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

[James Almer]

On 7/4/2023 5:41 PM, James Almer 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 at 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;

Changed this locally to AVERROR(ENOSYS), which is the more correct error 
value if nothing below is available.

Will send a v3 with that change after this one is reviewed anyway since 
this code is delicate and i want to push exactly what was approved.

>   
>   #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);
> +
>   /**
>    * @}
>    */