10. Core Services
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## 10. 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.

## 10.1 The AuthenticationManager, ProviderManager and AuthenticationProvider

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">
    	<constructor-arg>
    		<list>
    			<ref local="daoAuthenticationProvider"/>
    			<ref local="anonymousAuthenticationProvider"/>
    			<ref local="ldapAuthenticationProvider"/>
    		</list>
    	</constructor-arg>
    </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` Javadoc.

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.

### 10.1.1 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.

### 10.1.2 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 `GrantedAuthority` s. 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="passwordEncoder" ref="passwordEncoder"/>
    </bean>

The `PasswordEncoder` is optional. A `PasswordEncoder` provides encoding and decoding of passwords presented in the `UserDetails` object that is returned from the configured `UserDetailsService`. This will be discussed in more detail below.

## 10.2 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.

### 10.2.1 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

### 10.2.2 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.

## 10.3 Password Encoding

Spring Security’s `PasswordEncoder` interface is used to support the use of passwords which are encoded in some way in persistent storage. You should never store passwords in plain text. Always use a one-way password hashing algorithm such as bcrypt which uses a built-in salt value which is different for each stored password. Do not use a plain hash function such as MD5 or SHA, or even a salted version. Bcrypt is deliberately designed to be slow and to hinder offline password cracking, whereas standard hash algorithms are fast and can easily be used to test thousands of passwords in parallel on custom hardware. You might think this doesn’t apply to you since your password database is secure and offline attacks aren’t a risk. If so, do some research and read up on all the high-profile sites which have been compromised in this way and have been pilloried for storing their passwords insecurely. It’s best to be on the safe side. Using `org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder"` is a good choice for security. There are also compatible implementations in other common programming languages so it a good choice for interoperability too.

If you are using a legacy system which already has hashed passwords, then you will need to use an encoder which matches your current algorithm, at least until you can migrate your users to a more secure scheme (usually this will involve asking the user to set a new password, since hashes are irreversible). Spring Security has a package containing legacy password encoding implementation, namely, `org.springframework.security.authentication.encoding`. The `DaoAuthenticationProvider` can be injected with either the new or legacy `PasswordEncoder` types.

### 10.3.1 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.

### 10.3.2 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. People tend to choose similar passwords and huge dictionaries of these from previously hacked sites are available online. 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. 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).

Bcrypt automatically generates a random salt value for each password when it is encoded, and stores it in the bcrypt string in a standard format.

| Note
The legacy approach to handling salt was to inject a `SaltSource` into the `DaoAuthenticationProvider`, which would obtain a salt value for a particular user and pass it to the `PasswordEncoder`. Using bcrypt means you don’t have worry about the details of salt handling (such as where the value is stored), as it is all done internally. So we’d strongly recommend you use bcrypt unless you already have a system in place which stores the salt separately.

### 10.3.3 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 encoder is using hexadecimal strings (the default). 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. Using a standard like bcrypt will avoid these issues.

If you want to generate encoded passwords directly in Java for storage in your user database, then you can use the `encode` method on the `PasswordEncoder`.

## 10.4 Jackson Support

Spring Security has added Jackson Support for persisting Spring Security related classes. This can improve the performance of serializing Spring Security related classes when working with distributed sessions (i.e. session replication, Spring Session, etc).

To use it, register the `JacksonJacksonModules.getModules(ClassLoader)` as Jackson Modules.

    ObjectMapper mapper = new ObjectMapper();
    ClassLoader loader = getClass().getClassLoader();
    List<Module> modules = SecurityJackson2Modules.getModules(loader);
    mapper.registerModules(modules);

    // ... use ObjectMapper as normally ...
    SecurityContext context = new SecurityContextImpl();
    // ...
    String json = mapper.writeValueAsString(context);

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