System.Security.Cryptography.Crc32.cs

using System;
using System.Collections.Generic;
using System.Security.Cryptography;

namespace System.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;
                }
        }
}