ASP.NET Core Identity Token Providers – Under the Hood Part 2: Introducing Token Providers

Next up for my series on ASP.NET Core Identity I was interested in how the Identity library provides a way to create tokens which validate actions such as when a user first registers and we need to confirm their email address. This post took a lot longer to write than I expected it to as there are a lot of potential areas to cover. In the interests of making it reasonably digestible I’ve decided to introduce tokens and specifically look at the registration email confirmation token flow in this post. Other posts may follow as I dig deeper into the code and it’s uses.

As with part 1 let me prefix this post with two important notes.

  1. I am not a security expert. This series of posts records my own dive into the ASP.NET Identity Core code, publically available on GitHub which I’ve done for my own self-interest to try and understand how it works and what is available to me as a developer. Do not assume everything I have interpreted to be 100% accurate or any code samples as suitable production code.
  2. This is written whilst reviewing source mostly from the 3.0.0-rc1 release tag. I may stray into more recent dev code if implementations have changed considerably, but will try to highlight when I do so. One very important point here is that at the time of writing this post Microsoft have announced a renaming strategy for ASP.NET 5. Due to the brand new codebase this is now being called ASP.NET Core 1.0 and the underlying .NET Core will be .NET Core 1.0. This is going to result in namespace changes. I’ve used the anticipated new namespaces here (and will update if things change again).

What are tokens and why do we need them?

Tokens are something that an application or service can issue to a user and which they can later hand back as a way to prove their identity and often their authorisation for an action. We can use tokens in various places where we need to provide a mechanism to confirm something about them, such as that a phone number or email address actually belongs to them. They can also be used in other ways; Slack for example uses tokens to provide a magic sign in link on mobile devices.

Because of these potential uses it’s very important that they be secure and trustworthy since they could present a security hole into your application if used incorrectly. Mechanisms need to be in place to expire old or used tokens to prevent someone else using them should they gain access to them. ASP.NET Identity Core provides some basic tokens via token providers for common tasks. These are used by the default ASP.NET Web Application MVC template for some of the account and user management tasks on the AccountController and ManageController.

Now that I’ve explained what a token is let’s look at how we generate one.

Token providers

To get a token or validate one we use a token provider. ASP.NET Core Identity defines an IUserTokenProvider interface which any token providers should implement. This interface has been kept very simple and defines three methods:

Task<string> GenerateAsync(string purpose, UserManager<TUser> manager, TUser user);

This method will generate a token for a given purpose and user. The token is returned as a string.

Task<bool> ValidateAsync(string purpose, string token, UserManager<TUser> manager, TUser user);

This method will validate a token from a user. It will return true or false, indicating whether the token is valid or not.

Task<bool> CanGenerateTwoFactorTokenAsync(UserManager<TUser> manager, TUser user);

This indicates whether the token from this provider can be used for two factor authentication.

You can register as many token providers into your project as necessary to support your requirements. By default IdentityBuilder has a method AddDefaultTokenProviders() which you can chain onto your AddIdentity call from the startup file in your project. This will register the 3 default providers as per the code below. Token providers need to be registered with the DI container so they can be injected when required.

public virtual IdentityBuilder AddDefaultTokenProviders()
public virtual IdentityBuilder AddDefaultTokenProviders()
{
	var dataProtectionProviderType = typeof(DataProtectorTokenProvider<>).MakeGenericType(UserType);
	var phoneNumberProviderType = typeof(PhoneNumberTokenProvider<>).MakeGenericType(UserType);
	var emailTokenProviderType = typeof(EmailTokenProvider<>).MakeGenericType(UserType);
	return AddTokenProvider(TokenOptions.DefaultProvider, dataProtectionProviderType)
		.AddTokenProvider(TokenOptions.DefaultEmailProvider, emailTokenProviderType)
		.AddTokenProvider(TokenOptions.DefaultPhoneProvider, phoneNumberProviderType);
}

This code makes use of the TokenOptions class which defines a few common provider names and maintains a dictionary of the available providers, the key of which is the provider name. The value is the type for the provider being registered. The code for AddTokenProvider is as follows.

public virtual IdentityBuilder AddTokenProvider(string providerName, Type provider)
{
	if (!typeof(IUserTokenProvider<>).MakeGenericType(UserType).GetTypeInfo().IsAssignableFrom(provider.GetTypeInfo()))
	{
		throw new InvalidOperationException(Resources.FormatInvalidManagerType(provider.Name, "IUserTokenProvider", UserType.Name));
	}
	Services.Configure<IdentityOptions>(options =>
	{
		options.Tokens.ProviderMap[providerName] = new TokenProviderDescriptor(provider);
	});
	Services.AddTransient(provider);
	return this; 
}

Here you can see that the provider is being added into the ProviderMap dictionary and then registered with the DI container.

