PHP 7.4.0RC6 Released!

openssl_random_pseudo_bytes

(PHP 5 >= 5.3.0, PHP 7)

openssl_random_pseudo_bytesGenerate a pseudo-random string of bytes

Description

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

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)

See Also

  • random_bytes() - Generates cryptographically secure pseudo-random bytes
  • bin2hex() - Convert binary data into hexadecimal representation
  • crypt() - One-way string hashing
  • mt_rand() - Generate a random value via the Mersenne Twister Random Number Generator
  • uniqid() - Generate a unique ID
add a note add a note

User Contributed Notes 11 notes

up
37
nahun@telemako
6 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);
?>
up
10
powtac at gmx dot de
3 years 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
up
15
christophe dot weis at statec dot etat dot lu
8 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;
}
?>
up
6
Tyler Larson
10 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;
    }
?>
up
1
acatalept at gmail
8 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
up
1
mailjeffclayton [at] gmail
1 month ago
Getting an integer value from a given range with an even distribution:

This function I created to solve the problem of modulo results causing overlap of ranged results (which gave an uneven distribution).

What I mean for those not as familiar with the problem:

Using bytes for base 256 (base 16) and attempting to find a value in a range of values that may be for example 10-20 (a spread of 11) will not divide evenly, so values (using mod) will overlap and give more priority to some numbers than others.

Instead of calculating based on byte values, I used the byte values as keys to sort. This is very fast, and does not require large multiplications of data space that easily go over the value of Max Int.

Additionally: To make the user-supplied arguments not care about order I am using a handy swap function I found in the wild in conjunction with my function below.

// swap function

function swap(&$a,&$b) { list($a,$b)=array($b,$a); } // swap 2 variables-- no temp variable needed!

// function to get a random value within a given range of integers
   
function get_secure_random_ranged_value($max=99, $min=0) // handles 1 or 2 arguments, order does not matter
{
    $sortarray = array();
    $lo = (int)$min;
    $hi = (int)$max;
    if ($lo > $hi) swap($lo,$hi);
    $data_range = abs($hi - $lo) + 1; // +1 includes both the lowest 'zero' value and highest value of range
    $bytes_per_key = 4; // Max: ffff hex = 4,294,967,296 dec (over 4 billion) -- large span of random values covers massive datasets
    $num_bytes = $data_range * $bytes_per_key;
    $byte_string = (bin2hex(openssl_random_pseudo_bytes($num_bytes))); // only one call needed to get string of bytes
    $byte_blocksize = $bytes_per_key << 1; // shift multiply by 2 since a byte is 2 characters wide
   
    while ($key = substr($byte_string,0,$byte_blocksize)) { // get next byte block from string
        $byte_string = substr($byte_string,$byte_blocksize); // remove selected byte block from string
        $sortarray[]=$key; // populate the array with keys temporarily as array values
    }
       
    $sortarray = array_flip($sortarray); // swap to use the byte values as keys
    ksort($sortarray); // randomize by keys
    return array_shift($sortarray) + $lo; // grab top value from array and add it to the lowest value in the range   
}

//
// example getting values from 0 to 21:
//
   
for ($i=1;$i<=10;$i++) { $rnd = get_secure_random_ranged_value(21); echo "-> result: ".($rnd)." <br />\n";  }

//
// example getting values from 14 to 21:
//

for ($i=1;$i<=10;$i++) { $rnd = get_secure_random_ranged_value(14,21); echo "-> result: ".($rnd)." <br />\n";  }

//
// sample results from 14-21
//

-> result: 14
-> result: 18
-> result: 20
-> result: 15
-> result: 20
-> result: 16
-> result: 21
-> result: 15
-> result: 16
-> result: 17
up
1
crrodriguez at opensuse dot org
8 years ago
Remember to request at very least 8 bytes of entropy, ideally 32 or 64, to avoid possible theorical bruteforce attacks.
up
-1
umairkhi at hotmail dot com
1 year ago
After the fix of insecure number generation here:

http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-8867

This function as well as the text here needs an update. I believe this function is safe to use in FIPS compliant apps as well as it now used RAND_bytes instead of  the insecure RAND_pseudo_bytes().
up
-8
Anonymous
7 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;
}
up
-3
atesin () gmail ! com
1 year ago
if unavailable use this with core functions... maybe not as secure and optimized (any help?), but practical

<?php

$bytes
= '';
while (
strlen($bytes) < $lenght)
 
$bytes .= chr(mt_rand(0, 255));

?>
up
-11
Karsey
2 years ago
Why does bin2hex return twice as many characters as bytes?
To Top