<chapter xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="remember-me"
    xmlns:xlink="http://www.w3.org/1999/xlink">
    <info>
        <title>Remember-Me Authentication</title>
    </info>
    <section xml:id="remember-me-overview">
        <info>
            <title>Overview</title>
        </info>
        <para>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. </para>
        <para> Note that both implemementations require a
                <interfacename>UserDetailsService</interfacename>. If you are using an
            authentication provider which doesn't use a
                <interfacename>UserDetailsService</interfacename> (for example, the LDAP provider)
            then it won't work unless you also have a
                <interfacename>UserDetailsService</interfacename> bean in your application context.
        </para>
    </section>
    <section xml:id="remember-me-hash-token">
        <title>Simple Hash-Based Token Approach</title>
        <para>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:
            <programlisting>
    base64(username + ":" + expirationTime + ":" +
             md5Hex(username + ":" + expirationTime + ":" password + ":" + key))

    username:          As identifiable to the <interfacename>UserDetailsService</interfacename>
    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
        </programlisting></para>
        <para>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.</para>
        <para>If you are familiar with the topics discussed in the chapter on <link
                xlink:href="#ns-config">namespace configuration</link>, you can enable remember-me
            authentication just by adding the <literal>&lt;remember-me&gt;</literal> element: <programlisting><![CDATA[
  <http>
    ...
    <remember-me key="myAppKey"/>
  </http>
  ]]>
                </programlisting> The <interfacename>UserDetailsService</interfacename> 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
                <literal>user-service-ref</literal> attribute, where the value is the name of your
                <interfacename>UserDetailsService</interfacename> bean. </para>
    </section>
    <section xml:id="remember-me-persistent-token">
        <title>Persistent Token Approach</title>
        <para>This approach is based on the article <link
                xlink:href="http://jaspan.com/improved_persistent_login_cookie_best_practice"
                >http://jaspan.com/improved_persistent_login_cookie_best_practice</link> with some
            minor modifications <footnote><para>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.</para></footnote>.
            To use the this approach with namespace configuration, you would supply a datasource
            reference: <programlisting><![CDATA[
  <http>
    ...
    <remember-me data-source-ref="someDataSource"/>
  </http>
  ]]>
            </programlisting> The database should contain a
                <literal>persistent_logins</literal> table, created using the following SQL (or
            equivalent):
            <programlisting>
    create table persistent_logins (username varchar(64) not null, series varchar(64) primary key, token varchar(64) not null, last_used timestamp not null)
</programlisting></para>
        <!-- TODO: Add more info on the implementation and behaviour when tokens are stolen etc. Also some info for admins on invalidating tokens using key, or deleting info from db -->
    </section>
    <section xml:id="remember-me-impls">
        <info>
            <title>Remember-Me Interfaces and Implementations</title>
        </info>
        <para>Remember-me authentication is not used with basic authentication, given it is often
            not used with <literal>HttpSession</literal>s. Remember-me is used with
                <literal>UsernamePasswordAuthenticationFilter</literal>, and is implemented via
            hooks in the <literal>AbstractAuthenticationProcessingFilter</literal> superclass. The
            hooks will invoke a concrete <interfacename>RememberMeServices</interfacename> at the
            appropriate times. The interface looks like this:
            <programlisting language="java">
  Authentication autoLogin(HttpServletRequest request, HttpServletResponse response);
  void loginFail(HttpServletRequest request, HttpServletResponse response);
  void loginSuccess(HttpServletRequest request, HttpServletResponse response,
      Authentication successfulAuthentication);
    </programlisting>
            Please refer to the JavaDocs for a fuller discussion on what the methods do, although
            note at this stage that <literal>AbstractAuthenticationProcessingFilter</literal> only
            calls the <literal>loginFail()</literal> and <literal>loginSuccess()</literal> methods.
            The <literal>autoLogin()</literal> method is called by
                <classname>RememberMeAuthenticationFilter</classname> whenever the
                <classname>SecurityContextHolder</classname> does not contain an
                <interfacename>Authentication</interfacename>. 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.</para>
        <section>
            <title>TokenBasedRememberMeServices</title>
            <para> This implementation supports the simpler approach described in <xref
                    linkend="remember-me-hash-token"/>.
                    <classname>TokenBasedRememberMeServices</classname> generates a
                    <literal>RememberMeAuthenticationToken</literal>, which is processed by
                    <literal>RememberMeAuthenticationProvider</literal>. A <literal>key</literal> is
                shared between this authentication provider and the
                    <literal>TokenBasedRememberMeServices</literal>. In addition,
                    <literal>TokenBasedRememberMeServices</literal> requires A UserDetailsService
                from which it can retrieve the username and password for signature comparison
                purposes, and generate the <literal>RememberMeAuthenticationToken</literal> to
                contain the correct <interfacename>GrantedAuthority</interfacename>[]s. Some sort of
                logout command should be provided by the application that invalidates the cookie if
                the user requests this. <classname>TokenBasedRememberMeServices</classname> also
                implements Spring Security's <interfacename>LogoutHandler</interfacename> interface
                so can be used with <classname>LogoutFilter</classname> to have the cookie cleared
                automatically. </para>
            <para>The beans required in an application context to enable remember-me services are as
                follows: <programlisting language="xml"><![CDATA[
<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.web.authentication.rememberme.RememberMeAuthenticationProvider">
  <property name="key" value="springRocks"/>
</bean>
]]>
            </programlisting>Don't forget to add your
                    <interfacename>RememberMeServices</interfacename> implementation to your
                    <literal>UsernamePasswordAuthenticationFilter.setRememberMeServices()</literal>
                property, include the <literal>RememberMeAuthenticationProvider</literal> in your
                    <literal>AuthenticationManager.setProviders()</literal> list, and add
                    <classname>RememberMeAuthenticationFilter</classname> into your
                    <classname>FilterChainProxy</classname> (typically immediately after your
                    <literal>UsernamePasswordAuthenticationFilter</literal>).</para>
        </section>
        <section>
            <title>PersistentTokenBasedRememberMeServices</title>
            <para> This class can be used in the same way as
                    <classname>TokenBasedRememberMeServices</classname>, but it additionally needs
                to be configured with a <interfacename>PersistentTokenRepository</interfacename> to
                store the tokens. There are two standard implementations.
                                <itemizedlist><listitem><para><classname>InMemoryTokenRepositoryImpl</classname>
                            which is intended for testing
                                only.</para></listitem><listitem><para><classname>JdbcTokenRepositoryImpl</classname>
                            which stores the tokens in a database. </para></listitem></itemizedlist>
                The database schema is described above in <xref
                    linkend="remember-me-persistent-token"/>. </para>
        </section>
    </section>
</chapter>
