public string Authenticate(AuthenticateModel authData)
        {
            string ret = "";

            if (string.IsNullOrEmpty(authData.username) || string.IsNullOrEmpty(authData.password))
            {
                return(string.Format("The {0} can not be empty", string.IsNullOrEmpty(authData.username)?"user name":"password"));
            }

            Tenant tn = null;

            if (!string.IsNullOrEmpty(authData.tenant))
            {
                tn = dbCtx.Tenants.SingleOrDefault(t => t.Name.ToLower().Equals(authData.tenant.ToLower()));
                if (tn == null) // nonexisting tenant name provided
                {
                    return(string.Format("Tenant {0} not found", authData.tenant));
                }
            }

            // in this example application, we do not use the full ActiveDirectory identity features.
            // That, plus synchronizing the users in Izenda configuration DB and in the
            // authorization application DB, allows just to validate the user against the Active Directory.
            // In case you dont want to synchronize the users and/or not using the authorization DB
            // while still want to use the Active Directory - you need to implement
            // the full - featured identity mechanizm with ActiveDirectory support.
            //
            // So, here we just validate the active directory user if we set to use the active directory.
            ADUser   adUser = null;
            ADConfig adCfg  = Utilities.ADSettings();

            if (adCfg.UseActiveDirectory)
            {
                adUser = ADUtilities.ValidateADUser(authData.username, authData.password, adCfg);
            }

            if (adCfg.UseActiveDirectory && !adUser.IsValid)
            {
                return(string.Format("The user {0} is invalid or not found in the Active Directory.\nError:{1}", authData.username, adUser.Reserved));
            }

            // Check if the user exists in the authentication database.
            // For the Active Directory, we still will verify this too, even in case the user is valid on A.D.
            int?tnId = null;

            if (tn != null)
            {
                tnId = tn.Id;
            }
            AspNetUser user = dbCtx.AspNetUsers.SingleOrDefault(
                u => u.UserName.ToLower().Equals(authData.username.ToLower()) &&
                u.TenantId == tnId);

            // Tip.
            // At this point, if the AD user exists/valid (adUser.IsValid == true), it is possible to automatically
            // create the AD user in Izenda DB, if you'd like to. You can implement something like the following:
            // if ((adCfg.UseActiveDirectory && adUser.IsValid) && user == null) {
            //     user = (cast/retrieve to the AspNetUser)CreateUser(new CreateUserBindingModel { FirstName = "", LastName = "", Tenant = authData.tenant, Password = authData.password, IsAdmin = false });
            // }
            // See the article "Few aspects of Active Directory authentication" at Izenda Confluence board for details
            // as of now, we expect the A.D. user to exist in the database.
            if (user == null)
            {
                return(string.Format("User {0} not found {1}", authData.username, tn == null ? "":"for the tenant " + tn.Name));
            }

            if (!VerifyPassword(authData.password, user))
            {
                return("The password is incorrect");
            }

            // our sample (custom authenticacion) database does not have the user status flag.
            // we will use Izenda to find out if the user is active or not.
            string adminToken = IzendaTokenAuthorization.GetIzendaAdminToken();
            Task <IzendaBoundary.Models.UserDetail> getUser = IzendaUtilities.GetIzendaUserByTenantAndName(user.UserName, tn == null?null:tn.Name, adminToken);

            IzendaBoundary.Models.UserDetail userDetails = getUser.Result;
            if (userDetails == null)
            {
                return(string.Format("The user {0} not found in [Izenda database]. Contact your administrator please", user.UserName));
            }
            else if (!userDetails.Active)
            {
                return(string.Format("The user {0} was found but it is not active. Contact your administrator please", user.UserName));
            }
            return(ret);
        }