Ex, C# - rextester

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using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;
using System.Security;
using System.Security.Cryptography;
using System.Text;
using System.Threading.Tasks;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var test = EncryptPassword("TestUser", new SecureString(), "uu2cO7b7drhxHKItfRpcJ4#qk@230#$R");
            Console.WriteLine(test);
        }
        
        static string EncryptPassword(string UserName, SecureString Password, string SessionId)
        {
            // Password is MD5(sessionId + MD5(login + password))
            // Source: https://www.godo.dev/tutorials/csharp-md5/
            using (MD5 md5 = MD5.Create())
            {

string credentials = $"{UserName}{SecureStringHelper.ConvertToString(Password)}";
                // Hash credentials first
                md5.ComputeHash(Encoding.UTF8.GetBytes(credentials));
                var inputBuffer = Encoding.UTF8.GetBytes(SessionId).ToList();

string hexHash = BitConverter.ToString(md5.Hash).Replace("-", string.Empty).ToLowerInvariant();
                inputBuffer.AddRange(Encoding.UTF8.GetBytes(hexHash));
                //var inputBuffer = Encoding.UTF8.GetBytes(SessionId + credentialBuilder.ToString());

md5.ComputeHash(inputBuffer.ToArray());

//md5.TransformBlock(inputBuffer, 0, inputBuffer.Length, inputBuffer, 0);
                //md5.TransformFinalBlock(new byte[0], 0, 0);

// Get hash result after compute it  
                byte[] hashedCredentials = md5
                    .Hash;

StringBuilder strBuilder = new StringBuilder();
                for (int i = 0; i < hashedCredentials.Length; i++)
                {
                    //change it into 2 hexadecimal digits  
                    //for each byte  
                    strBuilder.Append(hashedCredentials[i].ToString("x2"));
                }
                return strBuilder.ToString();
            }
        }
    }

static class SecureStringHelper
    {
        public static string ConvertToString(SecureString s)
        {
            return "TestPassword";
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,47 sec, absolute running time: 0,09 sec, cpu time: 0,09 sec, average memory usage: 14 Mb, average nr of threads: 3, absolute service time: 0,58 sec