[[servlet-rememberme]]
== Remember-Me Authentication


[[remember-me-overview]]
=== Overview
Remember-me or persistent-login authentication refers to web sites being able to remember the identity of a principal between sessions.
This is typically accomplished by sending a cookie to the browser, with the cookie being detected during future sessions and causing automated login to take place.
Spring Security provides the necessary hooks for these operations to take place, and has two concrete remember-me implementations.
One uses hashing to preserve the security of cookie-based tokens and the other uses a database or other persistent storage mechanism to store the generated tokens.

Note that both implementations require a `UserDetailsService`.
If you are using an authentication provider which doesn't use a `UserDetailsService` (for example, the LDAP provider) then it won't work unless you also have a `UserDetailsService` bean in your application context.


[[remember-me-hash-token]]
=== Simple Hash-Based Token Approach
This approach uses hashing to achieve a useful remember-me strategy.
In essence a cookie is sent to the browser upon successful interactive authentication, with the cookie being composed as follows:

[source,txt]
----
base64(username + ":" + expirationTime + ":" +
md5Hex(username + ":" + expirationTime + ":" password + ":" + key))

username:          As identifiable to the UserDetailsService
password:          That matches the one in the retrieved UserDetails
expirationTime:    The date and time when the remember-me token expires, expressed in milliseconds
key:               A private key to prevent modification of the remember-me token
----

As such the remember-me token is valid only for the period specified, and provided that the username, password and key does not change.
Notably, this has a potential security issue in that a captured remember-me token will be usable from any user agent until such time as the token expires.
This is the same issue as with digest authentication.
If a principal is aware a token has been captured, they can easily change their password and immediately invalidate all remember-me tokens on issue.
If more significant security is needed you should use the approach described in the next section.
Alternatively remember-me services should simply not be used at all.

If you are familiar with the topics discussed in the chapter on <<ns-config,namespace configuration>>, you can enable remember-me authentication just by adding the `<remember-me>` element:

[source,xml]
----
<http>
...
<remember-me key="myAppKey"/>
</http>
----

The `UserDetailsService` will normally be selected automatically.
If you have more than one in your application context, you need to specify which one should be used with the `user-service-ref` attribute, where the value is the name of your `UserDetailsService` bean.

[[remember-me-persistent-token]]
=== Persistent Token Approach
This approach is based on the article http://jaspan.com/improved_persistent_login_cookie_best_practice[http://jaspan.com/improved_persistent_login_cookie_best_practice] with some minor modifications  footnote:[Essentially, the username is not included in the cookie, to prevent exposing a valid login name unecessarily.
There is a discussion on this in the comments section of this article.].
To use the this approach with namespace configuration, you would supply a datasource reference:

[source,xml]
----
<http>
...
<remember-me data-source-ref="someDataSource"/>
</http>
----

The database should contain a `persistent_logins` table, created using the following SQL (or equivalent):

[source,ddl]
----
create table persistent_logins (username varchar(64) not null,
								series varchar(64) primary key,
								token varchar(64) not null,
								last_used timestamp not null)
----

[[remember-me-impls]]
=== Remember-Me Interfaces and Implementations
Remember-me is used with `UsernamePasswordAuthenticationFilter`, and is implemented via hooks in the `AbstractAuthenticationProcessingFilter` superclass.
It is also used within `BasicAuthenticationFilter`.
The hooks will invoke a concrete `RememberMeServices` at the appropriate times.
The interface looks like this:

[source,java]
----
Authentication autoLogin(HttpServletRequest request, HttpServletResponse response);

void loginFail(HttpServletRequest request, HttpServletResponse response);

void loginSuccess(HttpServletRequest request, HttpServletResponse response,
	Authentication successfulAuthentication);
----

Please refer to the Javadoc for a fuller discussion on what the methods do, although note at this stage that `AbstractAuthenticationProcessingFilter` only calls the `loginFail()` and `loginSuccess()` methods.
The `autoLogin()` method is called by `RememberMeAuthenticationFilter` whenever the `SecurityContextHolder` does not contain an `Authentication`.
This interface therefore provides the underlying remember-me implementation with sufficient notification of authentication-related events, and delegates to the implementation whenever a candidate web request might contain a cookie and wish to be remembered.
This design allows any number of remember-me implementation strategies.
We've seen above that Spring Security provides two implementations.
We'll look at these in turn.

==== TokenBasedRememberMeServices
This implementation supports the simpler approach described in <<remember-me-hash-token>>.
`TokenBasedRememberMeServices` generates a `RememberMeAuthenticationToken`, which is processed by `RememberMeAuthenticationProvider`.
A `key` is shared between this authentication provider and the `TokenBasedRememberMeServices`.
In addition, `TokenBasedRememberMeServices` requires A UserDetailsService from which it can retrieve the username and password for signature comparison purposes, and generate the `RememberMeAuthenticationToken` to contain the correct `GrantedAuthority` s.
Some sort of logout command should be provided by the application that invalidates the cookie if the user requests this.
`TokenBasedRememberMeServices` also implements Spring Security's `LogoutHandler` interface so can be used with `LogoutFilter` to have the cookie cleared automatically.

The beans required in an application context to enable remember-me services are as follows:

[source,xml]
----
<bean id="rememberMeFilter" class=
"org.springframework.security.web.authentication.rememberme.RememberMeAuthenticationFilter">
<property name="rememberMeServices" ref="rememberMeServices"/>
<property name="authenticationManager" ref="theAuthenticationManager" />
</bean>

<bean id="rememberMeServices" class=
"org.springframework.security.web.authentication.rememberme.TokenBasedRememberMeServices">
<property name="userDetailsService" ref="myUserDetailsService"/>
<property name="key" value="springRocks"/>
</bean>

<bean id="rememberMeAuthenticationProvider" class=
"org.springframework.security.authentication.RememberMeAuthenticationProvider">
<property name="key" value="springRocks"/>
</bean>
----

Don't forget to add your `RememberMeServices` implementation to your `UsernamePasswordAuthenticationFilter.setRememberMeServices()` property, include the `RememberMeAuthenticationProvider` in your `AuthenticationManager.setProviders()` list, and add `RememberMeAuthenticationFilter` into your `FilterChainProxy` (typically immediately after your `UsernamePasswordAuthenticationFilter`).


==== PersistentTokenBasedRememberMeServices
This class can be used in the same way as `TokenBasedRememberMeServices`, but it additionally needs to be configured with a `PersistentTokenRepository` to store the tokens.
There are two standard implementations.

* `InMemoryTokenRepositoryImpl` which is intended for testing only.
* `JdbcTokenRepositoryImpl` which stores the tokens in a database.

The database schema is described above in <<remember-me-persistent-token>>.
