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LibCrypto: Implement a generic 16-bit CRC
This is mostly a 16-bit version of the 8-bit CRC, using the same general byte-LUT algorithm.
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@ -129,10 +129,19 @@ struct Traits<BigEndian<T>> : public GenericTraits<BigEndian<T>> {
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static constexpr bool is_trivially_serializable() { return Traits<T>::is_trivially_serializable(); }
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};
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constexpr u16 bitswap(u16 v)
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{
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v = ((v >> 1) & 0x5555) | ((v & 0x5555) << 1); // even & odd bits
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v = ((v >> 2) & 0x3333) | ((v & 0x3333) << 2); // pairs
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v = ((v >> 4) & 0x0F0F) | ((v & 0x0F0F) << 4); // nibbles
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return ((v >> 8) & 0x00FF) | ((v & 0x00FF) << 8); // bytes
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}
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}
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#if USING_AK_GLOBALLY
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using AK::BigEndian;
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using AK::bitswap;
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using AK::LittleEndian;
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using AK::NetworkOrdered;
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#endif
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82
Userland/Libraries/LibCrypto/Checksum/CRC16.h
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82
Userland/Libraries/LibCrypto/Checksum/CRC16.h
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@ -0,0 +1,82 @@
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/*
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* Copyright (c) 2023, kleines Filmröllchen <filmroellchen@serenityos.org>.
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#pragma once
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#include <AK/Endian.h>
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#include <AK/Format.h>
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#include <AK/Types.h>
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#include <LibCrypto/Checksum/ChecksumFunction.h>
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namespace Crypto::Checksum {
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// A generic 16-bit Cyclic Redundancy Check.
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// Just like CRC32, this class receives its polynomial little-endian.
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// For example, the polynomial x¹⁶ + x¹² + x⁵ + 1 is represented as 0x8408.
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template<u16 polynomial>
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class CRC16 : public ChecksumFunction<u16> {
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public:
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static constexpr u16 be_polynomial = bitswap(polynomial);
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// This is a big endian table, while CRC-32 uses a little endian table.
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static constexpr auto generate_table()
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{
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Array<u16, 256> data {};
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data[0] = 0;
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u16 value = 0x8000;
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auto i = 1u;
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do {
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if ((value & 0x8000) != 0) {
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value = be_polynomial ^ (value << 1);
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} else {
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value = value << 1;
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}
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for (auto j = 0u; j < i; ++j) {
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data[i + j] = value ^ data[j];
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}
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i <<= 1;
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} while (i < 256);
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return data;
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}
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static constexpr auto table = generate_table();
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virtual ~CRC16() = default;
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CRC16() = default;
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CRC16(ReadonlyBytes data)
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{
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update(data);
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}
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CRC16(u16 initial_state, ReadonlyBytes data)
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: m_state(initial_state)
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{
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update(data);
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}
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// FIXME: This implementation is naive and slow.
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// Figure out how to adopt the slicing-by-8 algorithm (see CRC32) for 16-bit polynomials.
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virtual void update(ReadonlyBytes data) override
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{
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for (size_t i = 0; i < data.size(); i++) {
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size_t table_index = ((m_state >> 8) ^ data.at(i)) & 0xFF;
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m_state = (table[table_index] ^ (static_cast<u32>(m_state) << 8)) & 0xFFFF;
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}
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}
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virtual u16 digest() override
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{
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return m_state;
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}
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private:
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u16 m_state { 0 };
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};
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}
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