 20 // over an insecure channel without disclosing the password. 21 // 22 // The password can be low entropy as authenticating with an attacker only 23 // gives the attacker a one-shot password oracle. No other information about 24 // the password is leaked. (However, you must be sure to limit the number of 25 // permitted authentication attempts otherwise they get many one-shot oracles.) 26 // 27 // The protocol requires several RTTs (actually two, but you shouldn't assume 28 // that.) To use the object, call GetNextMessage() and pass that message to the 29 // peer. Get a message from the peer and feed it into ProcessMessage. Then 30 // examine the return value of ProcessMessage: 31 // kResultPending: Another round is required. Call GetNextMessage and repeat. 32 // kResultFailed: The authentication has failed. You can get a human readable 33 // error message by calling error(). 34 // kResultSuccess: The authentication was successful. 35 // 36 // In each exchange, each peer always sends a message. 37 class CRYPTO_EXPORT P224EncryptedKeyExchange { 38 public: 39 enum Result { 40 kResultPending, 41 kResultFailed, 42 kResultSuccess, 43 }; 44 45 // PeerType's values are named client and server due to convention. But 46 // they could be called "A" and "B" as far as the protocol is concerned so 47 // long as the two parties don't both get the same label. 48 enum PeerType { 49 kPeerTypeClient, 50 kPeerTypeServer, 51 }; 52 53 // peer_type: the type of the local authentication party. 54 // password: secret session password. Both parties to the 55 // authentication must pass the same value. For the case of a 56 // TLS connection, see RFC 5705. 57 P224EncryptedKeyExchange(PeerType peer_type, 58 const base::StringPiece& password); 59 60 // GetNextMessage returns a byte string which must be passed to the other 61 // party in the authentication. 62 const std::string& GetNextMessage(); 63 64 // ProcessMessage processes a message which must have been generated by a 65 // call to GetNextMessage() by the other party. 66 Result ProcessMessage(const base::StringPiece& message); 67 68 // In the event that ProcessMessage() returns kResultFailed, error will 69 // return a human readable error message. 70 const std::string& error() const; 71 72 // The key established as result of the key exchange. Must be called 73 // at then end after ProcessMessage() returns kResultSuccess. 74 const std::string& GetKey() const; 75 76 // The key established as result of the key exchange. Can be called after 77 // the first ProcessMessage() 78 const std::string& GetUnverifiedKey() const; 79 80 private: 81 // The authentication state machine is very simple and each party proceeds 82 // through each of these states, in order. 83 enum State { 84 kStateInitial, 85 kStateRecvDH, 86 kStateSendHash, 87 kStateRecvHash, 88 kStateDone, 89 }; 90 91 FRIEND_TEST_ALL_PREFIXES(MutualAuth, ExpectedValues); 92 93 void Init(); 94 95 // Sets internal random scalar. Should be used by tests only. 96 void SetXForTesting(const std::string& x); 97 98 State state_; 99 const bool is_server_; 100 // next_message_ contains a value for GetNextMessage() to return. 101 std::string next_message_; 102 std::string error_; 103 104 // CalculateHash computes the verification hash for the given peer and writes 105 // |kSHA256Length| bytes at |out_digest|. 106 void CalculateHash(PeerType peer_type, 107 const std::string& client_masked_dh, 108 const std::string& server_masked_dh, 109 const std::string& k, 110 uint8_t* out_digest); 111 112 // x_ is the secret Diffie-Hellman exponent (see paper referenced in .cc 113 // file). 114 uint8_t x_[p224::kScalarBytes]; 115 // pw_ is SHA256(P(password), P(session))[:28] where P() prepends a uint32_t, 116 // big-endian length prefix (see paper referenced in .cc file). 117 uint8_t pw_[p224::kScalarBytes]; 118 // expected_authenticator_ is used to store the hash value expected from the 119 // other party. 120 uint8_t expected_authenticator_[kSHA256Length]; 121 122 std::string key_; 123 }; 124 125 } // namespace crypto 126 127 #endif // CRYPTO_P224_SPAKE_H_ 128