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Hashing and Salting Passwords with Spring Security 2 - Willie Wheeler
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Problem

You want to store passwords securely to prevent anybody—even yourself and other technical staff—from being able to view them.

Solution

In this recipe we're going to use Spring Security 2 to store passwords securely. We'll also show how to authenticate against secured passwords.

Spring Security makes it fairly easy to store passwords securely. The actual storage mechanism (database, LDAP or other) doesn't really matter since that's abstracted away, which is nice.

Before we jump into the how-to part of the recipe, let's briefly examine hash functions and how they work, since we'll need this background to understand what we're doing in the subsequent discussion.

Understanding hash functions

The idea behind this recipe is that we want to store passwords in encrypted form in the persistent store. There are in general two different approaches to encryption here: ciphers and hash functions. The two are similar in that each provides a way to encrypt data, but they differ in that only ciphers provide an easy way (assuming you know a secret key) to decrypt the data. Hash functions don't provide for easy decryption; hence they are sometimes referred to as one-way hash functions. All hash functions, however, are one-way.

When storing encrypted passwords, the typical practice is to use a hash function, such as MD5 or SHA-1, to encrypt the password before saving it. The reason for preferring a hash function to a cipher is that we don't usually want anybody to be able to recover the password: we never (or should never) display it on the screen or in e-mails, for example. A string of plaintext encrypted by a hash function is called a hash.

Here are a couple of examples of hashed text:

  • 4fe0e930dd0adf9daaba0b7219bbe1f1bbb7809c
  • 3af00c6cad11f7ab5db4467b66ce503e

The first string is an SHA-1 hash of the string dardy, and the second is an MD5 hash of the string friend. You can easily Google for free online hash calculators, both for MD5 and for SHA-1.

You might ask how we can use stored password hashes to authenticate users if we can't recover the plaintext password from the hash. The answer is that we don't compare plaintext passwords, but rather we compare the hashes. When the user submits a username/password pair, we hash the submitted password and compare that hash with the stored hash. If the two match, then we conclude that the submitted password was correct.

We can draw this conclusion due to a special property of hash functions: in general, different plaintext inputs map to different hashes. That is, in general, different plaintext inputs very, very rarely "collide." There are of course limits to this non-collision, but for practical purposes we can assume that different passwords will have different hashes. So if the hashes match, then the submitted password was correct.

Sound good? Let's start with the authentication side of things, and then we'll tackle user registrations.

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Comments (18)

Very nice article Willie... thanks
By Hema on Oct 12, 2008 at 8:44 PM PDT
Good work Dudes
By Mustafa Sait Özen on Oct 12, 2008 at 11:57 PM PDT

Nice article. Fills in a few gaps in the Spring Security documentation!

By Ross Duncan on Feb 25, 2009 at 9:35 AM PST

Hello Willie,

Do I need a custom UserDetailsServiceImpl for this?

Because I tried to do this with JdbcDaoImpl, like this:

<-- 2 -- >

class="org.springframework.security.userdetails.jdbc.JdbcDaoImpl"

p:userDao="accountDao" />

but I received a NotWritablePropertyException:

SEVERE: Servlet /FlexServer threw load() exception org.springframework.beans.NotWritablePropertyException: Invalid property 'userDao' of bean class [org.springframework.security.userdetails.jdbc.JdbcDaoImpl]: Bean property 'userDao' is not writable or has an invalid setter method. Does the parameter type of the setter match the return type of the getter? at org.springframework.beans.BeanWrapperImpl.setPropertyValue(BeanWrapperImpl.java:787) at org.springframework.beans.BeanWrapperImpl.setPropertyValue(BeanWrapperImpl.java:643)

Thanks in advance, and for this very interesting article (as usual),

Greets, Jochen

By Jochen Szostek on Jun 13, 2009 at 2:00 PM PDT

Correct me if i am wrong but the line number 5 in listing 4:

Object salt = saltSource.getSalt(userDetails); 

is bound to return in the variable salt the Long representing the id of the object account. So why not use the id directly. That way we would simplify the code to something like this:

public void registerAccount(Account account) {  
accountDao.save(account);  
account.setPassword(passwordEncoder.encodePassword(password,  account.getId())); 
accountDao.save(account);  
} 

Am I oversimplifying. Please correct me if I am wrong since I will be using this stuff in a project. Thanks for the nice article.

Stole

By stole on Aug 20, 2009 at 1:52 PM PDT

I was a little hasty while submitting the post previously. The code should have read:

public void registerAccount(Account account) {  
 accountDao.save(account);  
 account.setPassword(
    passwordEncoder.encodePassword(account.getPassword(), account.getId()));
 accountDao.save(account);
} 

Stole

By stole on Aug 20, 2009 at 2:27 PM PDT

Ah. Sometimes one has to say a lot more in order not to repeat oneself.

