# Core Services

Now that we have a high-level overview of the Spring Security architecture and its core classes, let's take a closer look at one or two of the core interfaces and their implementations, in particular the AuthenticationManager, UserDetailsService and the AccessDecisionManager. These crop up regularly throughout the remainder of this document so it's important you know how they are configured and how they operate.

## The AuthenticationManager, `ProviderManager` and `AuthenticationProvider`s

The AuthenticationManager is just an interface, so the implementation can be anything we choose, but how does it work in practice? What if we need to check multiple authentication databases or a combination of different authentication services such as a database and an LDAP server?

The default implementation in Spring Security is called `ProviderManager` and rather than handling the authentication request itself, it delegates to a list of configured `AuthenticationProvider`s, each of which is queried in turn to see if it can perform the authentication. Each provider will either throw an exception or return a fully populated Authentication object. Remember our good friends, UserDetails and UserDetailsService? If not, head back to the previous chapter and refresh your memory. The most common approach to verifying an authentication request is to load the corresponding UserDetails and check the loaded password against the one that has been entered by the user. This is the approach used by the `DaoAuthenticationProvider` (see below). The loaded UserDetails object - and particularly the `GrantedAuthority`s it contains - will be used when building the fully populated Authentication object which is returned from a successful authentication and stored in the `SecurityContext`.

If you are using the namespace, an instance of `ProviderManager` is created and maintained internally, and you add providers to it by using the namespace authentication provider elements (see the namespace chapter). In this case, you should not declare a `ProviderManager` bean in your application context. However, if you are not using the namespace then you would declare it like so:

    <bean id="authenticationManager"
         class="org.springframework.security.authentication.ProviderManager">
      <property name="providers">
        <list>
          <ref local="daoAuthenticationProvider"/>
          <ref local="anonymousAuthenticationProvider"/>
          <ref local="ldapAuthenticationProvider"/>
        </list>
      </property>
    </bean>

In the above example we have three providers. They are tried in the order shown (which is implied by the use of a `List`), with each provider able to attempt authentication, or skip authentication by simply returning `null`. If all implementations return null, the `ProviderManager` will throw a ProviderNotFoundException. If you're interested in learning more about chaining providers, please refer to the `ProviderManager` JavaDocs.

Authentication mechanisms such as a web form-login processing filter are injected with a reference to the ProviderManager and will call it to handle their authentication requests. The providers you require will sometimes be interchangeable with the authentication mechanisms, while at other times they will depend on a specific authentication mechanism. For example, `DaoAuthenticationProvider` and `LdapAuthenticationProvider` are compatible with any mechanism which submits a simple username/password authentication request and so will work with form-based logins or HTTP Basic authentication. On the other hand, some authentication mechanisms create an authentication request object which can only be interpreted by a single type of `AuthenticationProvider`. An example of this would be JA-SIG CAS, which uses the notion of a service ticket and so can therefore only be authenticated by a `CasAuthenticationProvider`. You needn't be too concerned about this, because if you forget to register a suitable provider, you'll simply receive a `ProviderNotFoundException` when an attempt to authenticate is made.

### Erasing Credentials on Successful Authentication

By default (from Spring Security 3.1 onwards) the `ProviderManager` will attempt to clear any sensitive credentials information from the Authentication object which is returned by a successful authentication request. This prevents information like passwords being retained longer than necessary.

This may cause issues when you are using a cache of user objects, for example, to improve performance in a stateless application. If the Authentication contains a reference to an object in the cache (such as a UserDetails instance) and this has its credentials removed, then it will no longer be possible to authenticate against the cached value. You need to take this into account if you are using a cache. An obvious solution is to make a copy of the object first, either in the cache implementation or in the AuthenticationProvider which creates the returned Authentication object. Alternatively, you can disable the `eraseCredentialsAfterAuthentication` property on `ProviderManager`. See the Javadoc for more information.

### `DaoAuthenticationProvider`

The simplest AuthenticationProvider implemented by Spring Security is `DaoAuthenticationProvider`, which is also one of the earliest supported by the framework. It leverages a UserDetailsService (as a DAO) in order to lookup the username, password and GrantedAuthoritys. It authenticates the user simply by comparing the password submitted in a `UsernamePasswordAuthenticationToken` against the one loaded by the UserDetailsService. Configuring the provider is quite simple:

    <bean id="daoAuthenticationProvider"
        class="org.springframework.security.authentication.dao.DaoAuthenticationProvider">
      <property name="userDetailsService" ref="inMemoryDaoImpl"/>
      <property name="saltSource" ref bean="saltSource"/>
      <property name="passwordEncoder" ref="passwordEncoder"/>
    </bean>

The PasswordEncoder and SaltSource are optional. A PasswordEncoder provides encoding and decoding of passwords presented in the UserDetails object that is returned from the configured UserDetailsService. A SaltSource enables the passwords to be populated with a "salt", which enhances the security of the passwords in the authentication repository. These will be discussed in more detail below.

## UserDetailsService Implementations

As mentioned in the earlier in this reference guide, most authentication providers take advantage of the UserDetails and UserDetailsService interfaces. Recall that the contract for UserDetailsService is a single method:

      UserDetails loadUserByUsername(String username) throws UsernameNotFoundException;

The returned UserDetails is an interface that provides getters that guarantee non-null provision of authentication information such as the username, password, granted authorities and whether the user account is enabled or disabled. Most authentication providers will use a UserDetailsService, even if the username and password are not actually used as part of the authentication decision. They may use the returned UserDetails object just for its `GrantedAuthority` information, because some other system (like LDAP or X.509 or CAS etc) has undertaken the responsibility of actually validating the credentials.

