Urgent: mp4decrypt copies file but doesn't decrypt

[SAVVYSCLUTTER]

I'm downloading from Channel 4 in Edge on Win10 in VirtualBox (Win7 host). I used WidevineProxy2 with the first CDM in https://forum.videohelp.com/threads/417425-Real-Device-L3-Cdms. First, I ran the N_m3u8DL-RE command from WidevineProxy2 followed by

.\mp4decrypt --key 00000000000000000000000012743272:80e6b93ac609c4d09 64b40b1f8080141 stream_2025-09-12_18-03-37.mkv countdown.mp4

, which created a 0 KB file. Next, I changed format=mkv to format=mp4 in the N_m3u8DL-RE command and ran mp4decrypt, which created a file the same size as the encrypted file. When I play it in WMP or VLC, it's scrambled. What did I do wrong?

[iamghost]

channel 4 URL?
yesterday show

--key 00000000000000000000000012753433:a74fb757cd32e468b1633404e9509037

[SAVVYSCLUTTER]

channel 4 URL?
yesterday show

--key 00000000000000000000000012753433:a74fb757cd32e468b1633404e9509037

https://www.channel4.com/programmes/countdown/on-demand/77001-112

[iamghost]

https://www.channel4.com/programmes/countdown/on-demand/77001-112

keys is fine, no problem with decrypt video
https://gofile.io/d/aXtQHO

[SAVVYSCLUTTER]

https://www.channel4.com/programmes/countdown/on-demand/77001-112

keys is fine, no problem with decrypt video
https://gofile.io/d/aXtQHO

Thanks. Please can you tell me what you did differently to me? I can upload the video that doesn't play if you like.