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From: Anton Khirnov <anton@khirnov.net>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
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Date: Tue, 04 Jul 2023 17:31:04 +0200
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Subject: Re: [FFmpeg-devel] [PATCH 1/2] avformat/hlsenc: fall back to
av_get_random_seed() when generating AES128 key
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Quoting James Almer (2023-07-04 16:37:03)
> On 7/4/2023 2:54 AM, Anton Khirnov 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.
>
> Maybe the gcrypt and openssl API calls used here can instead be moved to
> av_get_random_seed(), which would reduce (or outright remove) the cases
> /dev/random or get_generic_seed() are called and result in essentially
I see nothing wrong with using /dev/random, it's probably the most
trustworthy source on most machines. Though on linux it's probably
even better to use getrandom() where available.
--
Anton Khirnov
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