Re: [FFmpeg-devel] [PATCH v2 1/2] avutil/random_seed: add av_random()

[Hendrik Leppkes <h.leppkes@gmail.com>]

On Tue, Jul 4, 2023 at 10:41 PM James Almer <jamrial@gmail.com> wrote:
>
> Uses the existing code for av_get_random_seed() to return a buffer with
> cryptographically secure random data, or an error if none could be generated.
>
> Signed-off-by: James Almer <jamrial@gmail.com>
> ---
>  libavutil/random_seed.c | 54 ++++++++++++++++++++++++++++-------------
>  libavutil/random_seed.h | 12 +++++++++
>  2 files changed, 49 insertions(+), 17 deletions(-)
>
> diff --git a/libavutil/random_seed.c b/libavutil/random_seed.c
> index 66dd504ef0..0ed8f89cc6 100644
> --- a/libavutil/random_seed.c
> +++ b/libavutil/random_seed.c
> @@ -46,20 +46,22 @@
>  #define TEST 0
>  #endif
>
> -static int read_random(uint32_t *dst, const char *file)
> -{
>  #if HAVE_UNISTD_H
> +static int read_random(uint8_t *dst, size_t len, const char *file)
> +{
>      int fd = avpriv_open(file, O_RDONLY);
> -    int err = -1;
> +    ssize_t err = -1;
>
> +    if (len > SSIZE_MAX)
> +        return -1;
>      if (fd == -1)
>          return -1;
> -    err = read(fd, dst, sizeof(*dst));
> +    err = read(fd, dst, len);
>      close(fd);
> +    if (err == -1)
> +        return AVERROR(errno);
>
> -    return err;
> -#else
> -    return -1;
> +    return err == len;
>  #endif
>  }
>
> @@ -118,29 +120,47 @@ static uint32_t get_generic_seed(void)
>      return AV_RB32(digest) + AV_RB32(digest + 16);
>  }
>
> -uint32_t av_get_random_seed(void)
> +int av_random(uint8_t* buf, size_t len)
>  {
> -    uint32_t seed;
> +    int err = AVERROR_UNKNOWN;
>
>  #if HAVE_BCRYPT
>      BCRYPT_ALG_HANDLE algo_handle;
>      NTSTATUS ret = BCryptOpenAlgorithmProvider(&algo_handle, BCRYPT_RNG_ALGORITHM,
>                                                 MS_PRIMITIVE_PROVIDER, 0);
>      if (BCRYPT_SUCCESS(ret)) {
> -        NTSTATUS ret = BCryptGenRandom(algo_handle, (UCHAR*)&seed, sizeof(seed), 0);
> +        NTSTATUS ret = BCryptGenRandom(algo_handle, (PUCHAR)buf, len, 0);
>          BCryptCloseAlgorithmProvider(algo_handle, 0);
>          if (BCRYPT_SUCCESS(ret))
> -            return seed;
> +            return 0;
>      }
>  #endif
>
>  #if HAVE_ARC4RANDOM
> -    return arc4random();
> +    arc4random_buf(buf, len);
> +    return 0;
> +#endif
> +
> +#if HAVE_UNISTD_H
> +    err = read_random(buf, len, "/dev/urandom");
> +    if (err == 1)
> +        return 0;
> +    err = read_random(buf, len, "/dev/random");
> +    if (err == 1)
> +        return 0;
> +    if (err == 0)
> +           err = AVERROR_UNKNOWN;
>  #endif
>
> -    if (read_random(&seed, "/dev/urandom") == sizeof(seed))
> -        return seed;
> -    if (read_random(&seed, "/dev/random")  == sizeof(seed))
> -        return seed;
> -    return get_generic_seed();
> +    return err;
> +}
> +
> +uint32_t av_get_random_seed(void)
> +{
> +    uint32_t seed;
> +
> +    if (av_random((uint8_t *)&seed, sizeof(seed)) < 0)
> +        return get_generic_seed();
> +
> +    return seed;
>  }
> diff --git a/libavutil/random_seed.h b/libavutil/random_seed.h
> index 0462a048e0..ce982bb82f 100644
> --- a/libavutil/random_seed.h
> +++ b/libavutil/random_seed.h
> @@ -36,6 +36,18 @@
>   */
>  uint32_t av_get_random_seed(void);
>
> +/**
> + * Generate cryptographically secure random data, i.e. suitable for use as
> + * encryption keys and similar.
> + *
> + * @param buf buffer into which the random data will be written
> + * @param len size of buf in bytes
> + *
> + * @retval 0 success, and len bytes of random data was written into buf, or
> + *         a negative AVERROR code if random data could not be generated.
> + */
> +int av_random(uint8_t* buf, size_t len);

av_random seems like a pretty generic name for something thats
requiring to be cryptographically secure and otherwise fail. I would
expect a more specific name for that purpose. There is plenty other
uses of randomness in multimedia, noise, dithering, etc, which don't
need crypto security. The function doesn't have to handle those, but
maybe it should be specific in what it does handle?

- Hendrik
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