Masuit.Tools/Masuit.Tools.Abstractions/Security/Crc32.cs

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

namespace Masuit.Tools.Security
{
    /// <summary>
    /// 实现一个32位CRC哈希算法(兼容Zip)
    /// </summary>
    /// <remarks>
    /// Crc32仅应用于与旧文件格式和算法向后兼容。 对于新的应用程序，它不够安全。 如果需要多次调用同一数据，请使用HashAlgorithm接口
    /// </remarks>
    public sealed class Crc32 : HashAlgorithm
    {
        public const uint DefaultPolynomial = 0xedb88320u;
        public const uint DefaultSeed = 0xffffffffu;
        private static uint[] _defaultTable;
        private readonly uint _seed;
        private readonly uint[] _table;
        private uint _hash;

        public override int HashSize => 32;

        public Crc32() : this(DefaultPolynomial, DefaultSeed)
        {
        }

        public Crc32(uint polynomial, uint seed)
        {
            if (!BitConverter.IsLittleEndian)
            {
                throw new PlatformNotSupportedException("Big Endian 处理程序不支持");
            }

            _table = InitializeTable(polynomial);
            _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 static uint Compute(byte[] buffer)
        {
            return Compute(DefaultSeed, buffer);
        }

        public static uint Compute(uint seed, byte[] buffer)
        {
            return Compute(DefaultPolynomial, seed, buffer);
        }

        public static uint Compute(uint polynomial, uint seed, byte[] buffer)
        {
            return ~CalculateHash(InitializeTable(polynomial), seed, buffer, 0, buffer.Length);
        }

        private static uint[] InitializeTable(uint polynomial)
        {
            if (polynomial == DefaultPolynomial && _defaultTable != null)
            {
                return _defaultTable;
            }

            var createTable = new uint[256];
            for (uint i = 0; i < 256; i++)
            {
                var entry = i;
                for (var j = 0; j < 8; j++)
                {
                    if ((entry & 1) == 1)
                    {
                        entry = (entry >> 1) ^ polynomial;
                    }
                    else
                    {
                        entry >>= 1;
                    }
                }

                createTable[i] = entry;
            }

            if (polynomial == DefaultPolynomial)
            {
                _defaultTable = createTable;
            }

            return createTable;
        }

        private static uint CalculateHash(uint[] table, uint 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;
        }

        private static byte[] UInt32ToBigEndianBytes(uint uint32)
        {
            var result = BitConverter.GetBytes(uint32);
            if (BitConverter.IsLittleEndian)
            {
                Array.Reverse(result);
            }

            return result;
        }
    }
}