CascadiaPHP 2024


(PHP 5 >= 5.3.0, PHP 7, PHP 8)

openssl_random_pseudo_bytesGenerate a pseudo-random string of bytes


openssl_random_pseudo_bytes(int $length, bool &$strong_result = null): 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 strong_result parameter. It's rare for this to be false, but some systems may be broken or old.



The length of the desired string of bytes. Must be a positive integer less than or equal to 2147483647. PHP will try to cast this parameter to a non-null integer to use it.


If passed into the function, this will hold a bool 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


Returns the generated String of bytes.


openssl_random_pseudo_bytes() throws an Exception on failure.


Version Beschreibung
8.0.0 strong_result is nullable now.
7.4.0 The function no longer returns false on failure, but throws an Exception instead.


Beispiel #1 openssl_random_pseudo_bytes() example

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

"Lengths: Bytes: $i and Hex: " . strlen($hex) . PHP_EOL;

Das oben gezeigte Beispiel erzeugt eine ähnliche Ausgabe wie:

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

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

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

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

Siehe auch

  • random_bytes() - Get cryptographically secure random bytes
  • bin2hex() - Wandelt Binär-Daten in ihre hexadezimale Entsprechung um
  • crypt() - Einweg-String-Hashing
  • random_int() - Get a cryptographically secure, uniformly selected integer
add a note

User Contributed Notes 11 notes

10 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.

("Content-type: image/png");

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

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

$p = 0;
$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;

powtac at gmx dot de
8 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 and the corresponding source code at
christophe dot weis at statec dot etat dot lu
13 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.

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);
$min + $rnd;
mailjeffclayton [at] gmail
4 years 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
acatalept at gmail
13 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).

Tyler Larson
14 years ago
If you don't have this function but you do have OpenSSL installed, you can always fake it:

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);
crrodriguez at opensuse dot org
13 years ago
Remember to request at very least 8 bytes of entropy, ideally 32 or 64, to avoid possible theorical bruteforce attacks.
12 years ago
Another way to get random 32bit ints:
function myRand($max){
$result = floor($max*(hexdec(bin2hex(openssl_random_pseudo_bytes(4)))/0xffffffff));
}while($result == $max);
return $result;
umairkhi at hotmail dot com
6 years ago
After the fix of insecure number generation here:

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().
atesin () gmail ! com
6 years ago
if unavailable use this with core functions... maybe not as secure and optimized (any help?), but practical


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

6 years ago
Why does bin2hex return twice as many characters as bytes?
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