How to Decrypt and Play CENC protected contents in DASH?
I have a MPD clip which supports CENC,how to decrypt and play without using any specific DRM engine??Is there any decryption algorithm available to decrypt AES CTR 128 bit?If so will it be used to decrypt MPEG-DASH content?There are some third party libraries available in net like NACL,Openssl,crypto++,which can do AES ctr 128 decryption.Can i use one of them to decrypt DASH Content(CENC supported) ??
Answer
MPEG-CENC is simply AES-128 CTR encryption on an ISO BMFF (mp4) file. The specification for how this is applied to CENC is here: https://www.w3.org/TR/2014/WD-encrypted-media-20140828/cenc-format.html and https://www.iso.org/obp/ui/#iso:std:iso-iec:23001:-7:ed-1:v1
And a good explanation of AES-128 CTR https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Counter_.28CTR.29
To decrypt you will need the key and also the Initialization Vector (IV) that was used to encrypt the content. This is available from the senc and tenc boxes in the mp4 file. Once you will have those you will simply need to go through the samples or fragments of the file and decrypt and reassemble the mp4 (assuming you want to play it).
As mentioned by Bento tools mp4decrypt gives an example on how to do this: https://github.com/axiomatic-systems/Bento4/blob/master/Source/C%2B%2B/Apps/Mp4Decrypt/Mp4Decrypt.cpp