THUAI6/CAPI/cpp/grpc/include/absl/random/internal/seed_material.h

// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef ABSL_RANDOM_INTERNAL_SEED_MATERIAL_H_
#define ABSL_RANDOM_INTERNAL_SEED_MATERIAL_H_

#include <cassert>
#include <cstdint>
#include <cstdlib>
#include <string>
#include <vector>

#include "absl/base/attributes.h"
#include "absl/random/internal/fast_uniform_bits.h"
#include "absl/types/optional.h"
#include "absl/types/span.h"

namespace absl
{
    ABSL_NAMESPACE_BEGIN
    namespace random_internal
    {

        // Returns the number of 32-bit blocks needed to contain the given number of
        // bits.
        constexpr size_t SeedBitsToBlocks(size_t seed_size)
        {
            return (seed_size + 31) / 32;
        }

        // Amount of entropy (measured in bits) used to instantiate a Seed Sequence,
        // with which to create a URBG.
        constexpr size_t kEntropyBitsNeeded = 256;

        // Amount of entropy (measured in 32-bit blocks) used to instantiate a Seed
        // Sequence, with which to create a URBG.
        constexpr size_t kEntropyBlocksNeeded =
            random_internal::SeedBitsToBlocks(kEntropyBitsNeeded);

        static_assert(kEntropyBlocksNeeded > 0, "Entropy used to seed URBGs must be nonzero.");

        // Attempts to fill a span of uint32_t-values using an OS-provided source of
        // true entropy (eg. /dev/urandom) into an array of uint32_t blocks of data. The
        // resulting array may be used to initialize an instance of a class conforming
        // to the C++ Standard "Seed Sequence" concept [rand.req.seedseq].
        //
        // If values.data() == nullptr, the behavior is undefined.
        ABSL_MUST_USE_RESULT
        bool ReadSeedMaterialFromOSEntropy(absl::Span<uint32_t> values);

        // Attempts to fill a span of uint32_t-values using variates generated by an
        // existing instance of a class conforming to the C++ Standard "Uniform Random
        // Bit Generator" concept [rand.req.urng]. The resulting data may be used to
        // initialize an instance of a class conforming to the C++ Standard
        // "Seed Sequence" concept [rand.req.seedseq].
        //
        // If urbg == nullptr or values.data() == nullptr, the behavior is undefined.
        template<typename URBG>
        ABSL_MUST_USE_RESULT bool ReadSeedMaterialFromURBG(
            URBG* urbg, absl::Span<uint32_t> values
        )
        {
            random_internal::FastUniformBits<uint32_t> distr;

            assert(urbg != nullptr && values.data() != nullptr);
            if (urbg == nullptr || values.data() == nullptr)
            {
                return false;
            }

            for (uint32_t& seed_value : values)
            {
                seed_value = distr(*urbg);
            }
            return true;
        }

        // Mixes given sequence of values with into given sequence of seed material.
        // Time complexity of this function is O(sequence.size() *
        // seed_material.size()).
        //
        // Algorithm is based on code available at
        // https://gist.github.com/imneme/540829265469e673d045
        // by Melissa O'Neill.
        void MixIntoSeedMaterial(absl::Span<const uint32_t> sequence, absl::Span<uint32_t> seed_material);

        // Returns salt value.
        //
        // Salt is obtained only once and stored in static variable.
        //
        // May return empty value if optaining the salt was not possible.
        absl::optional<uint32_t> GetSaltMaterial();

    }  // namespace random_internal
    ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_RANDOM_INTERNAL_SEED_MATERIAL_H_