How do you encrypt a password?

I hate when people ask me this question, because I inevitably respond with a half-dozen questions of my own, which makes me seem like a bit of an arse.

To reduce that feeling, because the questions don’t seem to be going away any time soon, I thought I’d write some thoughts out.

Put enough locks on a thing, it's secure. Or it collapses under the weight of the locks.

Do you want those passwords in the first place?

Passwords are important objects – and because people naturally share IDs and passwords across multiple services, your holding on to a customer’s / user’s password means you are a necessary part of that user’s web of credential storage.

It will be a monumental news story when your password database gets disclosed or leaked, and even more of a story if you’ve chosen a bad way of protecting that data. You will lose customers and you will lose business; you may even lose your whole business.

Take a long hard look at what you’re doing, and whether you actually need to be in charge of that kind of risk.

Do you need those passwords to verify a user or to impersonate them?

If you are going to verify a user, you don’t need encrypted passwords, you need hashed passwords. And those hashes must be salted. And the salt must be large and random. I’ll explain why some other time, but you should be able to find much documentation on this topic on the Internet. Specifically, you don’t need to be able to decrypt the password from storage, you need to be able to recognise it when you are given it again. Better still, use an acknowledged good password hashing mechanism like PBKDF2. (Note, from the “2” that it may be necessary to update this if my advice is more than a few months old)

Now, do not read the rest of this section – skip to the next question.

Seriously, what are you doing reading this bit? Go to the heading with the next question. You don’t need to read the next bit.

<sigh/>

OK, if you are determined that you will have to impersonate a user (or a service account), you might actually need to store the password in a decryptable form.

First make sure you absolutely need to do this, because there are many other ways to impersonate an incoming user using delegation, etc, which don’t require you storing the password.

Explore delegation first.

Finally, if you really have to store the password in an encrypted form, you have to do it incredibly securely. Make sure the key is stored separately from the encrypted passwords, and don’t let your encryption be brute-forcible. A BAD way to encrypt would be to simply encrypt the password using your public key – sure, this means only you can decrypt it, but it means anyone can brute-force an encryption and compare it against the ciphertext.

A GOOD way to encrypt the password is to add some entropy and padding to it (so I can’t tell how long the password was, and I can’t tell if two users have the same password), and then encrypt it.

Password storage mechanisms such as keychains or password vaults will do this for you.

If you don’t have keychains or password vaults, you can encrypt using a function like Windows’ CryptProtectData, or its .NET equivalent, System.Security.Cryptography.ProtectedData.

[Caveat: CryptProtectData and ProtectedData use DPAPI, which requires careful management if you want it to work across multiple hosts. Read the API and test before deploying.]

[Keychains and password vaults often have the same sort of issue with moving the encrypted password from one machine to another.]

For .NET documentation on password vaults in Windows 8 and beyond, see: Windows.Security.Credentials.PasswordVault

For non-.NET on Windows from XP and later, see: CredWrite

For Apple, see documentation on Keychains

Can you choose how strong those passwords must be?

If you’re protecting data in a business, you can probably tell users how strong their passwords must be. Look for measures that correlate strongly with entropy – how long is the password, does it use characters from a wide range (or is it just the letter ‘a’ repeated over and over?), is it similar to any of the most common passwords, does it contain information that is obvious, such as the user’s ID, or the name of this site?

Maybe you can reward customers for longer passwords – even something as simple as a “strong account award” sticker on their profile page can induce good behaviour.

Length is mathematically more important to password entropy than the range of characters. An eight character password chosen from 64 characters (less than three hundred trillion combinations – a number with 4 commas) is weaker than a 64 character password chosen from eight characters (a number of combinations with 19 commas in it).

An 8-character password taken from 64 possible characters is actually as strong as a password only twice as long and chosen from 8 characters – this means something like a complex password at 8 characters in length is as strong as the names of the notes in a couple of bars of your favourite tune.

Allowing users to use password safes of their own makes it easier for them to use longer and more complex passwords. This means allowing copy and paste into password fields, and where possible, integrating with any OS-standard password management schemes

What happens when a user forgets their password?

Everything seems to default to sending a password reset email. This means your users’ email address is equivalent to their credential. Is that strength of association truly warranted?

In the process to change my email address, you should ask me for my password first, or similarly strongly identify me.

What happens when I stop paying my ISP, and they give my email address to a new user? Will they have my account on your site now, too?

Every so often, maybe you should renew the relationship between account and email address – baselining – to ensure that the address still exists and still belongs to the right user.

Do you allow password hints or secret questions?

Password hints push you dangerously into the realm of actually storing passwords. Those password hints must be encrypted as well as if they were the password themselves. This is because people use hints such as “The password is ‘Oompaloompah’” – so, if storing password hints, you must encrypt them as strongly as if you were encrypting the password itself. Because, much of the time, you are. And see the previous rule, which says you want to avoid doing that if at all possible.

Other questions that I’m not answering today…

How do you enforce occasional password changes, and why?

What happens when a user changes their password?

What happens when your password database is leaked?

What happens when you need to change hash algorithm?

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