ASP.NET Core Password Options and Custom Validators
ASP.NET Core provides a lot of identity feature out of the box when individual user accounts is selected during project creation. Using the default settings a user’s password is required to be at least 6 characters and contain a number, a lower case letter, an uppercase letter and a special character. This post is going to cover changing the the above options as well as creating custom validators.
Password Options
The following is the default registration of identity in the ConfigureServices  function of the Startup  class with the default settings mentioned above.
services.AddIdentity<ApplicationUser, IdentityRole>()
.AddEntityFrameworkStores<ApplicationDbContext>()
.AddDefaultTokenProviders();
AddIdentity  can accept options part of which allows control over the basic characteristics of what is required for user passwords. Here is the same AddIdentity but with all the options for passwords listed.
services.AddIdentity<ApplicationUser, IdentityRole>(options =>
{
options.Password.RequireDigit = true;
options.Password.RequireLowercase = true;
options.Password.RequireNonLetterOrDigit = true;
options.Password.RequireUppercase = true;
options.Password.RequiredLength = 6;
})
.AddEntityFrameworkStores<ApplicationDbContext>()
.AddDefaultTokenProviders();
All the options do what you would expect. One thing to note is if you change the required length by setting options.Password.RequiredLength then the new setting will only be validated on post back to the server, which is the case of most password validation anyway, but for pre-post validation on length then the string length data annotation needs to be updated on RegisterViewModel.Password, ResetPasswordViewModel.Password, ChangePasswordViewModel.NewPassword and SetPasswordViewModel.NewPassword.
Custom Password Validators
The above is great for changing simple aspects of password validation, but we all know password rules for organizations are not always simple enough to be covered by the above. Thankfully Microsoft has provided the AddPasswordValidator extension method to the IdentityBuilder class which is what is returned by AddIdentity.
AddPasswordValidator takes a type that implements IPasswordValidator. The custom validator only has to implement the  ValidateAsync defined by IPasswordValidator. The following validator checks to make sure that all the characters of the password are not the same and returns an IdentityResult based on the conditions passing. Forgive the contrived example, but I wanted to keep the class as simple as possible.
public class SameCharacterPasswordValidator<TUser>: IPasswordValidator<TUser>
where TUser : class
{
public Task<IdentityResult> ValidateAsync(UserManager<TUser> manager,
TUser user,
string password)
{
return Task.FromResult(password.Distinct().Count() == 1 ?
IdentityResult.Failed(new IdentityError
{
Code = "SameChar",
Description = "Passwords cannot be all the same character."
}) :
IdentityResult.Success);
}
}
If validation failed is the result then is added to the list of validation messages the user sees just like with the built in password validations.
Here is registration of identity with the custom password validation which is on the last line.
services.AddIdentity<ApplicationUser, IdentityRole>(options =>
{
options.Password.RequireDigit = true;
options.Password.RequireLowercase = true;
options.Password.RequireNonLetterOrDigit = true;
options.Password.RequireUppercase = true;
options.Password.RequiredLength = 6;
})
.AddEntityFrameworkStores<ApplicationDbContext>()
.AddDefaultTokenProviders()
.AddPasswordValidator<SameCharacterPasswordValidator<ApplicationUser>>();
Potential Use
Imagine you have a requirement to make sure a user doesn’t reuse the same password for a period of time. This would be a great place for a custom password validator. You could use dependency injection to get reference to a history of password hashes and use that to verify the user is not repeating the same password. Of course would have to first write the password hash history.
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