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="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.

From Spring Security 3.0.3, you can configure the ProviderManager will attempt to clear any sensitive credentials information from the Authentication object which is returned by a successful authentication request, to prevent information like passwords being retained longer than necessary. This feature is controlled by the eraseCredentialsAfterAuthentication property on ProviderManager. It is off by default. See the Javadoc for more information.

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.

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

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.

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.

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).

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)^[6]. 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.