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Date: Sat, 15 Feb 2014 12:50:32 +0000
Message-ID: <CAEKnhAEHWLaj9=k2ZG=+kSCFWbwvb+iDjb5QXgaGebpaE+UPmQ@mail.gmail.com>
To: =?ISO-8859-1?Q?P=E1draic_Brady?= <padraic.brady@gmail.com>
Cc: Andrey Andreev <narf@devilix.net>, Pierre Joye <pierre.php@gmail.com>, 
	PHP internals <internals@lists.php.net>
Content-Type: multipart/alternative; boundary=001a11c16f5e365d2604f2715eff
Subject: Re: [PHP-DEV] unify entropy source for all php related functions
From: bukka@php.net (Jakub Zelenka)

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On Sat, Feb 15, 2014 at 11:59 AM, P=E1draic Brady <padraic.brady@gmail.com>=
wrote:

> Hi,
>
> On 14 February 2014 15:30, Jakub Zelenka <bukka@php.net> wrote:
> > It uses /dev/urandom for seeding PRNG. It means that it reads only 32
> bytes
> > (256 bits) from it. If you call openssl_random_pseudo_bytes you will ge=
t
> > bytes generated by PRNG
> >
> >
> >>  - $crypto_strong will always be true on Linux (UNIX-whatever)
> >>
> >
> > Yes. The reason is that /dev/urandom is non-blocking and you will alway=
s
> > get enough entropy from it. You don't have to check the flag on Linux
> > because it will be always true!
> >
> > The point is that OpenSSL considers /dev/urandom strong enough for
> seeding
> > CSPRNG.
> >
> > Jakub
>
> There are the three categories:
>
> 1. Non-Cryptographically Deterministic PRNG
> 2. Entropy Input PRNG
> 3. Cryptographically Secure PRNG
>
> OpenSSL is the second. /dev/urandom is also the second. /dev/random is
> the third (after a warmup period!). Something like rand() would be in
> the first under the assumption that time is predictable.
>
> Those are the technical categories anyway... In reality, you can use a
> non-CSPRNG for cryptographic needs over the short term. If you can't,
> it means the damn thing is broken ;). If you have something like a
> long term or high value cryptographic product, you wont want to stint
> on entropy and you won't want to run afoul of any future issue with
> the entropy collection or mixing, and you're probably not going to
> even use a machine remotely susceptible in inactivity (which may limit
> the entropy sources).
>
> Be careful of taking claims at face value where definitions are in doubt.
>
> Paddy
>
>
>
Thanks for the explanation. It makes sense.

I thought that they considers the PRNG as Cryptographically Secure because
the documentation states that it produces cryptographically strong
pseudo-random bytes

http://www.openssl.org/docs/crypto/RAND_bytes.html

(RAND_bytes is equal to RAND_pseudo_bytes on Linux)

Jakub

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