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Securely Comparing Strings and Generating Random Numbers

2.6 version

Securely Comparing Strings and Generating Random Numbers

The Symfony Security component comes with a collection of nice utilities related to security. These utilities are used by Symfony, but you should also use them if you want to solve the problem they address.

Comparing Strings

The time it takes to compare two strings depends on their differences. This can be used by an attacker when the two strings represent a password for instance; it is known as a Timing attack.

Internally, when comparing two passwords, Symfony uses a constant-time algorithm; you can use the same strategy in your own code thanks to the StringUtils class:

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use Symfony\Component\Security\Core\Util\StringUtils;

// is some known string (e.g. password) equal to some user input?
$bool = StringUtils::equals($knownString, $userInput);

Caution

To avoid timing attacks, the known string must be the first argument and the user-entered string the second.

Generating a Secure random Number

Whenever you need to generate a secure random number, you are highly encouraged to use the Symfony SecureRandom class:

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use Symfony\Component\Security\Core\Util\SecureRandom;

$generator = new SecureRandom();
$random = $generator->nextBytes(10);

The nextBytes() method returns a random string composed of the number of characters passed as an argument (10 in the above example).

The SecureRandom class works better when OpenSSL is installed. But when it's not available, it falls back to an internal algorithm, which needs a seed file to work correctly. Just pass a file name to enable it:

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use Symfony\Component\Security\Core\Util\SecureRandom;

$generator = new SecureRandom('/some/path/to/store/the/seed.txt');

$random = $generator->nextBytes(10);
$hashedRandom = md5($random); // see tip below

Note

If you're using the Symfony Framework, you can get a secure random number generator via the security.secure_random service.

Tip

The nextBytes() method returns a binary string which may contain the \0 character. This can cause trouble in several common scenarios, such as storing this value in a database or including it as part of the URL. The solution is to hash the value returned by nextBytes() (to do that, you can use a simple md5() PHP function).

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.