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

[Marton Balint]

On Tue, 4 Jul 2023, 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 <jamr...@gmail.com>
---
TODO: APIChanges entry and minor version bump.

Also, if a new random.h header is prefered, i can move the prototype there.

libavutil/random_seed.c | 46 ++++++++++++++++++++++++++---------------
libavutil/random_seed.h | 12 +++++++++++
2 files changed, 41 insertions(+), 17 deletions(-)

diff --git a/libavutil/random_seed.c b/libavutil/random_seed.c
index 66dd504ef0..39fb27c5ad 100644
--- a/libavutil/random_seed.c
+++ b/libavutil/random_seed.c
@@ -46,20 +46,20 @@
#define TEST 0
#endif

-static int read_random(uint32_t *dst, const char *file)
-{
#if HAVE_UNISTD_H
+static ssize_t 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);

    return err;
-#else
-    return -1;
#endif
}

@@ -118,29 +118,41 @@ 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;
-
#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
+    if (read_random(buf, len, "/dev/urandom") == len)
+        return 0;
+    if (read_random(buf, len, "/dev/random")  == len)
+        return 0;
#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 AVERROR_INVALIDDATA;

Probably AVERROR(ENOSYS) is a more suiting error code for this.

Regards,
Marton
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