1 // Copyright 2017 The Abseil Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14
15 #ifndef ABSL_RANDOM_INTERNAL_SEED_MATERIAL_H_
16 #define ABSL_RANDOM_INTERNAL_SEED_MATERIAL_H_
17
18 #include <cassert>
19 #include <cstdint>
20 #include <cstdlib>
21 #include <string>
22 #include <vector>
23
24 #include "absl/base/attributes.h"
25 #include "absl/random/internal/fast_uniform_bits.h"
26 #include "absl/types/optional.h"
27 #include "absl/types/span.h"
28
29 namespace absl {
30 ABSL_NAMESPACE_BEGIN
31 namespace random_internal {
32
33 // Returns the number of 32-bit blocks needed to contain the given number of
34 // bits.
SeedBitsToBlocks(size_t seed_size)35 constexpr size_t SeedBitsToBlocks(size_t seed_size) {
36 return (seed_size + 31) / 32;
37 }
38
39 // Amount of entropy (measured in bits) used to instantiate a Seed Sequence,
40 // with which to create a URBG.
41 constexpr size_t kEntropyBitsNeeded = 256;
42
43 // Amount of entropy (measured in 32-bit blocks) used to instantiate a Seed
44 // Sequence, with which to create a URBG.
45 constexpr size_t kEntropyBlocksNeeded =
46 random_internal::SeedBitsToBlocks(kEntropyBitsNeeded);
47
48 static_assert(kEntropyBlocksNeeded > 0,
49 "Entropy used to seed URBGs must be nonzero.");
50
51 // Attempts to fill a span of uint32_t-values using an OS-provided source of
52 // true entropy (eg. /dev/urandom) into an array of uint32_t blocks of data. The
53 // resulting array may be used to initialize an instance of a class conforming
54 // to the C++ Standard "Seed Sequence" concept [rand.req.seedseq].
55 //
56 // If values.data() == nullptr, the behavior is undefined.
57 ABSL_MUST_USE_RESULT
58 bool ReadSeedMaterialFromOSEntropy(absl::Span<uint32_t> values);
59
60 // Attempts to fill a span of uint32_t-values using variates generated by an
61 // existing instance of a class conforming to the C++ Standard "Uniform Random
62 // Bit Generator" concept [rand.req.urng]. The resulting data may be used to
63 // initialize an instance of a class conforming to the C++ Standard
64 // "Seed Sequence" concept [rand.req.seedseq].
65 //
66 // If urbg == nullptr or values.data() == nullptr, the behavior is undefined.
67 template <typename URBG>
ReadSeedMaterialFromURBG(URBG * urbg,absl::Span<uint32_t> values)68 ABSL_MUST_USE_RESULT bool ReadSeedMaterialFromURBG(
69 URBG* urbg, absl::Span<uint32_t> values) {
70 random_internal::FastUniformBits<uint32_t> distr;
71
72 assert(urbg != nullptr && values.data() != nullptr);
73 if (urbg == nullptr || values.data() == nullptr) {
74 return false;
75 }
76
77 for (uint32_t& seed_value : values) {
78 seed_value = distr(*urbg);
79 }
80 return true;
81 }
82
83 // Mixes given sequence of values with into given sequence of seed material.
84 // Time complexity of this function is O(sequence.size() *
85 // seed_material.size()).
86 //
87 // Algorithm is based on code available at
88 // https://gist.github.com/imneme/540829265469e673d045
89 // by Melissa O'Neill.
90 void MixIntoSeedMaterial(absl::Span<const uint32_t> sequence,
91 absl::Span<uint32_t> seed_material);
92
93 // Returns salt value.
94 //
95 // Salt is obtained only once and stored in static variable.
96 //
97 // May return empty value if optaining the salt was not possible.
98 absl::optional<uint32_t> GetSaltMaterial();
99
100 } // namespace random_internal
101 ABSL_NAMESPACE_END
102 } // namespace absl
103
104 #endif // ABSL_RANDOM_INTERNAL_SEED_MATERIAL_H_
105