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

[Marton Balint <cus@passwd.hu>]

On Mon, 3 Jul 2023, 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()

It depends on OS and version most likely.

> 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.

I am no expert on the subject, but even the generic code seems reasonably 
secure. It gathers entropy, it uses a crypto hash to get the output. And 
as you said, even that only used for obscure cases.

>
> 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.

Sorry, seems a bit overdesign for me, so I won't be pursuing this further.

Regards,
Marton
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