Monday, 21 June 2010


Passwords are now so everyday that remembering a dozen or so long, complex password doesn't phase anyone. But why do passwords have to be so long and so complex? Well, you don't want to bad guys to be able to guess it or to apply brute force to 'crack' it and gain access to your private data, do you?

There are many factors that influence the strength of a password and each of those factors used together can help protect your credentials and thus your data. If you're still in doubt over the knowledge your users have around protecting their password and their password strength, then you can always educate them.

  • Show them your password policy and explain to them why it is how it is.
  • Explain to them the pitfalls of clicking on links in emails.
  • Explain installing random software can be invasive and dangerous.
  • Provide links to tools that can help them generate a strong password, or to essays or research papers. Or spend a day writing your own, summarising all the facts important to your business.

This post is designed to be a guide to how to help you decide how to implement safe storage of passwords. It is not meant to be complete one-size-fits-all policy document, though it could be used as a basis for creating a password policy specific to your needs. I've also attempted to give some positive and negative impacts for each area.

Length, or "size is important"

Password length is the most important factor in password strength. Forget entropy, make your password as long as you can. Many policies enforce a minimum length of mixed case, numbers and special characters, and some even impose a maximum length.

The minimum length is important, but please make it longer than 8, which is extremely common. I don't understand why anyone would want to restrict the maximum length of your password. At the end of the day the resultant hash is of a fixed length, so longer user passwords won't have any negative impact on storage concerns. Provide a large characters set from which to chose the password but there is no real need to force a user to use the upper/lower/numeric/special characters.

+ve longer passwords increase complexity, slowing down any brute force attack

-ve the longer the password the more likely it will be to be forgotten by the user resulting in more password resets which could result in more helpdesk calls.
-ve a user not being able to log in at a given time could impact time sensitive sales

Lifetime, or "all change please"

You should impose a maximum lifetime of a password. You could implement expiration such that after a given period of time you prompt the user to change their password. You could do this at login, or you could send them a mail prompting them to change their password, maybe providing a link to the reset page.
It may be worth adding historical knowledge of passwords used. When a user is prompted to change their password according to the lifetime policy the new password should not match any of the previous passwords used for the account. This eliminates the possibility of a stolen password database where the passwords have been expired and thus becoming stale, to then becoming valid again.

+ve any leaked password DB would only have a finite lifetime as the passwords would be stale after a given period of time
+ve informing the user to change their password may increase customer confidence and show you take security seriously.

-ve implementation of an expiration mechanism would have to be coded, which takes time and resource
-ve nagging the user may have a negative impact on user experience

Lockout, or "add some treacle"

It is quite common for hackers to brute force a live system. Obviously you've already implement a set of rules that will alert you to such attempts, along with your detection of cross-site scripting monitoring and alerting. You can take a couple of approaches here. You can lock accounts after a specified number of failed login attempts. Any account that has been locked out would require administrative intervention to re-enable the account.

+ve this will quickly stop any brute force attempts

-ve calls to helpdesk have a cost attributed to them, businesses are in business to make money

An alternative is to add some treacle. After a given number of failed attempts, apply a pause, a delay until the next login attempt can be made.

+ve slows the brute force attempt down

Salting, or "pass the salt"

Appending a salt to the password prior to applying the hash function has the effect of lengthening the password. You should generate a different salt for each password. If you can store the salt in a separate database, then great, that will slow a brute force attempt on a leaked database and storing salts in a different databases reduces the likelihood of the hash and the salt being stolen at the same time. Added the salt slows down the hash brute force, it may even take so long that the password has gone stale by the time it is cracked. There is no harm in rotating salts each time the user logs in. This could be implemented a an asynchronously (thus not affecting the user experience) strengthening the argument for storing the salt in a different database to the hash; if the theft of the two databases is not synchronised, then the hacker doesn't have the expected salt for a given user.  You can always append an additional, option application salt. This adds one more pieces to the puzzle that the attack must solve.  Store this somewhere that the salt, the final hash or the algorithm is not stored, thus requiring the attack to gain access to additional systems.

+ve slows down a brute force attempt

Key strengthening, or "stretch, and relax"

Employ a form of key stretching will strengthen any given password. Key stretching, by design executes slowly; introducing a delay to the hashing process introduces the same delay in the brute forcing process.

+ve hacker needs to know how the algorithm is implemented, and for that they would need access to source code
+ve significantly slows down any brute forcing

-ve slowing down the password verification _could_ impact user experience when logging in
initialise key
loop 100,000
        key = hash(key + password + salt)

Why hash all 3? This reduces the chances of collisions

Cryptographic functions, or "don't make a hash of it"

Choose your hashing function you probably want to use SHA-2, though judging by the rate of hash function breakage, you'll want to ensure you can quickly and easily upgrade to the latest SHA implementation; so prepare for SHA-3.

Complexity indicator, or "a show of strength"

It is always good to give your users a visual indication of how strong their password is. You can add a graphical indicator on the registration page and on password reset page. Though I would not retrospectively send my users an email telling them that they must change their password as it has found to be under strength. This would give the impression that somehow you have visibility of it.

I (don't) want to do whatever common people do

Common passwords have been collated for years. For example, 123456 was the #1 common password for years. There are multiple sources where you can download lists for the top 100, 1,000, 10,000, 100,000 passwords. Use one of these lists to ensure your users don't.

Sensible storage, or "Cover me, I need back up!"

Back up source code which will contain the algorithm, the per password salt, application salt and the resultant hash in isolation. There is no point is keeping them all separate in production if you then back themup to the same location. If you can, encrypt your back up.


There are many ways to increase the strength of passwords at rest and how you influence and educate your users in choosing strong passwords in the first place. There is no one right way and no one size fits all.
  • Educate your users/customers about secure passwords, they'll thank you for it.
  • Weigh up what is the right solution for your requirements.
  • Leave comments/suggestions/advice below (-;

No comments:

Post a Comment