Use crypt() for password storage

Update: This wiki has been rewritten to be in line with Yii 1.1.14. Since many of the detailed complexities are now handled by Yii, the article focuses on how the crypt() built-in function works and why it's important to use it correctly.

Storing passwords in php web apps

  1. Using PHP's crypt() to store passwords
  2. Example (simplistic)

There are many tutorials and examples that show storage of passwords in a table. Often the methods used are substandard and very easy to crack. For example, the "Agile Web Application Development with Yii1.1 and PHP5" book's example stores md5($password) in the DB and calls it "encryption". It is not. "The Yii Blog Tutorial", (prior to Yii version 1.1.13) was a little better in that it used a salt but it still used md5 and is easy to crack. (Since 1.1.14 Yii has a CPasswordHelper class which the Blog Tutorial uses.) The yii-user and yii-user-management extensions are similarly insecure. Examples of the same errors abound and are by no means limited to webapps implemented in Yii or PHP.

You cannot rely on a user to use a (practically) unguessable password or to not use that password in systems other than yours. And you should not assume that your server is so secure that an attacker cannot get hold of the password file/table or a backup of it.

A very common error I see in what I read and other people's code is fast hashes. MD5, for example, is very fast. As of Nov 2011 you can check 350 million keys per second on a commodity nVidia processor. (Update: two years later the technology for brute force password cracking has advanced to a frightening degree and is moving fast.) So no matter what you do with salts, the combination of short passwords and fast brute force checking means your system is open to intruders if you rely on a non-iterated message digest such as MD5 or any of the SHA algos. Most hash fuctions are indeed designed to be fast to compute.

The Blowfish hash function is currently considered pretty good. It is designed to be slow. The implementation in PHP's crypt() is easy to use. Set a cost parameter high enough to make a brute force attack really slow. I set it so that it takes about 250 ms on the production server which is fast enough for users to tolerate but slow enough to defeat a brute-force attack.

Each password should have its own salt. The salt's purpose is to make the dictionary size in a rainbow table or dictionary attack so large that the attack is not feasible. Salts used with the Blowfish hash do not need to be cryptographically secure random strings but they do need to be unique. A long enough string from an operating system's CSPRNG in non-blocking mode (e.g. /dev/urandom on Linux) is pretty good.

Some people advocate resalting every time a user logs in. I think this is only useful if you also limit the time interval between user logins, e.g. block an account if the user hasn't logged in in more than N weeks.

If your software will be in use for many years then you should increase the cost factor in line with increases in computer speed. You will need to rehash passwords when do.

Using PHP's crypt() to store passwords

People often get confused about how to use implement a password store using crypt(). It is actually very simple but it helps to know that:

  • It is safe to store the salt together with the password hash. An attacker cannot use it to make a dictionary attack easier.

  • The string crypt() returns is the concatenation of the salt you give it and the hash value.

  • crypt() ignores excess characters in the input salt string.

crypt() has function signature string crypt (string $str, string $salt) and the salt string format determines the hash method. For Blowfish hashing, the format is: "$2a$", a two digit cost parameter, "$", and 22 digits from the alphabet "./0-9A-Za-z". The cost must be between 04 and 31.

Notice how the first 29 characters are the same as the salt string:

crypt('EgzamplPassword', '$2a$10$1qAz2wSx3eDc4rFv5tGb5t')
    >> '$2a$10$1qAz2wSx3eDc4rFv5tGb5e4jVuld5/KF2Kpy.B8D2XoC031sReFGi'

The characters from position 30 onwards are the hash.

Notice also how anthing appended to the salt string argument has no effect on the result:

crypt('EgzamplPassword', '$2a$10$1qAz2wSx3eDc4rFv5tGb5t12345678901234567890')
    >> '$2a$10$1qAz2wSx3eDc4rFv5tGb5e4jVuld5/KF2Kpy.B8D2XoC031sReFGi'

crypt('EgzamplPassword', '$2a$10$1qAz2wSx3eDc4rFv5tGb5t$2a$10$1qAz2wSx3eDc4rFv5tGb5t')
    >> '$2a$10$1qAz2wSx3eDc4rFv5tGb5e4jVuld5/KF2Kpy.B8D2XoC031sReFGi'

And in particular, pass the value returned from crypt() back in as the salt argument:

crypt('EgzamplPassword', '$2a$10$1qAz2wSx3eDc4rFv5tGb5e4jVuld5/KF2Kpy.B8D2XoC031sReFGi')
    >> '$2a$10$1qAz2wSx3eDc4rFv5tGb5e4jVuld5/KF2Kpy.B8D2XoC031sReFGi'

So we can use crypt() to authenticate a user by passing the hash value it gave us previously back in as a salt when checking a password input.

Example (simplistic)

Say we have a user table like this

create table user (
    id int,
    email varchar(255),
    password_hash varchar(64)

From a user account generation form, assume that we have (already sanitized) user input in $form->email and $form->password. We generate the hash:

$salt = openssl_random_pseudo_bytes(22);
$salt = '$2a$%13$' . strtr(base64_encode($salt), array('_' => '.', '~' => '/'));
$password_hash = crypt($form->password, $salt);

And insert a row into user containing $form->email and $password_hash.

At user logon assume we again have sanitized user input in $form->email and $form->password. To authenticate these against the accounts in user we select the password_hash field from table user where email = $form->email and, with that value in $password_hash

if ($password_hash === crypt($form->password, $password_hash))
    // password is correct
    // password is wrong

So there is no need to store the salt in a separate column from the hash value because crypt() conveniently keeps it in the same string as the hash.

While this example shows how crypt() works, it is too simplistic for practical use. It glosses over several important details including: how to obtain a decent salt (the example assumes OpenSSL is available), what value to use for the cost parameter (the example arbitrarily uses 13), and what function to use to compare the retrieved database hash value with the computed value (=== is simple but might be vulnerable to timing attacks). The APIs in Yii's CSecurityManager and CPasswordHelper are intended to help the user deal with these matters.

In Yii

As of version 1.1.14, Yii has an API to help users with secure password storage: CPasswordHelper. The Blog Tutorial shows how it can be used.

Availability of crypt()'s Blowfish option

The crypt() function has ben part of PHP for a long time but not all PHP installations have all its options. The Blowfish hash option is available in all PHP systems since 5.3. It is also available in older PHPs if either the operating system has the option in its standard library crypt(3) function (e.g. many Unix and Linux systems) or if PHP has the Suhosin patch.

PHP's CRYPT_BLOWFISH constant is true if the system has Blowfish.

I have not found a solution that I can recommend to provide secure password storage when crypt()'s Blowfish option is absent. If you want to be secure you have to make this a requirement of your PHP runtime environemnt or take matters into your own hands.

Some people have commented that phpass has fallback algorithms when CRYPT_BLOWFISH is false and asked what's wrong with that. They are not sufficiently secure, in my opinion, to recommend and that's why I don't recommend phpass.

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Written by: fsb
Last updated by: fsb
Created on: Nov 27, 2012
Last updated: 9 years ago
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