Registration email confirmation

Now that I’ve covered what tokens are and how they are registered I think the best thing to do is to take a look at a token being generated and validated. I’ve chosen to step through the process which creates an email confirmation token. The user is sent a link to the ConfirmEmail action which includes the userid and their token as querystring parameters. The user is required to click the link which will then validate the token and then mark their email as confirmed.

Validating the email this way is good practice as it prevents people from registering with or adding mailboxes which do not belong to them. By sending a link to the email address requiring an action from the user before the email is activated, only the true owner of the mailbox can access the link and click it to confirm that they did indeed signup for the account. We are trusting the user’s action based on something secure we have provided to them. Because the tokens are encrypted they are protected against forgery.

Generating the token

ASP.NET Core Identity provides the classes necessary to generate the token to be issued to the user in their link. The actual use of the Identity system to request the token and to include it in the link is managed by the MVC site itself, calling into the Identity API as necessary.

In ASP.NET MVC projects the generation of the confirmation email is optional and it is not enabled by default. However the code is there, but commented out within the AccountController. The UserManager class within Identity provides all the methods needed to call for the generation of a token and to validate it again later on. Once we have the token back from the Identity library we are then able to use that token when we send our activation email.

In our example we can call GenerateEmailConfirmationTokenAsync(TUser user). We pass in the user for which the token will be generated.

public virtual Task<string> GenerateEmailConfirmationTokenAsync(TUser user)
{
	ThrowIfDisposed();
	return GenerateUserTokenAsync(user, Options.Tokens.EmailConfirmationTokenProvider, ConfirmEmailTokenPurpose);
}

GenerateUserTokenAsync requires the user, the name of the token provider to use (pulled from the Identity options) and the purpose for the token as a string. The ConfirmEmailTokenPurpose is a constant string defining the wording to use. In this case it is “EmailConfirmation”.

Each token is expected to carry a purpose so that they can be tied very closely to a specific action within your system. A token for one action would not be valid for another.

public virtual Task<string> GenerateUserTokenAsync(TUser user, string tokenProvider, string purpose)
{
	ThrowIfDisposed();
	if (user == null)
	{
		throw new ArgumentNullException("user");
	}
	if (tokenProvider == null)
	{
		throw new ArgumentNullException(nameof(tokenProvider));
	}
	if (!_tokenProviders.ContainsKey(tokenProvider))
	{
		throw new NotSupportedException(string.Format(CultureInfo.CurrentCulture, Resources.NoTokenProvider, tokenProvider));
	}

	return _tokenProviders[tokenProvider].GenerateAsync(purpose, this, user);
}

After the usual null checks what this boils down to is checking through the dictionary of token providers available to the UserManager based on the tokenProvider parameter passed into the method. Once the provider is found it’s GenerateAsync method is called.

At the moment all of the three default TokenOptions providers are set to use the default token provider so by default the DataProtectionTokenProvider is being called which has the following GenerateAsync method.

public class TokenOptions
{
	public static readonly string DefaultProvider = "Default";
	public static readonly string DefaultEmailProvider = "Email";
	public static readonly string DefaultPhoneProvider = "Phone";

	public Dictionary<string, TokenProviderDescriptor> ProviderMap { get; set; } = new Dictionary<string, TokenProviderDescriptor>();

	public string EmailConfirmationTokenProvider { get; set; } = DefaultProvider;
	public string PasswordResetTokenProvider { get; set; } = DefaultProvider;
	public string ChangeEmailTokenProvider { get; set; } = DefaultProvider;
}

NOTE: This setup is slightly confusing as it appears that certain token providers, although registered would never be called based on the way the options are setup by default. This could be modified by changing the options and I have tried to query why this is setup this way by default.

