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119 lines
3.9 KiB
119 lines
3.9 KiB
// Copyright (c) Damien Guard. All rights reserved.
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// Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
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// Originally published at http://damieng.com/blog/2006/08/08/calculating_crc32_in_c_and_net
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using System;
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using System.Collections.Generic;
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using System.Security.Cryptography;
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namespace DamienG.Security.Cryptography
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{
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/// <summary>
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/// Implements a 32-bit CRC hash algorithm compatible with Zip etc.
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/// </summary>
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/// <remarks>
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/// Crc32 should only be used for backward compatibility with older file formats
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/// and algorithms. It is not secure enough for new applications.
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/// If you need to call multiple times for the same data either use the HashAlgorithm
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/// interface or remember that the result of one Compute call needs to be ~ (XOR) before
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/// being passed in as the seed for the next Compute call.
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/// </remarks>
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public sealed class Crc32 : HashAlgorithm
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{
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public const UInt32 DefaultPolynomial = 0xedb88320u;
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public const UInt32 DefaultSeed = 0xffffffffu;
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static UInt32[] defaultTable;
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readonly UInt32 seed;
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readonly UInt32[] table;
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UInt32 hash;
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public Crc32()
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: this(DefaultPolynomial, DefaultSeed)
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{
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}
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public Crc32(UInt32 polynomial, UInt32 seed)
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{
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table = InitializeTable(polynomial);
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this.seed = hash = seed;
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}
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public override void Initialize()
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{
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hash = seed;
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}
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protected override void HashCore(byte[] array, int ibStart, int cbSize)
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{
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hash = CalculateHash(table, hash, array, ibStart, cbSize);
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}
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protected override byte[] HashFinal()
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{
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var hashBuffer = UInt32ToBigEndianBytes(~hash);
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HashValue = hashBuffer;
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return hashBuffer;
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}
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public override int HashSize { get { return 32; } }
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public static UInt32 Compute(byte[] buffer)
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{
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return Compute(DefaultSeed, buffer);
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}
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public static UInt32 Compute(UInt32 seed, byte[] buffer)
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{
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return Compute(DefaultPolynomial, seed, buffer);
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}
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public static UInt32 Compute(UInt32 polynomial, UInt32 seed, byte[] buffer)
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{
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return ~CalculateHash(InitializeTable(polynomial), seed, buffer, 0, buffer.Length);
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}
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static UInt32[] InitializeTable(UInt32 polynomial)
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{
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if (polynomial == DefaultPolynomial && defaultTable != null)
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return defaultTable;
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var createTable = new UInt32[256];
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for (var i = 0; i < 256; i++)
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{
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var entry = (UInt32)i;
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for (var j = 0; j < 8; j++)
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if ((entry & 1) == 1)
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entry = (entry >> 1) ^ polynomial;
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else
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entry = entry >> 1;
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createTable[i] = entry;
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}
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if (polynomial == DefaultPolynomial)
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defaultTable = createTable;
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return createTable;
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}
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static UInt32 CalculateHash(UInt32[] table, UInt32 seed, IList<byte> buffer, int start, int size)
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{
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var crc = seed;
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for (var i = start; i < size - start; i++)
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crc = (crc >> 8) ^ table[buffer[i] ^ crc & 0xff];
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return crc;
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}
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static byte[] UInt32ToBigEndianBytes(UInt32 uint32)
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{
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var result = BitConverter.GetBytes(uint32);
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if (BitConverter.IsLittleEndian)
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Array.Reverse(result);
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return result;
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}
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}
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} |