NFC hotel keys: when tag-types matter

Sometimes low-tech is better for security, particularly when compared to choosing the wrong type of latest technology. Growing use of NFC in hotel keys provides some instructive examples.

The hospitality industry started out with old-fashioned analog keys, where guests were given actual physical objects. Not surprisingly, they promptly proceeded to lose or misplace those keys, creating several problems for the hotel. First one is getting replacement keys made— certainly there are additional copies lying around, but they are still finite in number compared to the onslaught of careless guests. But more costly is having to potentially change that lock itself. With a key missing (and very likely attached to a key-tag bearing the hotel logo, along with room number) there is now someone else out there who could get into the room at will to burglarize the place. Upgrading to programmable card keys conveniently solves both problems. With a vast supply of cheap plastic stock, the hotel need not even charge guests for failing to return their key at checkout. Lost cards are not a security problem either, as the locking mechanism itself can be programmed to reject previous keys.

Until recently most of these cards used magnetic-stripes, the same 40-year old technology found in traditional credit-cards. It is trivial to clone such cards by reading the data encoded on the card and writing it over to a new blank card. Encrypting or authenticating card contents will not help— there is no need to reverse-engineer what those bits on the stripe represent or modify them. We are guaranteed that an identical card with same exact bits will open the door as long as the original does. (There may well be time-limitations, such as check-out date encoded in the card beyond which the door will not permit entry but those apply to both the original and clone.)

Jury is out on whether cards are easier to copy than old-fashioned keys. In both cases, some type of proximity is required. Reading magnetic stripe involves swiping the card through a reader; this can not be done with any stealth while the guest remains in possession of the card. Physical keys can also be copied at any home-improvement store but more surprisingly a clone can be made from a photograph of the key alone. The bitting, that pattern of bumps and ridges, is the abstract representation of the key that is sufficient to create a replica. This can be extracted from an image, as famously demonstrated for Diebold voting-machines. One paper from 2008 even used a high-powered zoom lenses to demonstrate feasibility of automatically extracting bitting from photographs taken 200ft away from the victim.

Next cycle of upgrades in the industry is replacing magnetic stripes by NFC. This blogger recently used such a key and decided to scan it using the handy NXP TagInfo application on Android:

Ultralight hotel key

How does this fare in comparison to the previous two options? It is arguably easiest to clone.

First notice the tag type is Ultralight. As the name implies, this is among the simplest types with least amount of memory. More importantly, Ultralight tags have no security mechanism against protecting their contents against reading. They are effectively 48 bytes worth of open books, available for anyone to read without authentication. Compare that to Mifare DESFire or even Mifare Classic, where each sector on the card has associated secret keys for read/write access. Data on those tags can only be accessed after NFC reader authenticates with the appropriate key.

So far this is not that different from magnetic stripes. The problem is NFC tags can be scanned surreptitiously while they are still on the person. It requires proximity; “N” does stand for “near” and typical read-ranges are limited to ~10cm. The catch is tags can be read through common materials such as cotton, plastic or leather. Metal shielding is required to effectively protect RFID-enabled objects against reading. It takes considerable skill to grab a card out of someone’s pocket, swipe it through a magnetic-stripe reader and return it undetected. It is much easier to gently bump into the same person while holding an ordinary phone with NFC capabilities.

The business case for upgrading a magnetic-stripe system to Ultralight is unclear. It certainly did not improve the security of guest rooms. By all indications it is also more expensive. Even the cheapest NFC tag still costs more than the ubiquitous magnetic stripe which has been around for decades. Same goes for readers mounted at every door required to process these cards. The only potential cost savings lie in using off-the-shelf mobile devices such as Android tablets for encoding cards at the reception desk, a very small piece of overall installation costs. (Speaking of mobile, this system also can not implement the more fancy use case of allowing customers to use their own phone as room key. While NFC hardware in most devices is capable of card-emulation, it can only emulate specific tag types. Ultralight is not one of them.) For a small incremental cost, the company providing this system could have used Ultralight C tags, which have three times as much memory, but also crucially, support 3DES-based authentication with readers that makes cloning non-trivial.

CP