For now though let’s look at the GenerateAsync method on the DataProtectionTokenProvider.

public virtual async Task<string> GenerateAsync(string purpose, UserManager<TUser> manager, TUser user)
{
	if (user == null)
	{
		throw new ArgumentNullException(nameof(user));
	}
	var ms = new MemoryStream();
	var userId = await manager.GetUserIdAsync(user);
	using (var writer = ms.CreateWriter())
	{
		writer.Write(DateTimeOffset.UtcNow);
		writer.Write(userId);
		writer.Write(purpose ?? "");
		string stamp = null;
		if (manager.SupportsUserSecurityStamp)
		{
			stamp = await manager.GetSecurityStampAsync(user);
		}
		writer.Write(stamp ?? "");
	}
	var protectedBytes = Protector.Protect(ms.ToArray());
	return Convert.ToBase64String(protectedBytes);
}

This method uses a memory stream to build up a byte array with the following elements:

  1. The current UTC time (converted to ticks within the extension method)
  2. The user id
  3. The purpose if not null
  4. The user security stamp if supported by the current user manager. The security stamp is a Guid stored in the database against the user. It gets updated when certain actions take place within the Identity UserManager class and provides a way to invalidate old tokens when an account has changed. The security stamp is changed for example when we change the username or email address of a user. By changing the stamp we prevent the same token being used to confirm the email again since the security stamp within the token will no longer match the user’s current security stamp.

These are then passed to the Protect method on an injected IDataProtector. For this post, going into detail about data protectors will be a bit deep and take me quite far off track. I do plan to look at them more in the future but for now it’s sufficient to say that the data protection library defines a cryptographic API for protecting data. Identity leverages this API from its token providers to encrypt and decrypt the tokens it has generated.

Once the protected bytes are returned they are base64 encoded and returned.

Validating the token

Once the user clicks on the link in their confirmation email it will take them to the ConfirmEmail action in the AccountController. That action takes in the userid and the code (protected token) from the link. This action will then call the ConfirmEmailAsync method on the UserManager which in turn will call a VerifyUserTokenAsync method. This method will get to appropriate token provider from the ProviderMap and call the ValidateAsync method.

Let’s step through the code which validates the token on the DataProtectorTokenProvider.

public virtual async Task<bool> ValidateAsync(string purpose, string token, UserManager<TUser> manager, TUser user)
{
	try
	{
		var unprotectedData = Protector.Unprotect(Convert.FromBase64String(token));
		var ms = new MemoryStream(unprotectedData);
		using (var reader = ms.CreateReader())
		{
			var creationTime = reader.ReadDateTimeOffset();
			var expirationTime = creationTime + Options.TokenLifespan;
			if (expirationTime < DateTimeOffset.UtcNow)
			{
				return false;
			}

			var userId = reader.ReadString();
			var actualUserId = await manager.GetUserIdAsync(user);
			if (userId != actualUserId)
			{
				return false;
			}
			var purp = reader.ReadString();
			if (!string.Equals(purp, purpose))
			{
				return false;
			}
			var stamp = reader.ReadString();
			if (reader.PeekChar() != -1)
			{
				return false;
			}

			if (manager.SupportsUserSecurityStamp)
			{
				return stamp == await manager.GetSecurityStampAsync(user);
			}
			return stamp == "";
		}
	}
	// ReSharper disable once EmptyGeneralCatchClause
	catch
	{
		// Do not leak exception
	}
	return false;
}

The token is first converted from the base64 string representation to a byte array. It is then passed to the IDataProtector to be decrypted. Once again the details of how this works are too detailed for this post. The decrypted contents are passed into a new memory stream to be read.

The creation time is first read out from the start of the token. The expiration time is calculated by taking the token creation time and adding the token lifespan defined in the DataProtectionTokenProviderOptions. By default this is set at 1 day. If the token has expired then the method returns false since it is no longer considered a valid token.

It then reads the userId string and compares it to the id of the user (this is based on the userId from the link they get sent in their email. The account controller first uses that id to load up a user from the database. This ensures that the token belongs to the user who is attempting to use it.

It next reads the purpose and checks that it matches the purpose for the validation that is occurring (this will be passed into the method by the caller). This ensures a token is valid against only a specific function.

It then reads in the security stamp and stores it in a local variable for use in a few moments.

It then calls PeekChar which tries to get (but not advance) the next character from the token. Since we should be at the end of the stream here it checks for -1 which indicates no more characters are available. Any other value indicates that this token has extra data and is therefore not valid.

Finally, if security stamps are supported by the current user manager the security stamp for the user is retrieved from the user store and compared to the stamp it read from the token. Assuming they match then we can now confirm that the token is indeed valid and return that response to the caller.

Other token providers

In addition to the DataProtectionTokenProvider there are other providers defined within the Identity namespace. As far as I can tell these are not yet used based on the way the options are setup. I have actually queried this in an issue on the Identity repo. It may still be an interesting exercise for me to dig into how they work and differ from the DataProtectionTokenProvider. There is also the concept of an SMS verification token in the default ManageController for a default MVC application which doesn’t use a token provider directly

It would also be quite simple to implement your own token provider if you need to implement some additional functionality or store additional data within the token.

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