openssl_random_pseudo_bytes

(PHP 5 >= 5.3.0, PHP 7)

openssl_random_pseudo_bytes — Generate a pseudo-random string of bytes

Description

string openssl_random_pseudo_bytes ( int $length [, bool &$crypto_strong ] )

Generates a string of pseudo-random bytes, with the number of bytes determined by the length parameter.

It also indicates if a cryptographically strong algorithm was used to produce the pseudo-random bytes, and does this via the optional crypto_strong parameter. It's rare for this to be FALSE, but some systems may be broken or old.

Parameters

length: The length of the desired string of bytes. Must be a positive integer. PHP will try to cast this parameter to a non-null integer to use it.
crypto_strong: If passed into the function, this will hold a boolean value that determines if the algorithm used was "cryptographically strong", e.g., safe for usage with GPG, passwords, etc. TRUE if it did, otherwise FALSE

Return Values

Returns the generated string of bytes on success, or FALSE on failure.

Examples

Example #1 openssl_random_pseudo_bytes() example


<?php
for ($i = -1; $i <= 4; $i++) {
    $bytes = openssl_random_pseudo_bytes($i, $cstrong);
    $hex   = bin2hex($bytes);

    echo "Lengths: Bytes: $i and Hex: " . strlen($hex) . PHP_EOL;
    var_dump($hex);
    var_dump($cstrong);
    echo PHP_EOL;
}
?>

The above example will output something similar to:

Lengths: Bytes: -1 and Hex: 0
string(0) ""
NULL

Lengths: Bytes: 0 and Hex: 0
string(0) ""
NULL

Lengths: Bytes: 1 and Hex: 2
string(2) "42"
bool(true)

Lengths: Bytes: 2 and Hex: 4
string(4) "dc6e"
bool(true)

Lengths: Bytes: 3 and Hex: 6
string(6) "288591"
bool(true)

Lengths: Bytes: 4 and Hex: 8
string(8) "ab86d144"
bool(true)

User Contributed Notes

nahun@telemako

2 years ago


Here's an example to show the distribution of random numbers as an image. Credit to Hayley Watson at the mt_rand page for the original comparison between rand and mt_rand.

rand is red, mt_rand is green and openssl_random_pseudo_bytes is blue.

NOTE: This is only a basic representation of the distribution of the data. Has nothing to do with the strength of the algorithms or their reliability.

<?php
header("Content-type: image/png");
$sizex=800;
$sizey=800;

$img = imagecreatetruecolor(3 * $sizex,$sizey);
$r = imagecolorallocate($img,255, 0, 0);
$g = imagecolorallocate($img,0, 255, 0);
$b = imagecolorallocate($img,0, 0, 255);
imagefilledrectangle($img, 0, 0, 3 * $sizex, $sizey, imagecolorallocate($img, 255, 255, 255));

$p = 0;
for($i=0; $i < 100000; $i++) {
    $np = rand(0,$sizex);
    imagesetpixel($img, $p, $np, $r);
    $p = $np;
}

$p = 0;
for($i=0; $i < 100000; $i++) {
    $np = mt_rand(0,$sizex);
    imagesetpixel($img, $p + $sizex, $np, $g);
    $p = $np;
}

$p = 0;
for($i=0; $i < 100000; $i++) {
    $np = floor($sizex*(hexdec(bin2hex(openssl_random_pseudo_bytes(4)))/0xffffffff));
    imagesetpixel($img, $p + (2*$sizex), $np, $b);
    $p = $np;
}

imagepng($img);
imagedestroy($img);
?>

acatalept at gmail

5 years ago


FYI, openssl_random_pseudo_bytes() can be incredibly slow under Windows, to the point of being unusable.  It frequently times out (>30 seconds execution time) on several Windows machines of mine.

Apparently, it's a known problem with OpenSSL (not PHP specifically).

See: http://www.google.com/search?q=openssl_random_pseudo_bytes+slow

christophe dot weis at statec dot etat dot lu

4 years ago


Another replacement for rand() using OpenSSL.

Note that a solution where the result is truncated using the modulo operator ( % ) is not cryptographically secure, as the generated numbers are not equally distributed, i.e. some numbers may occur more often than others.

A better solution than using the modulo operator is to drop the result if it is too large and generate a new one.

<?php
function crypto_rand_secure($min, $max) {
        $range = $max - $min;
        if ($range == 0) return $min; // not so random...
        $log = log($range, 2);
        $bytes = (int) ($log / 8) + 1; // length in bytes
        $bits = (int) $log + 1; // length in bits
        $filter = (int) (1 << $bits) - 1; // set all lower bits to 1
        do {
            $rnd = hexdec(bin2hex(openssl_random_pseudo_bytes($bytes, $s)));
            $rnd = $rnd & $filter; // discard irrelevant bits
        } while ($rnd >= $range);
        return $min + $rnd;
}
?>

Tyler Larson

6 years ago


If you don't have this function but you do have OpenSSL installed, you can always fake it:

<?php
    function openssl_random_pseudo_bytes($length) {
        $length_n = (int) $length; // shell injection is no fun
        $handle = popen("/usr/bin/openssl rand $length_n", "r");
        $data = stream_get_contents($handle);
        pclose($handle);
        return $data;
    }
?>

crrodriguez at opensuse dot org

5 years ago


Remember to request at very least 8 bytes of entropy, ideally 32 or 64, to avoid possible theorical bruteforce attacks.

Anonymous

3 years ago


Another way to get random 32bit ints:
function myRand($max){
    do{
        $result = floor($max*(hexdec(bin2hex(openssl_random_pseudo_bytes(4)))/0xffffffff));
    }while($result == $max);
    return $result;
}

powtac at gmx dot de

3 months ago


[Editor's note: the bug has been fixed as of PHP 5.4.44, 5.5.28 and PHP 5.6.12]



Until PHP 5.6 openssl_random_pseudo_bytes() did NOT use a "cryptographically strong algorithm"! 

See bug report https://bugs.php.net/bug.php?id=70014 and the corresponding source code at https://github.com/php/php-src/blob/php-5.6.10/ext/openssl/openssl.c#L5408