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, James Almer wrote:

> 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));

If randomize() were to be used for arbitrary lengths, I'd agree, but here 
it is only used for key generation (only 128-bit keys in the current 
code), so I think av_get_random_seed() for the whole keysize is fine, and 
more secure.

Regards,
Marton
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