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

From: Anton Khirnov <anton@khirnov.net>
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, 04 Jul 2023 07:54:06 +0200
Message-ID: <168845004614.542.18132678959456829324@lain.khirnov.net> (raw)
In-Reply-To: <20230703235057.GQ1093384@pb2>

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.

-- 
Anton Khirnov
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