avformat/dashdec: accept and relay CENC decryption key

Allows to process CENC-encrypted media segments.
Option arg syntax is same as that for option decryption_key in MOV demuxer

doc/demuxers.texi

@@ -274,6 +274,17 @@ which streams to actually receive.
 Each stream mirrors the @code{id} and @code{bandwidth} properties from the
 @code{<Representation>} as metadata keys named "id" and "variant_bitrate" respectively.
 
+@subsection Options
+
+This demuxer accepts the following option:
+
+@table @option
+
+@item cenc_decryption_key
+16-byte key, in hex, to decrypt files encrypted using ISO Common Encryption (CENC/AES-128 CTR; ISO/IEC 23001-7).
+
+@end table
+
 @section imf
 
 Interoperable Master Format demuxer.

libavformat/dashdec.c

@@ -150,6 +150,7 @@ typedef struct DASHContext {
     char *allowed_extensions;
     AVDictionary *avio_opts;
     int max_url_size;
+    char *cenc_decryption_key;
 
     /* Flags for init section*/
     int is_init_section_common_video;
@@ -1893,6 +1894,9 @@ static int reopen_demux_for_component(AVFormatContext *s, struct representation
     pls->ctx->pb = &pls->pb.pub;
     pls->ctx->io_open  = nested_io_open;
 
+    if (c->cenc_decryption_key)
+        av_dict_set(&in_fmt_opts, "decryption_key", c->cenc_decryption_key, AV_OPT_FLAG_DECODING_PARAM);
+
     // provide additional information from mpd if available
     ret = avformat_open_input(&pls->ctx, "", in_fmt, &in_fmt_opts); //pls->init_section->url
     av_dict_free(&in_fmt_opts);
@@ -2344,6 +2348,7 @@ static const AVOption dash_options[] = {
         OFFSET(allowed_extensions), AV_OPT_TYPE_STRING,
         {.str = "aac,m4a,m4s,m4v,mov,mp4,webm,ts"},
         INT_MIN, INT_MAX, FLAGS},
+    { "cenc_decryption_key", "Media decryption key (hex)", OFFSET(cenc_decryption_key), AV_OPT_TYPE_STRING, {.str = NULL}, INT_MIN, INT_MAX, .flags = FLAGS },
     {NULL}
 };