I know one person has successfully decoded it, and that it is just the header/index which is encrypted, but said person has chosen not to make the discovery public (presumably for legal reasons).
(No, it's not me. Like I could figure that out???)
Well no, I mean like presets (recolors) for instance that a user makes in game and then uploads to the exchange or they upload the Sims3Pack they exported to their site. Currently the Launcher takes the recolor and removes the manifest, changes the instance numbers of the XML and PNG, and then shoves it into dbcdb0.dbc or something like that in the DCCache folder. The dbcdb0.dbc file also contains an index resource of some sort that I haven't yet figured out. The Launcher does the same for patterns, but patterns can be extracted and stuck into the Mods/Packages folder and will show up in the game just fine (but not as custom content, acts as if it's EA made content.) If you do this for presets, though, it won't work. The PNG image will overwrite the default game thumbnail of the item, but you won't be able to select it to use it. That's what I'm referring to, has someone figured out a way to install presets without using the Launcher?
In theory we'll have the same problem with Sims3Packs that are created of objects and CAS Parts people make once that time comes, but I figure we'll cross that bridge when we come to it...