Re: [FFmpeg-devel] [PATCH 1/2] avformat/hlsenc: fall back to av_get_random_seed() when generating AES128 key

From: Kieran Kunhya <kierank@obe.tv>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH 1/2] avformat/hlsenc: fall back to av_get_random_seed() when generating AES128 key
Date: Tue, 4 Jul 2023 10:08:54 +0100
Message-ID: <CAK+ULv40PwAN2stC4r9aKrONq-DbmsYebxKOJWq8mW_B0BvD5g@mail.gmail.com> (raw)
In-Reply-To: <168845004614.542.18132678959456829324@lain.khirnov.net>

On Tue, 4 Jul 2023 at 06:54, Anton Khirnov <anton@khirnov.net> wrote:

> Quoting Michael Niedermayer (2023-07-04 01:50:57)
> > On Mon, Jul 03, 2023 at 11:09:54PM +0200, Anton Khirnov wrote:
> > > Quoting Marton Balint (2023-07-03 22:54:41)
> > > > On Mon, 3 Jul 2023, Anton Khirnov wrote:
> > > > My patch use av_get_random_seed() which uses what the underlying OS
> > > > provides, BCrypt for Windows, /dev/urandom for Linux, arc4random()
> for
> > > > BSD/Mac.
> > >
> > > IOW it's a jungle of various paths, some of which are not guaranteed to
> > > be cryptographically secure. I see no such guarantees for arc4random()
> > > from a brief web search, and the fallback get_generic_seed() certainly
> > > is not either. Granted it's only used on obscure architectures, but
> > > still.
> > >
> > > The doxy even says
> > > > This function tries to provide a good seed at a best effort bases.
> > >
> > > > You really think that these are significantly worse than
> > > > OpenSSL/GCrypt, so it should not be allowed to fallback to?
> > >
> > > I think we should be using cryptographically secure PRNG for generating
> > > encryption keys, or fail when they are not available. If you want to
> get
> > > rid of the openssl dependency, IMO the best solution is a new
> > >   int av_random(uint8_t* buf, size_t len);
> > > that guarantees either cryptographically secure randomness or an error.
> >
> > "guarantees cryptographically secure randomness" ?
> > If one defined "cryptographically secure" as "not broken publically as
> of today"
> >
> > Iam saying that as i think "guarantees" can be misleading in what it
> means
>
> I feel your snark is very much misplaced.
>
> I recall way more instances of broken crypto caused by overconfident
> non-experts with an attitude like yours ("those silly crypto libraries,
> broken all the time, how hard can it be really") than by actual
> vulnerabilities in actual crypto libraries.
>
> In fact the highest-profile break I remember (Debian key entropy bug)
> was caused precisely by non-experts fiddling with code they did not
> understand.
>

+1

Kieran
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