大数字字符串转换为短字符串(7位转4位)

/// <summary>
    /// 将数值字符串加密为短字符串
    /// </summary>
    public static class NumberToShortString
    {
        /// <summary>
        /// 共62个字符
        /// </summary>
        private static readonly string Keys = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

        /// <summary>
        /// 将数值字符串加密为短字符串(7位数字转4位字符)
        /// </summary>
        /// <param name="numberStr"></param>
        /// <returns></returns>
        static string IntToEnode62(string numberStr)
        {
            var iNumber = long.Parse(numberStr);
            var result = "";
            do
            {
                var m = iNumber % 62;
                result = Keys[(int)m] + result;
                iNumber = (iNumber - m) / 62;
            }
            while (iNumber > 0);

            return result.PadLeft(4, '0');
        }

        /// <summary>
        /// 将加密后短字符串转换为原数值(4位字符转7位数字)
        /// </summary>
        /// <param name="shortStr"></param>
        /// <returns></returns>
        static long Encode62ToInt(string shortStr)
        {
            long num = 0;
            for (var i = 0; i < shortStr.Length; i++)
            {
                var n = shortStr.Length - i - 1;
                num += Convert.ToInt64(Keys.IndexOf(shortStr[i]) * Math.Pow(62, n));
            }

            var temp = num.ToString().PadLeft(7, '0');
            long.TryParse(temp, out num);

            return num;
        }

        /// <summary>
        /// 将加密后短字符串转换为原数值字符串
        /// </summary>
        /// <param name="shortStr">密后短字符串</param>
        /// <returns>原数值字符串</returns>
        public static string StrToNumber(string shortStr)
        {
            var id = "";
            for (var i = shortStr.Length - 4; i > -4; i = i - 4) //从最后往前以4字节为一组读取字符  
            {
                var offset = i < 0 ? 0 : i;
                var len = i < 0 ? shortStr.Length % 4 : 4;
                var str = Encode62ToInt(shortStr.Substring(offset, len)).ToString();
                if (offset > 0)
                {
                    //若不是第一组，则不足7位补0  
                    str = str.PadLeft(7, '0');
                }

                id = str + id;
            }

            return id;
        }

        /// <summary>
        /// 将数值字符串加密为短字符串
        /// </summary>
        /// <param name="numberStr">原数值字符串,数值范围在[-9223372036854775808,9223372036854775807]</param>
        /// <returns>数字或字母加密短字符结果</returns>
        public static string NumberToStr(string numberStr)
        {
            long iNumber = 0l;
            if (!long.TryParse(numberStr, out iNumber))
            {
                throw new Exception("数值字符串不在有效范围内!数值范围在[-9223372036854775808,9223372036854775807]");
            }

            var result = "";
            for (var i = numberStr.Length - 7; i > -7; i -= 7)
            {
                var offset1 = (i < 0) ? 0 : i;
                var offset2 = i + 7;
                var num = IntToEnode62(numberStr.Substring(offset1, offset2 - offset1));
                result = num + result;
            }

            return result;
        }
    }