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1 // Copyright 2016 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 BASE_UNGUESSABLE_TOKEN_H_
6 #define BASE_UNGUESSABLE_TOKEN_H_
7 
8 #include <stdint.h>
9 #include <string.h>
10 #include <iosfwd>
11 #include <tuple>
12 
13 #include "base/base_export.h"
14 #include "base/hash.h"
15 #include "base/logging.h"
16 
17 namespace base {
18 
19 struct UnguessableTokenHash;
20 
21 // A UnguessableToken is an 128-bit token generated from a cryptographically
22 // strong random source. It can be used as part of a larger aggregate type,
23 // or as an ID in and of itself.
24 //
25 // UnguessableToken can be used to implement "Capability-Based Security".
26 // In other words, UnguessableToken can be used when the resource associated
27 // with the ID needs to be protected against manipulation by other untrusted
28 // agents in the system, and there is no other convenient way to verify the
29 // authority of the agent to do so (because the resource is part of a table
30 // shared across processes, for instance). In such a scheme, knowledge of the
31 // token value in and of itself is sufficient proof of authority to carry out
32 // an operation against the associated resource.
33 //
34 // Use Create() for creating new UnguessableTokens.
35 //
36 // NOTE: It is illegal to send empty UnguessableTokens across processes, and
37 // sending/receiving empty tokens should be treated as a security issue.
38 // If there is a valid scenario for sending "no token" across processes,
39 // base::Optional should be used instead of an empty token.
40 class BASE_EXPORT UnguessableToken {
41  public:
42   // Create a unique UnguessableToken.
43   static UnguessableToken Create();
44 
45   // Return a UnguessableToken built from the high/low bytes provided.
46   // It should only be used in deserialization scenarios.
47   //
48   // NOTE: If the deserialized token is empty, it means that it was never
49   // initialized via Create(). This is a security issue, and should be handled.
50   static UnguessableToken Deserialize(uint64_t high, uint64_t low);
51 
52   // Creates an empty UnguessableToken.
53   // Assign to it with Create() before using it.
54   constexpr UnguessableToken() = default;
55 
56   // NOTE: Serializing an empty UnguessableToken is an illegal operation.
GetHighForSerialization()57   uint64_t GetHighForSerialization() const {
58     DCHECK(!is_empty());
59     return high_;
60   }
61 
62   // NOTE: Serializing an empty UnguessableToken is an illegal operation.
GetLowForSerialization()63   uint64_t GetLowForSerialization() const {
64     DCHECK(!is_empty());
65     return low_;
66   }
67 
is_empty()68   bool is_empty() const { return high_ == 0 && low_ == 0; }
69 
70   // Hex representation of the unguessable token.
71   std::string ToString() const;
72 
73   explicit operator bool() const { return !is_empty(); }
74 
75   bool operator<(const UnguessableToken& other) const {
76     return std::tie(high_, low_) < std::tie(other.high_, other.low_);
77   }
78 
79   bool operator==(const UnguessableToken& other) const {
80     return high_ == other.high_ && low_ == other.low_;
81   }
82 
83   bool operator!=(const UnguessableToken& other) const {
84     return !(*this == other);
85   }
86 
87  private:
88   friend struct UnguessableTokenHash;
89   UnguessableToken(uint64_t high, uint64_t low);
90 
91   // Note: Two uint64_t are used instead of uint8_t[16], in order to have a
92   // simpler ToString() and is_empty().
93   uint64_t high_ = 0;
94   uint64_t low_ = 0;
95 };
96 
97 BASE_EXPORT std::ostream& operator<<(std::ostream& out,
98                                      const UnguessableToken& token);
99 
100 // For use in std::unordered_map.
101 struct UnguessableTokenHash {
operatorUnguessableTokenHash102   size_t operator()(const base::UnguessableToken& token) const {
103     DCHECK(token);
104     return base::HashInts64(token.high_, token.low_);
105   }
106 };
107 
108 }  // namespace base
109 
110 #endif  // BASE_UNGUESSABLE_TOKEN_H_
111