[PHP] Re: mcrypt_create_iv - why so slow?

[Nathan Nobbe]

Interesting, using MCRYPT_DEV_URANDOM instead of MCRYPT_DEV_RANDOM seems
practically instantaneous.

Another less elegant solution I've found is to simply str_pad to the length
returned by mcrypt_get_iv_size.

Still begs the question though, any idea what's holding up the show w/
MCRYPT_DEV_RANDOM?  #morbidcuriosity

-nathan


On Fri, May 31, 2013 at 12:40 AM, Nathan Nobbe quickshif...@gmail.comwrote:

 Hi folks,

 This code:

 ?php
 $iv = mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256,
 MCRYPT_MODE_CBC), MCRYPT_DEV_RANDOM);
 var_dump($iv);

 Takes just over a minute to run on my laptop and roughly 45 seconds on a
 capable server, any idea why?

 time php test-iv.php
 string(32) '???H??y?PJ?U?1O;6???ѧ

 real 0m44.917s
 user 0m0.024s
 sys 0m0.036s

 Also, I've noticed the mcrypt_encypt  mcrypt_decrypt complain with

 The IV parameter must be as long as the blocksize

 when not using mcrypt_create_iv, however, if the value of the IV parameter
 is consistent in both calls, the decryption seems to succeed despite the
 warning.

 So wondering:
 * can the call to mcrypt_create_iv be sped up
 * is there an alternative (faster) way to create a proper iv
 * how big a risk is it to 'ride dirty' here and not use mcrypt_create_iv

 thanks,

 -nathan

[PHP] Re: mcrypt_create_iv - why so slow?

[Matt Graham]

From: Nathan Nobbe
 Interesting, using MCRYPT_DEV_URANDOM instead of MCRYPT_DEV_RANDOM
 seems practically instantaneous. Still [raises] the question though,
 any idea what's holding up the show w/ MCRYPT_DEV_RANDOM?

/dev/random is a high quality entropy source and requires more time to
generate output, as it has to retrieve random stuff from keyboard interrupts,
mouse interrupts, and other sources, and make sure it's got *really* random
bits.  /dev/urandom will just throw out a pile of pseudo-random bits of
potentially lower quality immediately.  You can see that by doing time dd
if=/dev/random of=/dev/null bs=16k count=5 and repeating the same command
with /dev/urandom.  1.312 seconds vs. 0.019 seconds here.

Not much to do with PHP, though, just the way the Linux kernel people did
things.  /dev/urandom is probably the way to go for most normal random data
needs.

-- 
Matt G / Dances With Crows
The Crow202 Blog:  http://crow202.org/wordpress/
There is no Darkness in Eternity/But only Light too dim for us to see


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