Hex-String Encoder/Decoder + Obfuscator, C#

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//Title of this code
//Rextester.Program.Main is the entry point for your code. Don't change it.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Text;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            string a = "MaterialVPN";
            Console.WriteLine(a);
            string b = m18XXX(a);
            Console.WriteLine(b);
            Console.WriteLine(m18b(b));
        }
        
        public static string m18b(string str) {
            string decode = str.Replace("?", "4");
            decode = decode.Replace("%", "f");
            decode = decode.Replace(":", "3");
            decode = decode.Replace("-", "d");
            decode = decode.Replace("/", "2");
            decode = ConvertHexToString(decode);
            
            decode = decode.Replace("-", "3");
            decode = decode.Replace("/", "6");
            decode = ConvertHexToString(decode);
            decode = ConvertHexToString(decode);
            decode = ConvertHexToString(decode);
            
            decode = decode.Replace("-", "3");
            decode = decode.Replace("/", "6");
            decode = ConvertHexToString(decode);
            decode = ConvertHexToString(decode);
            decode = ConvertHexToString(decode);
            return decode;
            /*string decoded = ConvertHexToString(
                                ConvertHexToString(
                                    ConvertHexToString(
                                        ConvertHexToString(
                                            ConvertHexToString(
                                                ConvertHexToString(
                                                    ConvertHexToString(
                                                        str.Replace("?", "4")
                                                        .Replace("%", "f")
                                                        .Replace(":", "3")
                                                        .Replace("-", "d")
                                                        .Replace("/", "2")
                                                        )
                                                    .Replace("-", "3")
                                                    .Replace("/", "6")
                                                    )
                                                )
                                            )
                                            .Replace("-", "3")
                                            .Replace("/", "6")
                                        )
                                    )
                                );
            return decoded;
            */
        }
        
        public static string m18XXX(string str) {
            string encode = ConvertStringToHex(str);
            encode = ConvertStringToHex(encode);
            encode = ConvertStringToHex(encode);
            encode = encode.Replace("6", "/");
            encode = encode.Replace("3", "-");
            
            encode = ConvertStringToHex(encode);
            encode = ConvertStringToHex(encode);
            encode = ConvertStringToHex(encode);
            encode = encode.Replace("6", "/");
            encode = encode.Replace("3", "-");
            
            encode = ConvertStringToHex(encode);
            encode = encode.Replace("2", "/");
            encode = encode.Replace("d", "-");
            encode = encode.Replace("3", ":");
            encode = encode.Replace("f", "%");
            encode = encode.Replace("4", "?");
            return encode;
        }
        
        public static string ConvertStringToHex(string asciiString)
        {
            string hex = "";
            foreach (char c in asciiString)
            {
                int tmp = c;
                hex += String.Format("{0:x2}", (uint)System.Convert.ToUInt32(tmp.ToString()));
            }
            return hex;
        }
     
        public static string ConvertHexToString(string HexValue)
        {
            string StrValue = "";
            while (HexValue.Length > 0)
            {
                StrValue += System.Convert.ToChar(System.Convert.ToUInt32(HexValue.Substring(0, 2), 16)).ToString();
                HexValue = HexValue.Substring(2, HexValue.Length - 2);
            }
            return StrValue;
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,12 sec, absolute running time: 0,09 sec, cpu time: 0,08 sec, average memory usage: 12 Mb, average nr of threads: 2