How to check valid Kid:Key?

[Musito]

Hi!

Is there a way to check that a given kid:key pair is valid with a specific manifest?

For example, download the init.mp4 or a segment of the video and try to decrypt it with that key pair and return if the keys are correct or not.

Thanks!

[pteque]

From a mathematical point of view, any key (of the right length) is valid, so standard decryption tools (e.g. mp4decrypt) don't tell you whether a key is valid because they don't check that the media streams they produce look plausible.

What you can do is ask ffmpeg to play the decrypted media stream and check the number of warnings/errors that it generates. Here is some example code in tests for the Rust library dash-mpd-rs (which is used in the download tool dash-mpd-cli).