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Physically Unclonable Function

Types of PUFs

PUFs using explicitly introduced randomness

Optical PUF

An optical PUF consists of a transparent material that is doped with light scattering particles. When a laser beam shines on the material, a random and unique speckle pattern will arise. The placement of the light scattering particles is an uncontrolled process and the interaction between the laser and the particles is very complex. Therefore, it is very hard to duplicate the optical PUF such that the same speckle pattern will arise. We say the optical PUF is practically unclonable.

Coating PUF

A coating PUF can be built in the top layer of an IC. Above a normal IC, a network of metal wires is laid out in a comb shape. The space between and above the comb structure is filled with an opaque material and randomly doped with dielectric particles. Because of the random placement, size and dielectric strength of the particles, the capacitance between each couple of metal wires will be random up to a certain extent. This unique randomness can be used to obtain a unique identifier for the device carrying the Coating PUF. Moreover, the placement of this opaque PUF in the top layer of an IC protects the underlying circuits from being inspected by an attacker, e.g. for reverse-engineering. When an attacker tries to remove (a part of) the coating, the capacitance between the wires is bound to change and the original unique identifier will be destroyed. In it was shown how an unclonable RFID Tag is be built with Coating PUFs.

PUFs using intrinsic randomness

Silicon PUF

A silicon PUF exploits the random variations in delays of wires and gates. Given an input challenge, a race condition is set up in the circuit, and two transitions that propagate along different paths are compared to see which comes first. An arbiter, typically implemented as a latch, produces a 1 or a 0, depending on which transition comes first. Many circuits realizations are possible and at least two have been fabricated. When a circuit with the same layout mask is fabricated on different chips, the logic function implemented by the circuit is different for each chip due to the random variations of delays.

A PUF based on a delay loop, i.e., a ring oscillator with logic, is described in . A multiplexor-based PUF is described in . A secure processor design using a PUF is described in . A multiplexor-based PUF with an RF interface for use in RFID anti-counterfeiting applications is described in .

SRAM PUF

A Physically Unclonable Function is a function that relies on variations which inadvertently exist for the material used for hardware. They produce - given an input - an output that will be different from other instances of said hardware, thus providing a countermeasure against creation of exact clones for a specific product. They are present in all ICs having SRAM memory on board. Their behavior and application for anti-counterfeiting purposes were investigated in detail in and in On top of this they permit the implementation of secure secret key storage without storing the key in digital form.

An example would be an RFID tag, which can easily be cloned. When equipped with a PUF however, creating a clone in a reasonable timeframe can be next to impossible.

Butterfly PUF

Recently a new PUF was introduced : the Butterfly PUF. The Butterfly PUF is based on cross-coupling of two latches or flip-flops. The mechanism being this PUF is similar to the one behind the SRAM PUF but has the advantage that it can be implemented on any SRAM FPGA.

Magnetic PUF

A magnetic PUF exists on a magnetic stripe card. The physical structure of the magnetic media applied to a card is fabricated by blending billions of particles of barium ferrite together in a slurry during the manufacturing process. The particles have many different shapes and sizes. The slurry is applied to a receptor layer. The particles land in a random fashion, much like pouring a handful of wet magnetic sand onto a carrier. To pour the sand to land in the exact same pattern a second time is physically impossible due to the inexactness of the process, the sheer number of particles, and the random geometry of their shape and size. The randomness introduced during the manufacturing process cannot be controlled. This is a classic example of a PUF using intrinsic randomness.

When the slurry dries, the receptor layer is sliced into strips and applied to plastic cards, but the random pattern on the magnetic stripe remains and cannot be changed. Because of their physically unclonable functions, it is highly improbable that two magnetic stripe cards will ever be identical. In fact, using a standard size card, the odds of any two cards having an exact matching magnetic PUF are calculated to be 1 in 900 million.[citation needed] Further, because the PUF is magnetic, we know that each card will carry a distinctive, repeatable and readable magnetic signal.

Personalizing the PUF

The personal data encoded on the magnetic stripe contributes another layer of randomness. When the card is encoded with personal identifying information, the odds of two encoded magstripe cards having an identical magnetic signature are approximately 1 in 10 Billion.[citation needed] The encoded data can be used as a marker to locate significant elements of the PUF. This signature can be digitized and is generally called a magnetic fingerprint. An example of its use is in the Magneprint brand system.

Stimulating the PUF

The magnetic head acts as a stimulus on the PUF and amplifies the random magnetic signal. Because of the complex interaction of the magnetic head, influenced by speed, pressure, direction and acceleration, with the random components of the PUF, each swipe of the head over the magnetic PUF will yield a stochastic, but very distinctive signal. Think of it as a song with thousands of notes. The odds of the same notes recurring in an exact pattern from a single card swiped many times are 1 in 100 million, but overall the melody remains very recognizable.

Uses for a Magnetic PUF

The stochastic behavior of the PUF in concert with the stimulus of the head makes the magnetic stripe card an excellent tool for Dynamic Token Authentication, Forensic Identification, Key generation, One-Time Passwords, and Digital Signatures.

