[FFmpeg-devel] [PATCH v4 2/4] avutil/random_seed: add av_random_bytes()

[James Almer <jamrial@gmail.com>]

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 | 33 ++++++++++++++++++++++-----------
 libavutil/random_seed.h | 13 +++++++++++++
 2 files changed, 35 insertions(+), 11 deletions(-)

diff --git a/libavutil/random_seed.c b/libavutil/random_seed.c
index a51149235b..f5c291263e 100644
--- a/libavutil/random_seed.c
+++ b/libavutil/random_seed.c
@@ -46,7 +46,7 @@
 #define TEST 0
 #endif
 
-static int read_random(uint32_t *dst, const char *file)
+static int read_random(uint8_t *dst, size_t len, const char *file)
 {
 #if HAVE_UNISTD_H
     FILE *fp = avpriv_fopen_utf8(file, "r");
@@ -54,10 +54,10 @@ static int read_random(uint32_t *dst, const char *file)
 
     if (!fp)
         return AVERROR_UNKNOWN;
-    err = fread(dst, 1, sizeof(*dst), fp);
+    err = fread(dst, 1, len, fp);
     fclose(fp);
 
-    if (err != sizeof(*dst))
+    if (err != len)
         return AVERROR_UNKNOWN;
 
     return 0;
@@ -121,27 +121,38 @@ 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_bytes(uint8_t* buf, size_t len)
 {
-    uint32_t seed;
+    int err;
 
 #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 (!read_random(&seed, "/dev/urandom"))
-        return seed;
-    return get_generic_seed();
+    err = read_random(buf, len, "/dev/urandom");
+
+    return err;
+}
+
+uint32_t av_get_random_seed(void)
+{
+    uint32_t seed;
+
+    if (av_random_bytes((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..e67e6e38c4 100644
--- a/libavutil/random_seed.h
+++ b/libavutil/random_seed.h
@@ -36,6 +36,19 @@
  */
 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, len bytes of random data was written
+ *                                   into buf
+ * @retval "a negative AVERROR code" random data could not be generated
+ */
+int av_random_bytes(uint8_t *buf, size_t len);
+
 /**
  * @}
  */
-- 
2.41.0

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