By stole on Aug 20, 2009 at 10:19 PM PDT

The answer as to why you shouldn't change the code is simple. It may seam as a simplification to you, but in fact doing what you're proposing will make it harder to change the salt afterwards.

In case you want to switch from the id to something else as the salt you could simply change it in your security config.

p:userPropertyToUse="id" /> 
By muskatus on Sep 15, 2009 at 9:25 AM PDT

The main reason I would offer is this. There are two cases where you access the password: during account creation, and during authentication. A DaoAuthenticationProvider handles the authentication case and to use that, you need to inject it with a salt source. By using that same salt source during registration, you don't have to worry about manually syncing the two cases in the event that you decide to use something other than the ID: you would just change the XML configuration of the salt source itself.

Know that response is pretty late but hopefully it helps anyway.

By Willie Wheeler on Sep 16, 2009 at 12:40 AM PDT

...another way to say the same thing is that this is basically a DRY move.

By Willie Wheeler on Sep 16, 2009 at 12:45 AM PDT

I initially found your article because I couldn't find any documentation in Spring on how they mix the salt with the password. Thanks for letting me know about the braces e.g. password {salt}. By the way, how did you find that out? I'm frustrated that I couldn't find that from my reading of the documentation.

Decided to read your article and it cleared up a lot of confusion I had about the configuration - especially surrounding the authentication provider. I was declaring an authentication manager bean and the necessary list of providers. Nice to know I didn't need to do that but simply had to use the security:custom-authentication-provider tag!

Great article - thank you.

PUK

By PUK on Oct 27, 2009 at 8:02 AM PDT

I've got a question about configuration. I've got two configuration files: security in one xml config (referenced from web.xml), and another xml config for the main dispatcher servlet (dispatcher-servlet.xml).

I already have a userDAO which is configured in the dispatcher-servlet.xml. This needs to be dependency injected into the UserDetailsService so I've repeated the userDAO bean in the security xml config too. This repitition is bad.

Additionally, I want to use the password encoder and salt source within a controller bean. Therefore I need to DI the password encoder and salt source beans into the controller bean. They're defined in different xml files.

How does someone get around this problem?

Thanks,

PUK

By PUK on Oct 27, 2009 at 8:20 AM PDT

@PUK: Glad to know the article helped. Yeah, on the namespace config, that's a pretty nice feature. Behind the scenes it's still creating all the beans you'd normally create, but in effect the namespace config amounts to a domain-specific language that makes config more convenient and intuitive.

As far as finding out about the braces, it's been far enough back in time that I can't say for sure, but I'm about 90% confident that I had to dig into the source for that little gem. :-)

On your UserDao question and password encoder questions, I agree with you that DRY applies here. What I would do in the case you describe is have three separate bean config files:

  1. an applicationContext.xml for high-reuse object like the UserDao
  2. an applicationContext-security.xml for security objects so you can take advantage of the XML default namespace feature (so you don't have to repeat the "security:" prefix everywhere, and
  3. a main-servlet.xml (or whatever you want to call it) for your web config. The web config will be able to see your applicationContext.xml but not vice versa.

Just make sure you load the app configs from the context loader instead of from the DispatcherServlet.

By Willie Wheeler on Oct 27, 2009 at 11:48 AM PDT

@Willie: I really appreciate the immediate response! Previous to reading it I had arrived at this solution:

  1. applicationContext.xml (basically empty)
  2. commonBeans.xml (userDao and a couple of other beans)
  3. applicationContext-security.xml (import on commonBeans.xml followed by security configs)
  4. dispatcher-servlet.xml (import on commonBeans.xml followed by servlet configs)

This solution was born out of my lack of knowledge and assumptions of the contexts and configuration files. I had figured that each xml file might only be able to reference beans known to its file. I was also unsure about the hierarchy of the contexts.

Obviously I was worried about my "solution" because of the duplication of beans in memory and this was set to grow because security and servlet both need message sources from i18n, etc).

The bit in your response about the web config being able to see the applicationContext.xml was a revelation. The common stuff is in applicationContext.xml and it all works very well.

I know you must be very busy with books, blogs, and your own development so thanks again!

PUK

By PUK on Nov 1, 2009 at 2:04 AM PST

Hi, I want my password to be hashed and stored in a xml file using a simple hash function may be using MD5. Request you to please provide me some sample code for the same to hash my passowrd and store it in a xml file. Thanks Amit

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