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

[James Almer]

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;
+}
+
+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);
+
 /**
  * @}
  */
-- 
2.41.0

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