AspNet Membership Encrypted Password Generation - asp.net

Question

Before we begin I want to make it clear that I know how bad this question is. It's a terrible situation but I'm being constrained by some very strange specifications.
I know that not only should you not try to write your own wrappers for this kind of stuff, but Microsoft have made it clear this shouldn't even be used. So before you start writing replies saying "Why are you even doing this" please try to understand that I have had these conversations with my superiors already, but the push for new features and lack of time means that despite it being atrocious; nevertheless - here I am.
We have an ASP Net membership database, started some time well before my time at this company and now hosting some 40k users. We have a platform in .Net 3.5 which lets users log in.
My job currently is to write an API in .Net Core 2.1 part of which is to allow for user creation and updating but there in lies the problem - migrating from Membership to Identity is not an option so I've been told to create a wrapper for the stored procedures in the Membership database.
This has been mostly successful with the only issue being; the subject of this question. Creating a user via aspnet_Membership_CreateUser I need to submit the data in such a way that it can be successfully validated in our platform.
I had originally followed this post but found that it's been designed for PasswordFormat 1 - Hashed; I then found that our user base used PasswordFormat 2 - Encrypted and as such the users I was creating would not validate.
The code looked something like this
public bool CreateUser(string userName, string password, string email, string securityQuestion, string securityAnswer, bool isApproved)
{
bool success = false;
//Just so I can attempt to login afterwards
password = "Hello World!";
//Our password and password salt need to be base64 encoded before we can save them to the DB
string salt = Guid.NewGuid().ToString();
string encryptedSalt = salt.Base64Encode();
//Concatenate our salt and password
IEnumerable<byte> saltedpass = salt.GetBytes(Encoding.UTF8).Concat(password.GetBytes(Encoding.UTF8));
//Use SHA1 to hash more - equivilant to the HASHBYTES('SHA1' T-SQL
byte[] sha1HashedPass = PasswordHelper.HashBytes(_validationMethod, saltedpass.ToArray(), _validationKey);
string hashedPass = sha1HashedPass.ToBase64String();
int errorCode = MembershipCreateUser(_applicationName, userName, hashedPass, encryptedSalt, email, securityQuestion, securityAnswer, isApproved);
if (errorCode == 0)
{
success = true;
}
return success;
}
Worth noting that _validationKey is the machine key shared across the applications which use this database, which I'm passing into the SHA1 mechanism.
So, intentionally and woefully bad security practice aside;
Is there a way in C# to generate an encrypted (not hashed) passwords and salts in this way?

Answer 1

Thank you for the comments - thankfully we were able to support Hashed passwords in our platform; the issue was with my code and not ASP Membership.
As mentioned I was taking a post that was originally written in T-SQL and trying to build a C# implementation of it. My implementation of this code was incorrect and as such the passwords and salts I was generating were not able to be validated by ASP Net Membership, this was not obvious in my original post because I had obfuscated the method which was SHA1 hashing my data.
//Using hard coded just for example
string username = "joeborder";
string password = "Hello World!";
string salt = "TastySalt";
Encoding encoder = Encoding.Unicode; //Encoding was also incorrect
//Our password and password salt need to be base64 encoded before we can save them to the DB
string encryptedSalt = salt.Base64Encode();
//Concatenate our salt and password
IEnumerable<byte> saltedpass = salt.GetBytes(encoder).Concat(password.GetBytes(encoder));
//Use SHA1 to hash more - equivilant to the HASHBYTES('SHA1') T-SQL
var SHA1Hasher = new SHA1CryptoServiceProvider(); //Originally I was using HMACSHA1 which was producing a different output
byte[] sha1HashedPass = SHA1Hasher.ComputerHash(saltedpass.ToArray());
string hashedPass = sha1HashedPass.ToBase64String();
/*
EXEC aspnet_Membership_CreateUser
#ApplicationName = "MyApp",
#UserName = username,
#Password = hashedPass,
#PasswordSalt = encryptedSalt,
...Etc
*/
Then in our .Net 3.5 application the following code would work
string username = "joeborder";
string password = "Hello World!";
if (Membership.ValidateUser(username, password))
{
Console.WriteLine("You've gotta be kidding me thats a clutch");
}