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

[Anton Khirnov <anton@khirnov.net>]

Quoting James Almer (2023-07-03 21:33:04)
> On 7/2/2023 4:30 PM, Marton Balint wrote:
> > It should be OK to use av_get_random_seed() to generate the key instead of
> > using openSSL/Gcrypt functions. This removes the hard dependancy of those libs
> > for key generation functionality.
> > 
> > Fixes ticket #10441.
> > 
> > Signed-off-by: Marton Balint <cus@passwd.hu>
> > ---
> >   libavformat/hlsenc.c | 18 ++++++++----------
> >   1 file changed, 8 insertions(+), 10 deletions(-)
> > 
> > diff --git a/libavformat/hlsenc.c b/libavformat/hlsenc.c
> > index 1e0848ce3d..0b22c71186 100644
> > --- a/libavformat/hlsenc.c
> > +++ b/libavformat/hlsenc.c
> > @@ -40,6 +40,7 @@
> >   #include "libavutil/intreadwrite.h"
> >   #include "libavutil/opt.h"
> >   #include "libavutil/log.h"
> > +#include "libavutil/random_seed.h"
> >   #include "libavutil/time.h"
> >   #include "libavutil/time_internal.h"
> >   
> > @@ -710,18 +711,18 @@ fail:
> >       return ret;
> >   }
> >   
> > -static int randomize(uint8_t *buf, int len)
> > +static void randomize(uint8_t *buf, int len)
> >   {
> >   #if CONFIG_GCRYPT
> >       gcry_randomize(buf, len, GCRY_VERY_STRONG_RANDOM);
> > -    return 0;
> > +    return;
> >   #elif CONFIG_OPENSSL
> >       if (RAND_bytes(buf, len))
> > -        return 0;
> > -#else
> > -    return AVERROR(ENOSYS);
> > +        return;
> >   #endif
> > -    return AVERROR(EINVAL);
> > +    av_assert0(len % 4 == 0);
> > +    for (int i = 0; i < len; i += 4)
> > +        AV_WB32(buf + i, av_get_random_seed());
> 
> Maybe instead use a PRNG, like the following:
> 
> AVLFG c;
> av_lfg_init(&c, av_get_random_seed());
> for (int i = 0; i < len; i += 4)
>      AV_WB32(buf + i, av_lfg_get(&c));

We really REALLY should not be taking any shortcuts when generating
keys.

Ideally we shouldn't be generating them ourselves in the first place, as
we are not a crypto library. This patch seems like a step backward to
me.

-- 
Anton Khirnov
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