(PHP 5 >= 5.3.0, PHP 7, PHP 8)
openssl_random_pseudo_bytes —Generate a pseudo-random string of bytes
Generates astring of pseudo-random bytes, with the number of bytes determined by thelength parameter.
It also indicates if a cryptographically strong algorithm was used to produce the pseudo-random bytes, and does this via the optionalstrong_result parameter. It's rare for this to befalse, but some systems may be broken or old.
length The length of the desired string of bytes. Must be a positive integer less than or equal to2147483647. PHP will try to cast this parameter to a non-null integer to use it.
strong_result If passed into the function, this will hold abool value that determines if the algorithm used was "cryptographically strong", e.g., safe for usage with GPG, passwords, etc.true if it did, otherwisefalse
Returns the generatedstring of bytes.
openssl_random_pseudo_bytes() throws anException on failure.
| Version | Description |
|---|---|
| 8.0.0 | strong_result is nullable now. |
| 7.4.0 | The function no longer returnsfalse on failure, but throws anException instead. |
Example #1openssl_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);
echoPHP_EOL;
}
?>The above example will outputsomething similar to:
Lengths: Bytes: 1 and Hex: 2string(2) "42"bool(true)Lengths: Bytes: 2 and Hex: 4string(4) "dc6e"bool(true)Lengths: Bytes: 3 and Hex: 6string(6) "288591"bool(true)Lengths: Bytes: 4 and Hex: 8string(8) "ab86d144"bool(true)
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);
?>[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 reporthttps://bugs.php.net/bug.php?id=70014 and the corresponding source code athttps://github.com/php/php-src/blob/php-5.6.10/ext/openssl/openssl.c#L5408Another 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
functioncrypto_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+$rnd;
}
?>Getting an integer value from a given range with an even distribution:
This function I created to solve the problem of modulo results causing overlap of ranged results (which gave an uneven distribution).
What I mean for those not as familiar with the problem:
Using bytes for base 256 (base 16) and attempting to find a value in a range of values that may be for example 10-20 (a spread of 11) will not divide evenly, so values (using mod) will overlap and give more priority to some numbers than others.
Instead of calculating based on byte values, I used the byte values as keys to sort. This is very fast, and does not require large multiplications of data space that easily go over the value of Max Int.
Additionally: To make the user-supplied arguments not care about order I am using a handy swap function I found in the wild in conjunction with my function below.
// swap function
function swap(&$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: 17FYI, 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+slowIf you don't have this function but you do have OpenSSL installed, you can always fake it:
<?php
functionopenssl_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;
}
?>Remember to request at very least 8 bytes of entropy, ideally 32 or 64, to avoid possible theorical bruteforce attacks.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().