This is a great example of warning signs. Note the number of times sizes (bits) and algorithm (AES/Rijndael) are mentioned. Note the lack of clarity in the specification (is "+" addition, XOR, or concatenation?) Note the overly-large UID space which has nothing to do with security (will you really have more than 4 billion users?)

Now look at how useful this design is. You've created a single-purpose authenticator that just says "someone at some point in time has seen this exact UID/authenticator pair". All other features (expiration, privileges, source of the authenticator, version, system name the UID is for) are left out.

Maybe it's "secure", but it doesn't do anything.