<chapter xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="anonymous">
    <info>
        <title>Anonymous Authentication</title>
    </info>
    <section xml:id="anonymous-overview">
        <info>
            <title>Overview</title>
        </info>
        <para>It's generally considered good security practice to adopt a
                <quote>deny-by-default</quote> where you explicitly specify what is allowed and
            disallow everything else. Defining what is accessible to unauthenticated users is a
            similar situation, particularly for web applications. Many sites require that users must
            be authenticated for anything other than a few URLs (for example the home and login
            pages). In this case it is easiest to define access configuration attributes for these
            specific URLs rather than have for every secured resource. Put differently, sometimes it
            is nice to say <literal>ROLE_SOMETHING</literal> is required by default and only allow
            certain exceptions to this rule, such as for login, logout and home pages of an
            application. You could also omit these pages from the filter chain entirely, thus
            bypassing the access control checks, but this may be undesirable for other reasons,
            particularly if the pages behave differently for authenticated users.</para>
        <para>This is what we mean by anonymous authentication. Note that there is no real
            conceptual difference between a user who is <quote>anonymously authenticated</quote> and
            an unauthenticated user. Spring Security's anonymous authentication just gives you a
            more convenient way to configure your access-control attributes. Calls to servlet API
            calls such as <methodname>getCallerPrincipal</methodname>, for example, will still
            return null even though there is actually an anonymous authentication object in the
                <classname>SecurityContextHolder</classname>.</para>
        <para>There are other situations where anonymous authentication is useful, such as when an
            auditing interceptor queries the <classname>SecurityContextHolder</classname> to
            identify which principal was responsible for a given operation. Classes can be authored
            more robustly if they know the <classname>SecurityContextHolder</classname> always
            contains an <interfacename>Authentication</interfacename> object, and never
                <literal>null</literal>.</para>
    </section>
    <section xml:id="anonymous-config">
        <info>
            <title>Configuration</title>
        </info>
        <para>Anonymous authentication support is provided automatically when using the HTTP
            configurain Spring Security 3.0 and can be customized (or disabled) using the
                <literal>&lt;anonymous></literal> element. You don't need to configure the beans
            described here unless you are using traditional bean configuration.</para>
        <para>Three classes that together provide the anonymous authentication feature.
                <literal>AnonymousAuthenticationToken</literal> is an implementation of
                <interfacename>Authentication</interfacename>, and stores the
                <interfacename>GrantedAuthority</interfacename>s which apply to the anonymous
            principal. There is a corresponding <literal>AnonymousAuthenticationProvider</literal>,
            which is chained into the <literal>ProviderManager</literal> so that
                <literal>AnonymousAuthenticationToken</literal>s are accepted. Finally, there is an
                <classname>AnonymousAuthenticationFilter</classname>, which is chained after the
            normal authentication mechanisms and automatically adds an
                <literal>AnonymousAuthenticationToken</literal> to the
                <classname>SecurityContextHolder</classname> if there is no existing
                <interfacename>Authentication</interfacename> held there. The definition of the
            filter and authentication provider appears as follows:</para>
        <para>
            <programlisting>
<![CDATA[
<bean id="anonymousAuthFilter"
    class="org.springframework.security.web.authentication.AnonymousAuthenticationFilter">
  <property name="key" value="foobar"/>
  <property name="userAttribute" value="anonymousUser,ROLE_ANONYMOUS"/>
</bean>

<bean id="anonymousAuthenticationProvider"
    class="org.springframework.security.authentication.AnonymousAuthenticationProvider">
  <property name="key" value="foobar"/>
</bean>]]>
    </programlisting>
        </para>
        <para>The <literal>key</literal> is shared between the filter and authentication provider,
            so that tokens created by the former are accepted by the latter<footnote><para>The use
                    of the <literal>key</literal> property should not be regarded as providing any
                    real security here. It is merely a book-keeping exercise. If you are sharing a
                        <classname>ProviderManager</classname> which contains an
                        <classname>AnonymousAuthenticationProvider</classname> in a scenario where
                    it is possible for an authenticating client to construct the
                        <interfacename>Authentication</interfacename> object (such as with RMI
                    invocations), then a malicious client could submit an
                        <classname>AnonyousAuthenticationToken</classname> which it had created
                    itself (with chosen username and authority list). If the <literal>key</literal>
                    is guessable or can be found out, then the token would be accepted by the
                    anonymous provider. This isn't a problem with normal usage but if you are using
                    RMI you would be best to use a customized <classname>ProviderManager</classname>
                    which omits the anonymous provider rather than sharing the one you use for your
                    HTTP authentication mechanisms.</para></footnote>. The
                <literal>userAttribute</literal> is expressed in the form of
                <literal>usernameInTheAuthenticationToken,grantedAuthority[,grantedAuthority]</literal>.
            This is the same syntax as used after the equals sign for
                <literal>InMemoryDaoImpl</literal>'s <literal>userMap</literal> property.</para>
        <para>As explained earlier, the benefit of anonymous authentication is that all URI patterns
            can have security applied to them. For example:</para>
        <para>
            <programlisting>
<![CDATA[
<bean id="filterSecurityInterceptor"
    class="org.springframework.security.web.access.intercept.FilterSecurityInterceptor">
  <property name="authenticationManager" ref="authenticationManager"/>
  <property name="accessDecisionManager" ref="httpRequestAccessDecisionManager"/>
  <property name="securityMetadata">
    <security:filter-security-metadata-source>
      <security:intercept-url pattern='/index.jsp' access='ROLE_ANONYMOUS,ROLE_USER'/>
      <security:intercept-url pattern='/hello.htm' access='ROLE_ANONYMOUS,ROLE_USER'/>
      <security:intercept-url pattern='/logoff.jsp' access='ROLE_ANONYMOUS,ROLE_USER'/>
      <security:intercept-url pattern='/login.jsp' access='ROLE_ANONYMOUS,ROLE_USER'/>
      <security:intercept-url pattern='/**' access='ROLE_USER'/>
    </security:filter-security-metadata-source>" +
  </property>
</bean>]]>
    </programlisting>
        </para>
    </section>
    <section xml:id="anonymous-auth-trust-resolver">
        <title><interfacename>AuthenticationTrustResolver</interfacename></title>
        <para> Rounding out the anonymous authentication discussion is the
                <interfacename>AuthenticationTrustResolver</interfacename> interface, with its
            corresponding <literal>AuthenticationTrustResolverImpl</literal> implementation. This
            interface provides an <literal>isAnonymous(Authentication)</literal> method, which
            allows interested classes to take into account this special type of authentication
            status. The <classname>ExceptionTranslationFilter</classname> uses this interface in
            processing <literal>AccessDeniedException</literal>s. If an
                <literal>AccessDeniedException</literal> is thrown, and the authentication is of an
            anonymous type, instead of throwing a 403 (forbidden) response, the filter will instead
            commence the <interfacename>AuthenticationEntryPoint</interfacename> so the principal
            can authenticate properly. This is a necessary distinction, otherwise principals would
            always be deemed <quote>authenticated</quote> and never be given an opportunity to login
            via form, basic, digest or some other normal authentication mechanism. </para>
        <para> You will often see the <literal>ROLE_ANONYMOUS</literal> attribute in the above
            interceptor configuration replaced with <literal>IS_AUTHENTICATED_ANONYMOUSLY</literal>.
            This is an example of the use of the <classname>AuthenticatedVoter</classname> which
            will see in ???. It uses an <interfacename>AuthenticationTrustResolver</interfacename>
            to process this particular configuration attribute and grant access to aonymous users.
        </para>
    </section>
</chapter>
