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

[Marton Balint]

On Tue, 4 Jul 2023, James Almer wrote:

> On 7/3/2023 6:52 PM, Marton Balint wrote:
>>
>>
>>  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.
>
> I just sent a patchset implementing a new av_random() function that uses the 
> existing av_get_random_seed() entropy sources, and adds the ones used by 
> hlsenc too.
> If that goes in, then this patchset can go in as well afterwards.

Thanks! I will rework the patch to use av_random() then.

Regards,
Marton