[FFmpeg-devel] [PATCH] avformat/ac3dec: detect fairplay encryption

[Sebastian Hubbard]

Added id3v2_buf and id3v2_buf_size fields to AVProbeData. When an id3v2
tag is found at the start of probed content, id3v2_buf is filled with
a pointer to the start of the tag bytes, and id3v2_buf_size with the
number of bytes in the tag.

ac3_eac3_probe checks the tag when present for a private frame with an
owner identifier of com.apple.streaming.audioDescription, which
signals that the file is encrypted with an Apple DRM scheme. The CRC
checksums for such files are for the content post-decryption, and with
this change they are skipped when Apple DRM is detected. This enables
ffprobe to detect the stream type, even though it will not be playable.

Signed-off-by: Sebastian Hubbard <sebh at google.com>
---
 libavformat/ac3dec.c   | 22 ++++++++++++++++++++--
 libavformat/avformat.h |  2 ++
 libavformat/format.c   |  6 ++++--
 3 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/libavformat/ac3dec.c b/libavformat/ac3dec.c
index 1f87939388..c29c6fb27b 100644
--- a/libavformat/ac3dec.c
+++ b/libavformat/ac3dec.c
@@ -24,17 +24,35 @@
 #include "libavcodec/ac3_parser.h"
 #include "avformat.h"
 #include "rawdec.h"
+#include "id3v2.h"
+#include "avio_internal.h"
 
 static int ac3_eac3_probe(const AVProbeData *p, enum AVCodecID expected_codec_id)
 {
     int max_frames, first_frames = 0, frames;
     const uint8_t *buf, *buf2, *end;
     enum AVCodecID codec_id = AV_CODEC_ID_AC3;
+    AVIOContext ioctx;
+    AVDictionary *metadata = NULL;
+    ID3v2ExtraMeta *id3v2_extra_meta = NULL;
+    int skip_crc = 0;
 
     max_frames = 0;
     buf = p->buf;
     end = buf + p->buf_size;
 
+    if (p->id3v2_buf && p->id3v2_buf_size > 0) {
+        ffio_init_context(&ioctx, p->id3v2_buf, p->id3v2_buf_size, 0, NULL, NULL, NULL, NULL);
+        ff_id3v2_read_dict(&ioctx, &metadata, ID3v2_DEFAULT_MAGIC, &id3v2_extra_meta);
+        if (!ff_id3v2_parse_priv_dict(&metadata, &id3v2_extra_meta)) {
+            if (av_dict_get(metadata, "id3v2_priv.com.apple.streaming.audioDescription", NULL, 0)) {
+                skip_crc = 1;
+            }
+        }
+        ff_id3v2_free_extra_meta(&id3v2_extra_meta);
+        av_dict_free(&metadata);
+    }
+
     for(; buf < end; buf++) {
         if(buf > p->buf && !(buf[0] == 0x0B && buf[1] == 0x77)
                         && !(buf[0] == 0x77 && buf[1] == 0x0B) )
@@ -72,10 +90,10 @@ static int ac3_eac3_probe(const AVProbeData *p, enum AVCodecID expected_codec_id
                     buf3[i  ] = buf2[i+1];
                     buf3[i+1] = buf2[i  ];
                 }
-                if (av_crc(av_crc_get_table(AV_CRC_16_ANSI), 0, buf3 + 2, frame_size - 2))
+                if (!skip_crc && av_crc(av_crc_get_table(AV_CRC_16_ANSI), 0, buf3 + 2, frame_size - 2))
                     break;
             } else {
-                if (av_crc(av_crc_get_table(AV_CRC_16_ANSI), 0, buf2 + 2, frame_size - 2))
+                if (!skip_crc && av_crc(av_crc_get_table(AV_CRC_16_ANSI), 0, buf2 + 2, frame_size - 2))
                     break;
             }
             if (bitstream_id > 10)
diff --git a/libavformat/avformat.h b/libavformat/avformat.h
index 9b9b634ec3..afd1b67b5c 100644
--- a/libavformat/avformat.h
+++ b/libavformat/avformat.h
@@ -448,6 +448,8 @@ typedef struct AVProbeData {
     unsigned char *buf; /**< Buffer must have AVPROBE_PADDING_SIZE of extra allocated bytes filled with zero. */
     int buf_size;       /**< Size of buf except extra allocated bytes */
     const char *mime_type; /**< mime_type, when known. */
+    unsigned char *id3v2_buf; /**< Bytes of id3v2 tag if one was found at start of file. */
+    int id3v2_buf_size; /**< Size of id3v2_buf */
 } AVProbeData;
 
 #define AVPROBE_SCORE_RETRY (AVPROBE_SCORE_MAX/4)
diff --git a/libavformat/format.c b/libavformat/format.c
index c47490c8eb..cd510976a6 100644
--- a/libavformat/format.c
+++ b/libavformat/format.c
@@ -149,8 +149,10 @@ ff_const59 AVInputFormat *av_probe_input_format3(ff_const59 AVProbeData *pd, int
         if (lpd.buf_size > id3len + 16) {
             if (lpd.buf_size < 2LL*id3len + 16)
                 nodat = ID3_ALMOST_GREATER_PROBE;
-            lpd.buf      += id3len;
-            lpd.buf_size -= id3len;
+            lpd.id3v2_buf      = lpd.buf;
+            lpd.id3v2_buf_size = id3len;
+            lpd.buf           += id3len;
+            lpd.buf_size      -= id3len;
         } else if (id3len >= PROBE_BUF_MAX) {
             nodat = ID3_GREATER_MAX_PROBE;
         } else
-- 
2.25.0.265.gbab2e86ba0-goog