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6eb5ba5691
Split the repeated `SipHash` v[0..3] initialization into a small `SipHashState` helper that is used by both `CSipHasher` and `PresaltedSipHasher`. Added explanatory comments to clarify behavior, documenting the equivalence of `PresaltedSipHasher` `operator()` overloads to `CSipHasher` usage. Co-authored-by: Ryan Ofsky <ryan@ofsky.org>
73 lines
2.4 KiB
C++
73 lines
2.4 KiB
C++
// Copyright (c) 2016-present The Bitcoin Core developers
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// Distributed under the MIT software license, see the accompanying
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// file COPYING or http://www.opensource.org/licenses/mit-license.php.
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#ifndef BITCOIN_CRYPTO_SIPHASH_H
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#define BITCOIN_CRYPTO_SIPHASH_H
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#include <array>
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#include <cstdint>
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#include <span>
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class uint256;
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/** Shared SipHash internal state v[0..3], initialized from (k0, k1). */
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class SipHashState
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{
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static constexpr uint64_t C0{0x736f6d6570736575ULL}, C1{0x646f72616e646f6dULL}, C2{0x6c7967656e657261ULL}, C3{0x7465646279746573ULL};
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public:
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explicit SipHashState(uint64_t k0, uint64_t k1) noexcept : v{C0 ^ k0, C1 ^ k1, C2 ^ k0, C3 ^ k1} {}
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std::array<uint64_t, 4> v{};
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};
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/** General SipHash-2-4 implementation. */
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class CSipHasher
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{
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SipHashState m_state;
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uint64_t m_tmp{0};
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uint8_t m_count{0}; //!< Only the low 8 bits of the input size matter.
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public:
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/** Construct a SipHash calculator initialized with 128-bit key (k0, k1). */
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CSipHasher(uint64_t k0, uint64_t k1);
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/** Hash a 64-bit integer worth of data.
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* It is treated as if this was the little-endian interpretation of 8 bytes.
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* This function can only be used when a multiple of 8 bytes have been written so far.
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*/
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CSipHasher& Write(uint64_t data);
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/** Hash arbitrary bytes. */
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CSipHasher& Write(std::span<const unsigned char> data);
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/** Compute the 64-bit SipHash-2-4 of the data written so far. The object remains untouched. */
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uint64_t Finalize() const;
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};
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/**
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* Optimized SipHash-2-4 implementation for uint256.
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*
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* This class caches the initial SipHash v[0..3] state derived from (k0, k1)
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* and implements a specialized hashing path for uint256 values, with or
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* without an extra 32-bit word. The internal state is immutable, so
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* PresaltedSipHasher instances can be reused for multiple hashes with the
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* same key.
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*/
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class PresaltedSipHasher
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{
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const SipHashState m_state;
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public:
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explicit PresaltedSipHasher(uint64_t k0, uint64_t k1) noexcept : m_state{k0, k1} {}
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/** Equivalent to CSipHasher(k0, k1).Write(val).Finalize(). */
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uint64_t operator()(const uint256& val) const noexcept;
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/**
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* Equivalent to CSipHasher(k0, k1).Write(val).Write(extra).Finalize(),
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* with `extra` encoded as 4 little-endian bytes.
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*/
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uint64_t operator()(const uint256& val, uint32_t extra) const noexcept;
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};
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#endif // BITCOIN_CRYPTO_SIPHASH_H
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