library JDEncr;

uses
  SysUtils,
  Classes,
  Windows,
  StrUtils,
  Encryption in '..\JDCommon\Encryption.pas';
  //Pre-made encryption unit

{$R *.res}

//Note: use PChar and not String, and Bool, not Boolean
//stdcall goes at the end of each declaration

function CompareStrings(PlainText, EncryptedText: PChar;
  EncType: Integer; CaseSensitive: Bool): Bool; stdcall;
var
  S: String;
  Enc: TEncType;
begin
  Enc:= TEncType.Create(EncType, 1);
  try
    Result := False;
    S:= DecryptString(EncryptedText, Enc);
    if CaseSensitive then begin
      if S = PlainText then
        Result:= True;
    end else begin
      if UpperCase(S) = UpperCase(PlainText) then
        Result:= True;
    end;
  finally
    Enc.Free;
  end;
end;

//exports section goes at end of unit before the begin/end.
//List everything you want to export, just like in the uses
exports
   CompareStrings;

begin

end.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Runtime.InteropServices;
//use InteropServices to use DllImport

namespace JDWeb
{
    public class JDEncr
    {
        //Uses Ansi CharSet
        //Uses StdCall CallingConvention (like in Delphi)
        //uses String instead of PChar
        //Parameters recognize types only, and names can differ from original function in DLL
        [DllImport("JDEncr.dll", CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Ansi)]
        public static extern bool CompareStrings(String PlainText, String EncryptedText, int EncType, bool CaseSensitive);
    }
}

  bool P = JDEncr.CompareStrings(EnteredPassword, RegisteredPassword, EncryptionType, true);