Code hopping RFID cards

I have been recently reading up on field programming RFID cards (changing their ID values after they have been manufactured), and while I am still quite sketchy on how the entire process works, it seems to present some interesting flexibility that current RFID technology does not allow. Today, if a person copies and retransmits the "unique" ID from a RFID card, there is no way to verify that the intended device sent that signal, and by recording a RFID entry card while the person is on a subway, it would be easy to gain unauthorized access.

Based on the above information, my question is whether it is possible to use a reading/reprogramming device to read a unique ID, look up in in a database whether that ID is authorized, and if so reprogram the RFID chip to a new, random ID and then grant entry. This is a similar system to the one used by garage door openers, and while the RFID system (where the reader does the reprogramming instead of both transmitter and receiver hopping codes in a publicly unreproducible, but secretly reproducible system) is less secure than the garage door example, it would provide some benefit and would prevent repeated unauthorized access.

A couple potential drawbacks, but in my mind minor compared to the security gains:

• Because the RFID chip would be constantly changing values, it would only work on a single reader system. This system could include multiple entry points, but they would all need to sync back to the same database. It could not, for example, be used for work entrances and then also for the keyless entry in a car.
• It would not actually prevent the situation on a subway where someone reads your key and then immediately uses it, for the system would just reprogram their faked card and they would be home free. The fact that the legitimate card no longer works, however, might raise red flags and help aid in detainment of the unauthorized person.
• The read than write than read again process would take considerably more time than a simple read process to complete, potentially delaying entry. This could be alleviated by the system blinking yellow while the updating process is occurring, or simply not showing any sign of activity until the update was complete, encouraging the user to keep the card near the reader.
• If the user removes the card during the writing process, it is possible the card could be corrupted. A simple alarm or beep/light sequence could alert the user to this, allowing them to hold the card nearby to be correctly reprogrammed, but it is possible people would disregard the warning and be unable to use the RFID card in the future.

I think this is a really interesting technology, and I am very interested in learning more about how the reprogramming works and whether it is possible to implement this on a small scale as a way to enter my house and/or automobile

Great site for background images

A few months ago I found a website I really like as a source for background images, it is called "InterfaceLift" (a totally weird name, I know) and have am subscribed to the new wallpaper RSS feed. This pulls down top rated images from IFL's voting system (known as "The Loupe") and displays them, enabling me to easily view and download images I like. I would highly recommend subscribing to it if you are interested in completely utilizing windows 7's background slideshow, as I now have over 550 images in my rotation, and all of them are awesome!

Side note: while it is not immediately obvious, if you sign up as a member you can view and vote on Loupe submissions, which are often at least as interesting as the ones that finally make it through, though downloading the images is a little bit more of a hassle.