自定义Base64转换工具类

    /// <summary>自定义包含指定字符的base64工具</summary>
    public static class Base64Helper
    {
        static readonly string base64Table = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_-";
        static readonly int[] base64Index = new int[]
        {
            -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
            -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
            -1,63,-1,-1,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-1,-1,-1,-1,0,1,
            2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,
            -1,-1,-1,62,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,
            43,44,45,46,47,48,49,50,51,-1,-1,-1,-1,-1,-1
        };
        public static byte[] FromBase64String(string inData)
        {
            int inDataLength = inData.Length;
            int lengthmod4 = inDataLength % 4;
            int calcLength = (inDataLength - lengthmod4);
            byte[] outData = new byte[inDataLength / 4 * 3 + 3];
            int j = 0;
            int i;
            int num1, num2, num3, num4;

            for (i = 0; i < calcLength; i += 4, j += 3)
            {
                num1 = base64Index[inData[i]];
                num2 = base64Index[inData[i + 1]];
                num3 = base64Index[inData[i + 2]];
                num4 = base64Index[inData[i + 3]];