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// 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_RANDEN_TRAITS_H_
#define ABSL_RANDOM_INTERNAL_RANDEN_TRAITS_H_
// HERMETIC NOTE: The randen_hwaes target must not introduce duplicate
// symbols from arbitrary system and other headers, since it may be built
// with different flags from other targets, using different levels of
// optimization, potentially introducing ODR violations.
#include <cstddef>
#include "absl/base/config.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace random_internal {
// RANDen = RANDom generator or beetroots in Swiss German.
// 'Strong' (well-distributed, unpredictable, backtracking-resistant) random
// generator, faster in some benchmarks than std::mt19937_64 and pcg64_c32.
//
// High-level summary:
// 1) Reverie (see "A Robust and Sponge-Like PRNG with Improved Efficiency") is
// a sponge-like random generator that requires a cryptographic permutation.
// It improves upon "Provably Robust Sponge-Based PRNGs and KDFs" by
// achieving backtracking resistance with only one Permute() per buffer.
//
// 2) "Simpira v2: A Family of Efficient Permutations Using the AES Round
// Function" constructs up to 1024-bit permutations using an improved
// Generalized Feistel network with 2-round AES-128 functions. This Feistel
// block shuffle achieves diffusion faster and is less vulnerable to
// sliced-biclique attacks than the Type-2 cyclic shuffle.
//
// 3) "Improving the Generalized Feistel" and "New criterion for diffusion
// property" extends the same kind of improved Feistel block shuffle to 16
// branches, which enables a 2048-bit permutation.
//
// Combine these three ideas and also change Simpira's subround keys from
// structured/low-entropy counters to digits of Pi (or other random source).
// RandenTraits contains the basic algorithm traits, such as the size of the
// state, seed, sponge, etc.
struct RandenTraits {
// Size of the entire sponge / state for the randen PRNG.
static constexpr size_t kStateBytes = 256; // 2048-bit
// Size of the 'inner' (inaccessible) part of the sponge. Larger values would
// require more frequent calls to RandenGenerate.
static constexpr size_t kCapacityBytes = 16; // 128-bit
// Size of the default seed consumed by the sponge.
static constexpr size_t kSeedBytes = kStateBytes - kCapacityBytes;
// Assuming 128-bit blocks, the number of blocks in the state.
// Largest size for which security proofs are known.
static constexpr size_t kFeistelBlocks = 16;
// Ensures SPRP security and two full subblock diffusions.
// Must be > 4 * log2(kFeistelBlocks).
static constexpr size_t kFeistelRounds = 16 + 1;
// Size of the key. A 128-bit key block is used for every-other
// feistel block (Type-2 generalized Feistel network) in each round.
static constexpr size_t kKeyBytes = 16 * kFeistelRounds * kFeistelBlocks / 2;
};
// Randen key arrays. In randen_round_keys.cc
extern const unsigned char kRandenRoundKeys[RandenTraits::kKeyBytes];
extern const unsigned char kRandenRoundKeysBE[RandenTraits::kKeyBytes];
} // namespace random_internal
ABSL_NAMESPACE_END
} // namespace absl
#endif // ABSL_RANDOM_INTERNAL_RANDEN_TRAITS_H_
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