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Request for Comments: Timing attack safe string comparison function
- Version: 0.2
- Date: 2013-12-22
- Author: Rouven Weßling, me@rouvenwessling
- Status: Under Discussion
- First Published at: http://wiki.php.net/rfc/timing_attack
Timing attacks are not a widely recognized problem, since they require significant skill and resources of the attacker. To prevent these attacks constant-time string comparisons can be used. These implementations are rather hard to get right, without leaking timing information.
While high profile applications and frameworks already ship with time-constant string comparison functions, this RFC aims to make it simpler for PHP developers to protect their applications. Additionally there's less chance of unexpected timing differences in lower level languages.
The password_verify function implemented in PHP 5.5 already does a time-constant string comparison, however in many projects bcrypt can not be used for some reason (legacy project, interfacing with other systems, etc.), but they'd still benefit from this functionality.
Implement a new function called hash_compare.
Signature bool hash_compare(string knownString, string userString)
The time this function takes is based only on the length of the user supplied string.
Users have to be mindful, as it is important that the user supplied string (or a hash of that string) is used as the the second parameter not the first.
Backward Incompatible Changes
Proposed PHP Version(s)
Next PHP 5.x
Impact to Existing Extensions
- Decide on a name for the function.
Patches and Tests
Patch including tests: https://github.com/realityking/php-src/compare/timing_attack
- 0.1 Initial publication
- 0.2 Renamed to hash_compare, added link to Zend Framework 2, removed information leak when knownString is empty (Thank you Tjerk)