Before you execute any mp4decrypt.exe download, run through this mental checklist:

Cause: You downloaded a standalone .exe without the full GPAC libraries. Solution: Never use a standalone copy. Install the full GPAC suite from the verified source above. The required .dll files (like libgpac.dll) are inside the suite.

mp4decrypt.exe --key 1:deadbeef... encrypted.mp4 decrypted.mp4

(The key must match the KID:KEY used during encryption.)

Note: This post explains what MP4Decrypt.exe is, how to obtain it safely, how to verify downloads, and how to use it responsibly. Do not use tools to circumvent DRM or access content you don’t have rights to.