sammyk

Sammy Kaye Powers

leigh

Leigh

Contents

PHP RFC: Easy User-land CSPRNG

Introduction

This RFC proposes adding a user-land API for an easy to use and reliable CSPRNG in PHP.

The Problem

By default PHP does not provide an easy mechanism for accessing cryptographically strong random numbers in user-land. Users have a few options like openssl_random_pseudo_bytes(), mcrypt_create_iv() or directly opening /dev/*random devices to obtain high quality pseudo-random bytes, but unfortunately system support for these functions and extensions varies between platforms and each come with their own set of problems

  • The mcrypt_create_iv() function has no dependency on MCrypt lib yet it requires the MCrypt extension to be installed before it can be used. Users are forced to include an entire library for no reason.
  • openssl_random_pseudo_bytes() is provided by the OpenSSL lib. This function comes with a $crypto_strong the meaning of which may just confuse users.
  • Falling back to /dev/urandom is OS-specific.

In addition users may attempt to generate their own streams of random bytes relying on rand() or mt_rand(), and this is something we absolutely want to avoid.

Proposal

There should be a user-land API to easily return an arbitrary length of cryptographically secure pseudo-random bytes directly and work on any supported platform.

The initial proposal is to add two user-land functions that return the bytes as binary and integer. Arbitrary length strings of random bytes are important for salts, keys and initialisation vectors. Integers based on CS random are important for applications where unbiased results are critical (i.e. shuffling a Poker deck).

Signatures:

random_bytes(int length);
random_int(int min, int max);

Examples:

$randomStr = random_bytes($length = 16);
 
$randomInt = random_int($min = 0, $max = 127);

The sources of random used are as follows:

  • On windows CryptGenRandom is used exclusively
  • arc4random_buf() is used if it is available (generally BSD specific)
  • /dev/arandom is used where available
  • /dev/urandom is used where none of the above is available
  • An error is thrown in the event that a sufficient source of randomness is unavailable.

Backward Incompatible Changes

Any user-land code that defines a random_bytes() or random_int() function would generate a fatal error, however it is likely that these functions provide the same or similar functionality as desired.

Proposed PHP Version(s)

PHP 7

RFC Impact

To SAPIs

This RFC should not impact the SAPI's.

To Existing Extensions

No existing extensions are affected.

To Opcache

Opcache is unaffected.

New Constants

There would be no new constants.

php.ini Defaults

There would be no new php.ini defaults.

Open Issues

  • Nothing yet

Unaffected PHP Functionality

This change does not affect any of the existing rand() or mt_rand() functionality.

Future Scope

The concepts from the RFC could be used to:

  • Deprecate mcrypt_create_iv()
  • Improve session_id randomness generation
  • Detect LibreSSL-portable for arc4random() on Linux

Patches and Tests

The current patch can be found here: https://github.com/php/php-src/pull/1119

Proposed Voting Choices

The voting choices are yes (in favor for accepting this RFC for PHP 7) or no (against it).

Acknowledgements

Big thanks to Anthony Ferrara, Daniel Lowrey, E. Smith and all the kids in the PHP room for all the help with this one!

Versions

Version Changed Date
0.5 Updated the function header for random_int() to reflect all args as required. - SammyK
0.4 Added BC info. Updated patch link to point to PR. - SammyK
0.3 Changed -PHP_INT_MAX to ~PHP_INT_MAX (thanks @trevorsuarez) - SammyK
0.2 Condensed the problem domain into something more focused. Added function sigs. - Leigh.
0.1 Mmmm drafty - Leigh
0.0 Initial draft - need Leigh's input - SammyK

Votes

An option needs 50%+1 votes to win

Reliable user-land CSPRNG (100% approved)
User Vote
aharvey Yes
auroraeosrose Yes
bishop Yes
crodas Yes
Damien Tournoud Yes
derick Yes
dragoonis Yes
eliw Yes
galvao Yes
gasolwu Yes
hywan Yes
ircmaxell Yes
jedibc Yes
jmikola Yes
jpauli Yes
jwage Yes
kinncj Yes
klaussilveira Yes
lcobucci Yes
leigh Yes
leszek Yes
lstrojny Yes
mbeccati Yes
mike Yes
nikic Yes
omars Yes
pajoye Yes
pauloelr Yes
ralphschindler Yes
ramsey Yes
rdlowrey Yes
rdohms Yes
rmf Yes
salathe Yes
stas Yes
stelianm Yes
svpernova09 Yes
thekid Yes
weierophinney Yes
yohgaki Yes
zeev Yes