Given UserDetailsService is so simple to implement, it should be easy for users to retrieve authentication information using a persistence strategy of their choice. Having said that, Spring Security does include a couple of useful base implementations, which we'll look at below.

### In-Memory Authentication

Is easy to use create a custom UserDetailsService implementation that extracts information from a persistence engine of choice, but many applications do not require such complexity. This is particularly true if you're building a prototype application or just starting integrating Spring Security, when you don't really want to spend time configuring databases or writing UserDetailsService implementations. For this sort of situation, a simple option is to use the `user-service` element from the security namespace:

      <user-service id="userDetailsService">
        <user name="jimi" password="jimispassword" authorities="ROLE_USER, ROLE_ADMIN" />
        <user name="bob" password="bobspassword" authorities="ROLE_USER" />
      </user-service>

This also supports the use of an external properties file:

      <user-service id="userDetailsService" properties="users.properties"/>

The properties file should contain entries in the form

    username=password,grantedAuthority[,grantedAuthority][,enabled|disabled]

For example

     jimi=jimispassword,ROLE_USER,ROLE_ADMIN,enabled
     bob=bobspassword,ROLE_USER,enabled

### `JdbcDaoImpl`

Spring Security also includes a UserDetailsService that can obtain authentication information from a JDBC data source. Internally Spring JDBC is used, so it avoids the complexity of a fully-featured object relational mapper (ORM) just to store user details. If your application does use an ORM tool, you might prefer to write a custom UserDetailsService to reuse the mapping files you've probably already created. Returning to `JdbcDaoImpl`, an example configuration is shown below:

    <bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
      <property name="driverClassName" value="org.hsqldb.jdbcDriver"/>
      <property name="url" value="jdbc:hsqldb:hsql://localhost:9001"/>
      <property name="username" value="sa"/>
      <property name="password" value=""/>
    </bean>

    <bean id="userDetailsService"
          class="org.springframework.security.core.userdetails.jdbc.JdbcDaoImpl">
      <property name="dataSource" ref="dataSource"/>
    </bean>

You can use different relational database management systems by modifying the `DriverManagerDataSource` shown above. You can also use a global data source obtained from JNDI, as with any other Spring configuration.

#### Authority Groups

By default, `JdbcDaoImpl` loads the authorities for a single user with the assumption that the authorities are mapped directly to users (see the database schema appendix). An alternative approach is to partition the authorities into groups and assign groups to the user. Some people prefer this approach as a means of administering user rights. See the `JdbcDaoImpl` Javadoc for more information on how to enable the use of group authorities. The group schema is also included in the appendix.

## Password Encoding

Spring Security's PasswordEncoder interface is used to support the use of passwords which are encoded in some way in persistent storage. This will normally mean that the passwords are hashed using a digest algorithm such as MD5 or SHA.

### What is a hash?

Password hashing is not unique to Spring Security but is a common source of confusion for users who are not familiar with the concept. A hash (or digest) algorithm is a one-way function which produces a piece of fixed-length output data (the hash) from some input data, such as a password. As an example, the MD5 hash of the string password (in hexadecimal) is

        5f4dcc3b5aa765d61d8327deb882cf99

A hash is one-way in the sense that it is very difficult (effectively impossible) to obtain the original input given the hash value, or indeed any possible input which would produce that hash value. This property makes hash values very useful for authentication purposes. They can be stored in your user database as an alternative to plaintext passwords and even if the values are compromised they do not immediately reveal a password which can be used to login. Note that this also means you have no way of recovering the password once it is encoded.

### Adding Salt to a Hash

One potential problem with the use of password hashes that it is relatively easy to get round the one-way property of the hash if a common word is used for the input. For example, if you search for the hash value `5f4dcc3b5aa765d61d8327deb882cf99` using google, you will quickly find the original word password. In a similar way, an attacker can build a dictionary of hashes from a standard word list and use this to lookup the original password. One way to help prevent this is to have a suitably strong password policy to try to prevent common words from being used. Another is to use a salt when calculating the hashes. This is an additional string of known data for each user which is combined with the password before calculating the hash. Ideally the data should be as random as possible, but in practice any salt value is usually preferable to none. Spring Security has a SaltSource interface which can be used by an authentication provider to generate a salt value for a particular user. Using a salt means that an attacker has to build a separate dictionary of hashes for each salt value, making the attack more complicated (but not impossible).

### Hashing and Authentication

When an authentication provider (such as Spring Security's `DaoAuthenticationProvider` needs to check the password in a submitted authentication request against the known value for a user, and the stored password is encoded in some way, then the submitted value must be encoded using exactly the same algorithm. It's up to you to check that these are compatible as Spring Security has no control over the persistent values. If you add password hashing to your authentication configuration in Spring Security, and your database contains plaintext passwords, then there is no way authentication can succeed. Even if you are aware that your database is using MD5 to encode the passwords, for example, and your application is configured to use Spring Security's `Md5PasswordEncoder`, there are still things that can go wrong. The database may have the passwords encoded in Base 64, for example while the enocoder is using hexadecimal strings (the default) You can configure the encoder to use Base 64 instead of hex by setting the `encodeHashAsBase64` property to `true`. Check the Javadoc for `MessageDigestPasswordEncoder` and its parent classes for more information. . Alternatively your database may be using upper-case while the output from the encoder is lower-case. Make sure you write a test to check the output from your configured password encoder with a known password and salt combination and check that it matches the database value before going further and attempting to authenticate through your application. For more information on the default method for merging salt and password, see the Javadoc for `BasePasswordEncoder`. If you want to generate encoded passwords directly in Java for storage in your user database, then you can use the `encodePassword` method on the PasswordEncoder.