Two way encryption from int to short string

I am creating a tiny web application where our clients can update some information about their company. Currently, clients do not have a username / password and we do not want to check their registration, so we want to give them an automatically generated password / key to enter the website.

Our plan is to encrypt their customerId and provide it to the client so that he can enter this key into the webapp so that we can decrypt the key into his id.

There are about 10k customers and they don't have all the emails, so some will receive an email with a URL and a code. This means that the client must enter a code, so the code can be no more than 8 characters (preferably 6).

Here's an empty template:

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

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            int passwordLength = 6;
            int customerId = 12345;
            string encrypted = Crypter.Encrypt(customerId, "secretKey", passwordLength);
            if (customerId == Crypter.Decrypt(encrypted, "secretKey"))
            {
                Console.WriteLine("It worked! Well done!");
            }
        }

    }

    public static class Crypter
    {
        public static string Encrypt(int input, string key, int passwordLength)
        {
            string encryptedString = "";
            //do encrypt stuffz here

            return encryptedString;
        }
        public static int Decrypt(string encryoted, string key)
        {
            int decrypted = 0;
            //do decrypt stuffz here

            return decrypted;
        }
    }
}

      

        
        
        
      

    

=> Can anyone link me with more information on how to do this?

I'm not looking for "plz send me tez codez", but if someone else has done something similar already, feel free to share.

Thanks in advance for any information.