Justin,
I have tested today a docid to read it by decoding it via base64... and it doesn't work.
Are you 100% of what you said?
I know for a fact, that the docid for security reasons they use the user ID for the user requesting the docid to "encrypt" the docid.
So if userA gets a docid, and gives it to userB, then userB cannot use the docid, since he is not userA.
For me this is like an encryption of the docid using the userA "key", and only userA can decrypt it, since he has his secret key.
Now what is the exact algorithm used, I have no idea. But from my simple test it doesn't seems to be base64, at least when I use base64 I receive an error saying that there are garbage characters.
If you have more juicy info, I am all hears :-D