Description:
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For some reason, using mcrypt_create_iv on my system (WinXP, Apache 1.3.28, PHP 5.0.3, mcrypt 2.5.7) usually returns the same value (specifically, when base64_encoded, the value "qSqamZGHYAIBHnH8KiVcFwNNgrXbLL7R5Fl6lc4xjwA=") each time called, with some exceptions.

Exceptions: if the code I have included is executed on my machine again before approx 15 seconds has elapsed, another number, also fitting into an expected value, is returned.

To explain:

If I execute the code below once, then wait 15 or 16 seconds, it will return the same value both times, ie "qSqamZGHYAIBHnH8KiVcFwNNgrXbLL7R5Fl6lc4xjwA=". If I execute the code, then a second later I execute the code again, then a second later I execute the code a third time, THEN I wait 15 seconds and repeat the execution 3 times, I will get the following base64_encoded values:

qSqamZGHYAIBHnH8KiVcFwNNgrXbLL7R5Fl6lc4xjwA=
DqwXXwbWS0Nkm/7+18Y12zx1JIns87nfTITMx5oOWXI=
pytYv/yDzGw5JlyRm7timBu98nR3vTR7udWwlUVG6gI=
qSqamZGHYAIBHnH8KiVcFwNNgrXbLL7R5Fl6lc4xjwA=
DqwXXwbWS0Nkm/7+18Y12zx1JIns87nfTITMx5oOWXI=
pytYv/yDzGw5JlyRm7timBu98nR3vTR7udWwlUVG6gI=

Obviously I have no idea why waiting approx 15 seconds restarts the cycle, but the net effect of this is that I generally end up with an $iv value of "qSqamZGHYAIBHnH8KiVcFwNNgrXbLL7R5Fl6lc4xjwA=", since there is usually a much longer period between requests to create an initialization vector value than 15 seconds (at least, it's usually much longer between in the site I have designed).

I hope this makes sense -- if not, please email me and I will try to provide what information I can.

Note: I also tried "srand((double) microtime() * 1000000);" instead of just "srand();" in case srand() was not being initialized properly for MCRYPT_RAND, but this made no difference.

Reproduce code:
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$td = mcrypt_module_open('rijndael-256','','cbc','');
srand();
$iv = mcrypt_create_iv(mcrypt_get_iv_size('rijndael-256','cbc'), MCRYPT_RAND);
echo base64_encode($iv)."<br>";


Expected result:
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I expect $iv to be a different (ie random) value each time the code is run.

Actual result:
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If run approx 15 seconds apart, $iv will always be the same. if run more frequently than 15 seconds apart, the value will follow an expected progression, rather than being truly random.