It's been found  that Xbox360 Live Arcade downloads files with the extension .xcp over unencrypted http on port 3074 for content distribution.
- XCP stands for Xbox Content Package (from http://forums.xbox-scene.com/index.php?s=&showtopic=462911&view=findpost&p=3067263)
- XCP files are (in-order) encrypted and compressed (with zlib). Underneath, they are STFS containers.
- XCP files from some categories ("Full Game Demos" being one of them) are not encrypted
- It may be possible to use the avatar downloads to glean more info about the file format.
- The communication is pure http and you can redirect traffic to a local webserver, however it does checksum the file
- Content downloaded with licenses are locked to a 360 at purchase
- Box A downloads trial
- Box B can run the same downloaded trial
- Box A upgrades trial to full
- Box B can only run in trial mode (if available)
- Content is downloaded and extracted into place
- When upgrading to full license the 360 dials home and then some modification is made to lock content to the particular box
First Generation Xbox
xcp files are not new to the Xbox 360.
- Are encrypted Cabinet files
- Encrypted with a public and symmetric key.
- Keys needed for extraction.
- Need tools to create our own without the Xbox SDK