Edit report at https://bugs.php.net/bug.php?id=55519&edit=1
ID: 55519
Updated by: ahar...@php.net
Reported by: sergio dot bobillier at gmail dot com
Summary: mt_rand only generetes even 64 bits numbers
-Status: Open
+Status: Assigned
Type: Bug
Package: Math related
Operating System: Linux (Ubuntu 11.04 x64)
PHP Version: Irrelevant
-Assigned To:
+Assigned To: aharvey
Block user comment: N
Private report: N
New Comment:
Verified on 64-bit OS X, at least beyond a certain set of mt_rand() bounds. Not
entirely sure what those actual bounds are yet.
Previous Comments:
------------------------------------------------------------------------
[2011-08-27 09:09:30] sergio dot bobillier at gmail dot com
Description:
------------
I was doing a lame RSA implementation when I discovered that mt_rand only
generates even numbers when the number is a 64 bits integer.
The Test script shows the script I used to generate two 64 bits integers p and
q with mt_rand. I was doing primality test only with odd numbers since even
numbers are not prime but the script never do the primality test because all
the generated numbers are even.
I thought this might be an integer overflow problem but I'm doing this on a 64
bits system and the var_dump function shows that the variables are actually
integers like so:
int(2117698586505379840)
int(1690740540401778688)
int(3279295862706012160)
int(4526730875131396096)
int(3188107862534520832)
int(1914802535743356928)
int(4172596828035874816)
int(4559249790717657088)
int(1938449129751445504)
int(1859647742213619712)
Test script:
---------------
function is_prime($num)
{
// See if the number is prime
echo "Primality test";
return true;
}
function gen_keys()
{
// 1. find two 64 bits primes p and q
$p = 0;
$q = 0;
while(!$p)
{
$p = mt_rand(72057594037927936, 4611686018427387904);
// debugging:
var_dump($p);
if($p % 2 == 0)
{
$p = 0;
}
else
{
// Primality test in another function
// (not important here)
echo "odd";
if(!is_prime($p))
$p = 0;
}
}
echo $p;
}
Expected result:
----------------
I expect the mt_rand to generate some odd numbers and make the script go
through the primality test or at least print "odd"
Actual result:
--------------
The script doesn't reach the primality or print "odd" test because all
generated numbers are even.
------------------------------------------------------------------------
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Edit this bug report at https://bugs.php.net/bug.php?id=55519&edit=1
