Usage of decrypt-cenc for an encrypted DASH Stream

[completafurdc]

Hi! I have an encrypted DASH Stream with its Key. I want to decrypt this stream and convert it back to DASH Format. So, What I did was to download a segment and its corresponding init file. They I merged both the files init + segment. I then tried to run
go run .\examples\decrypt-cenc\main.go -i .\input.mp4 -o .\output.mp4 -k KEY which gave me an error

2023/01/22 19:20:38 decode mdat: unexpected EOF exit status 1 whereas when I decrypt the segment using mp4decrypt.exe it works well. Is there some issue with they way I am concating the init and the segment?

Also, how do I remove the init or the moov atoms after decryption so that it is compatible again for DASH streaming?

[tobbee]

Hi @completafurdc,

Thanks for trying out the software. The example code is named so to signal that it is rather specific code, and may not be able to handle arbitrary input.

From you description, it is hard to know what goes wrong, but the error message indicates that the decoder is trying to read too far in the mdat box. That is the part of the media file that contains the (encrypted) media. Why it reads too far is hard to tell without further analysis, but it may be due to some offset not being handled properly.

One weak point in the code is the geting the offset decryptSamplesInPlace as described in the TODO comments. That may be related.

You may also want to check if the tool mp4ff-info can parse and provide information about the combined file. That may give you some helpful information.

Regarding splitting a file created by concatenating an init segment with a media segment, you can read the list of top level boxes to find where the media segment starts (with an styp or moof box). The top level of mp4 boxes is just a list of boxes where each box has the structure [4 byte size][4 byte name][size-8 bytes content]. Use mp4ff-into to get a better view of the structure. You can then split the file using "tail -c ...", or write a small program that does the split. An alternative would be to rewrite the example code so that it outputs just the media segment without the init segment.

[completafurdc]

Hi! Thanks for prompt response. Is it possible I can share the sample files with further more description related to this issue in private? Due to company's security policy, I cannot share the assets or screenshots in public. If that's okay then we may discuss it here,

[tobbee]

Hi @completafurdc,

I'm afraid I cannot help you directly, but since two weeks, I'm working as a consultant at Eyevinn Technology, so I may be able to help you professionally. If you want to discuss that possibility, feel free to contact me at [email protected].

[tobbee]

The new mp4ff-decrypt tool should handle most cases, since it can decrypt both cenc and cbcs schemes.