Storing Passwords Safely

Steal passwordIf your application deals with user accounts, it has to deal with passwords. Storing passwords in plain text would be a bad idea; a data breach could allow an attacker access to every account. The obvious answer is to encrypt the passwords. However, using cryptography without understanding could give you a false sense of security—if you make the inappropriate choice, you could make things easier for an attacker without realizing it. This article will focus on getting you up to speed with the best ways to use cryptography to secure passwords.

Fundamentals

When people think encryption, they usually think of schemes that allow you to send a file encrypted with a key and decrypt it with the same or a different key. This is useful for most data, but not with passwords, because you don’t want anyone to decrypt them. Instead, use one-way algorithms; better known as hashing.

The recommended hashing algorithms nowadays are SHA-2 and SHA-3; while many people think of MD5 and SHA-1, these have weaknesses that make them not recommended for new applications. With hashes, your users only need to provide the original value, and you store only the hashed value. Then you hash the value provided by the user and compare it to the hashed value you stored earlier. If the hashed values match, it’s valid.

Simply hashing passwords isn’t enough. If you just have hashes, someone could calculate hashes for common passwords ahead of time and see which ones matched. Large lists of pre-computed hashes are called rainbow tables, and are often handed around. You should also add a salt (an arbitrary secret string) in addition to the password before hashing. By storing both the salt and the hashed password, it makes it harder for attackers to be able to guess the contents of the password, because they need to include the salt as well.

Algorithms designed for passwords

Hashing algorithms can also be tuned for passwords. Most hashing algorithms are designed for speed in addition to security, because they’re often used for checking the integrity of lots of large files. For password hashing, you don’t want speed. Validating a hash on such a small amount of data doesn’t need to be quick. If it is quick, then it makes it easier for attackers to try to generate more and more passwords. Thus, algorithms designed for passwords will try to do things like running multiple iterations behind the scenes. The recommended algorithms for password hashing are argon2 and bcrypt, which take password-related factors into account. When you have the choice, argon2 is better than bcrypt due its taking into account new advances in cryptography research.

It’s worth noting that you shouldn’t write your own crypto code unless you know exactly what you’re doing. It’s very easy to introduce subtleties that can compromise the security of the system. When possible, leave the implementation to the experts. Most languages, libraries, or OSes will include a facility to easily do a specific task like hashing passwords. If they don’t, they’ll give you lower-level ways to access modern cryptography algorithms; still, take caution when doing so.

How to hash passwords in PHP

PHP provides built-in functions to make password hashing easy, secure, and error-free. You create passwords with password_hash, and verify them with password_verify. These functions are hard to misuse and do the right thing by default, including upgrading to better algorithms when available. For example, let’s make a password:

In: var_dump(password_hash("hunter2", PASSWORD_DEFAULT)); 
Out: string(60) "$2y$10$Hw1zOtl.AW0rXgNJAoZ2JufKoifJJM.8AcA3Ox/xe2D/uE4WE0F/."

When you call password_hash(), it returns a single value you can simply stuff inside a database. This value packs all the information needed to verify the password later; the algorithm, version, salt, and hash. PHP will even generate the salt string for you, so you don’t need to risk reusing salts. It will also pick an algorithm. Later, when you verify a password, all you need is the password the user gave you, and the result of password_hash that you stored somewhere, passed to password_verify():

In: var_dump(password_verify("hunter2", '$2y$10$Hw1zOtl.AW0rXgNJAoZ2JufKoifJJM.8AcA3Ox/xe2D/uE4WE0F/.'));
Out: bool(true)
In: var_dump(password_verify("hunter3", '$2y$10$Hw1zOtl.AW0rXgNJAoZ2JufKoifJJM.8AcA3Ox/xe2D/uE4WE0F/.'));
Out: bool(false)

With this approach, there’s no risk you’ll use raw hashing primitives and accidentally get it wrong. Even if your user table is somehow compromised by an attacker, they can’t easily take all the passwords with it.

Protect your passwords the right way

By using trusted hashing functions that reflect the latest security principles, you can stay a step ahead of attackers who would like to have your passwords. Don’t try to write your own encryption or hashing functions, though. Professional solutions are available–use them.

If you would like more information, or if your application might benefit from a security audit, get in touch.

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