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44725c2 Jun 22, 2018
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// Copyright (c) Damien Guard. All rights reserved.
// 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 http://www.apache.org/licenses/LICENSE-2.0
using System;
using System.Collections.Generic;
using System.Security.Cryptography;
namespace DamienG.Security.Cryptography
{
/// <summary>
/// Implements a 32-bit CRC hash algorithm compatible with Zip etc.
/// </summary>
/// <remarks>
/// Crc32 should only be used for backward compatibility with older file formats
/// and algorithms. It is not secure enough for new applications.
/// If you need to call multiple times for the same data either use the HashAlgorithm
/// interface or remember that the result of one Compute call needs to be ~ (XOR) before
/// being passed in as the seed for the next Compute call.
/// </remarks>
public sealed class Crc32 : HashAlgorithm
{
public const UInt32 DefaultPolynomial = 0xedb88320u;
public const UInt32 DefaultSeed = 0xffffffffu;
static UInt32[] defaultTable;
readonly UInt32 seed;
readonly UInt32[] table;
UInt32 hash;
public Crc32()
: this(DefaultPolynomial, DefaultSeed)
{
}
public Crc32(UInt32 polynomial, UInt32 seed)
{
if (!BitConverter.IsLittleEndian)
throw new PlatformNotSupportedException("Not supported on Big Endian processors");
table = InitializeTable(polynomial);
this.seed = hash = seed;
}
public override void Initialize()
{
hash = seed;
}
protected override void HashCore(byte[] array, int ibStart, int cbSize)
{
hash = CalculateHash(table, hash, array, ibStart, cbSize);
}
protected override byte[] HashFinal()
{
var hashBuffer = UInt32ToBigEndianBytes(~hash);
HashValue = hashBuffer;
return hashBuffer;
}
public override int HashSize { get { return 32; } }
public static UInt32 Compute(byte[] buffer)
{
return Compute(DefaultSeed, buffer);
}
public static UInt32 Compute(UInt32 seed, byte[] buffer)
{
return Compute(DefaultPolynomial, seed, buffer);
}
public static UInt32 Compute(UInt32 polynomial, UInt32 seed, byte[] buffer)
{
return ~CalculateHash(InitializeTable(polynomial), seed, buffer, 0, buffer.Length);
}
static UInt32[] InitializeTable(UInt32 polynomial)
{
if (polynomial == DefaultPolynomial && defaultTable != null)
return defaultTable;
var createTable = new UInt32[256];
for (var i = 0; i < 256; i++)
{
var entry = (UInt32)i;
for (var j = 0; j < 8; j++)
if ((entry & 1) == 1)
entry = (entry >> 1) ^ polynomial;
else
entry = entry >> 1;
createTable[i] = entry;
}
if (polynomial == DefaultPolynomial)
defaultTable = createTable;
return createTable;
}
static UInt32 CalculateHash(UInt32[] table, UInt32 seed, IList<byte> buffer, int start, int size)
{
var hash = seed;
for (var i = start; i < start + size; i++)
hash = (hash >> 8) ^ table[buffer[i] ^ hash & 0xff];
return hash;
}
static byte[] UInt32ToBigEndianBytes(UInt32 uint32)
{
var result = BitConverter.GetBytes(uint32);
if (BitConverter.IsLittleEndian)
Array.Reverse(result);
return result;
}
}
}