References

^ Physical One-Way Functions

^ R. Pappu, B. Recht, J. Taylor, and N. Gershenfeld. Physical One-Way functions. Science, 297(5589):2026-2030, Sep 2002. http://dx.doi.org/10.1126/science.1074376.

^ B. Skoric, S. Maubach, T. Kevenaar, and P. Tuyls. Information-theoretic analysis of capacitive physical unclonable functions. J. Appl. Phys., 100(2):024902, Jul 2006. http://dx.doi.org/10.1063/1.2209532

^ B. Skoric, G.-J. Schrijen, W. Ophey, R. Wolters, N. Verhaegh, and J. van Geloven. Experimental hardware for coating PUFs and optical PUFs. In P. Tuyls, B. Skoric, and T. Kevenaar, editors, Security with Noisy Data - On Private Biometrics, Secure Key Storage and Anti-Counterfeiting, pages 255-268. Springer London, 2008. http://dx.doi.org/10.1007/978-1-84628-984-2_15

^ Pim Tuyls, Geert-Jan Schrijen, Boris Skoric, Jan van Geloven, Nynke Verhaegh and Rob Wolters: "Read-proof hardware from protective coatings", CHES 2006, p 369- 383.

^ Pim Tuyls, Lejla Batina RFID-Tags for Anti-counterfeiting. CT-RSA, 2006, pp:115-131

^ B. Gassend, D. Clarke, M. van Dijk and S. Devadas. Silicon Physical Random Functions. Proceedings of the Computer and Communications Security Conference, November 2002

^ D. Lim, J-W. Lee, B. Gassend, M. van Dijk, E. Suh, and S. Devadas. Extracting Secret Keys from Integrated Circuits, IEEE Transactions on VLSI Systems, volume 13, Number 10, pages 12001205, October 2005

^ G. E. Suh, C. W. O'Donnell, and S. Devadas. Aegis: A Single-Chip secure processor. IEEE Design and Test of Computers, 24(6):570-580, Nov 2007. http://dx.doi.org/10.1109/MDT.2007.179

^ S. Devadas, V. Khandelwal, S. Paral, R. Sowell, E. Suh, T. Ziola, Design and Implementation of `Unclonable' RFID ICs for Anti-Counterfeiting and Security Applications, RFID World 2008, March 2008

^ Jorge Guajardo, Sandeep S. Kumar, Geert-Jan Schrijen, Pim Tuyls, PGA Intrinsic PUFs and Their Use for IP Protection, Workshop on Cryptographic Hardware and Embedded Systems (CHES), Sep 10-13, 2007, Vienne, Austria

^ Jorge Guajardo, Sandeep S. Kumar, Geert-Jan Schrijen, Pim Tuyls, hysical Unclonable Functions and Public-Key Crypto for FPGA IP Protection, International Conference on Field Programmable Logic and Applications (FPL), Aug 27-29, 2007, Amsterdam, The Netherlands.

^ L. Bolotnyy and G. Robins. Physically unclonable Function-Based security and privacy in RFID systems. In 5th IEEE Int. Conf. on Pervasive Computing and Communications (PERCOM), pages 211-220, Washington, DC, USA, 2007. IEEE Computer Society. http://dx.doi.org/10.1109/PERCOM.2007.26

^ S. Kumar, J. Guajardo, R. Maes, G.J. Schrijen qnd P. Tuyls, The Buttefly PUF: Protecting IP on every FPGA, In IEEE International Workshop on Hardware Oriented Security and Trust, Anaheim 2008.

^ http://www.aip.org/dbis/stories/2005/14405.html

^ Tony Fitzpatrick, Nov. 11, 2004, "Magneprint technology licensed to TRAX Systems, Inc." http://news-info.wustl.edu/tips/page/normal/4159.html

^ Patrick L. Thimangu,January 7, 2005, "Washington U. cashing in with MagnePrint licensing," St. Louis Business Journal http://www.bizjournals.com/stlouis/stories/2005/01/10/story7.html?jst=s_cn_hl

http://people.csail.mit.edu/rudolph/Teaching/Lectures/Security/Lecture-Security-PUFs-2.pdf

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Airline Carry-on dilemma?

I have to go on a trip to London in about 3 days. I want to take my laptop, a tote bag, and my purse, but I am only allowed one personal item and one bag on the plane and I want all of them to go on it. My purse can fit in my tote bag, but will that count as two items or one? Do I have to pay extra money if I bring an extra carry-on or will they just plain-out not allow me to have three things on the plane? My laptop can also fit inside my tote bag, but not with the purse in it.
((American Airlines))
((When I say I have to go I mean I'm a minor and my parents are forcing me to go with them on the trip. haha.))

Your purse can go in your tote bag and they won't care. I've gone through security with a backpack a purse and a bag of cookies in my hand before and they don't really care. Just one time when I had a suitcase and a backpack I was stopped, but that was because my suitcase was too big.

FYI you can't pay money for an extra carry on. If you really had too much stuff(which you don't) you would have to check a bag.