METAPUF A Challenge Response Pair Generator

Received Jul 23, 2018 Revised Sept 2, 2018 Accepted Oct 16, 2018 Physically unclonable function (PUF) is a hardware security module preferred for hardware feature based random number and secret key generation. Security of a cryptographic system relies on the quality of the challengeresponse pair, it is necessary that the key generation mechanism must unpredictable and its response should constant under different operating condition. Metastable state in CMOS latch is undesirable since it response becomes unpredictable, this feature used in this work to generate a unique response. A feedback mechanism is developed which forces the latch into the metastable region; after metastable state, latch settle to high or state depends on circuit internal condition and noise which cannot be predicted. Obtained inter hamming variation for 8 PUF is 51.43% and average intra hamming distance is 99.76% with supply voltage variation and 96.22% with temperature variation. Keyword:


Introduction
Security in a cryptographic system represented as how much secured the Response is! Kerckchoff's principle of cryptographic says "A cryptosystem should be secure even if everything about the system, except the key is public knowledge" [5] it is necessary that key must originate in a secured environment.The response is derived from random number processes by error correcting code to remove biasing error.The randomness of the random number is the strength of the key, classical algorithm based random number was deterministic in nature; it starts with an initial seed and starts repeating after sequence length.It is already proven that based on history upcoming random number can be predicted statically, which ultimately weakens the Response.Another method is physical variable based on a random number generator like thermal noise.In 2007 GE suh and S devadas [1] presented architecture of physically unclonable function (PUF) is an emerging technology for authentication and Response generation.PUF is a computational unit which output (response) is a function of input (challenge) as a well-selected hardware feature.A unique feature of hardware added into the computation [2,20].Two silicon ICs cannot have a similar feature; the silicon-based integrated circuit is the preferred choice for implementation of PUF circuit.
PUF circuit is unclonable and its output cannot be predicted.In 2013 NXP semiconductor has quoted that "any physical device characteristic that fluctuates can be turned into PUF".S.Devdas[1] have introduced delay-based PUF, delay unit implemented with a multiplexer, input signal raced between two parallel paths; at the destination, arbiter decides which decide which path is faster accordingly generate response bit [3].A ring oscillator PUF introduced in 2009;s election of operating frequency depaend on a large size multiplexer, two counters operate parallel on selected different ring oscillator.Response bit is high if value of counter1>counter2.[8][9][10][11].In 2015 PUF found application to securing the internet of things (IOT) device.Quality of PUF response is judged by 3 parameters [1, 7,13,17,18].A) Uniformity-Probability of occurrence of bit '1' and a bit '0' in the key should equal, half of the key length.Ideal value is 50%.B) Reliability-PUF should same response for a challenge in a different operating condition like variable supply voltage, high temperature, and noisy environment.Ideal value is 100%.C) Uniqueness-For same challenge different PUF must produce a different response.Ideal value is 50%.
The remaining section of the paper as follows.Section 2 reviews metastability state of CMOS latch.Section 3 describes the schematic details of metastable PUF and their performance and Section 4 concludes with future directions 1.1.Metastable CMOS latch Metastable in the system is undesirable because system behavior uncontrollable and its response are unpredictable [14] where as this unpredictable behavior enable to orginate randomness of PUF.

METASTABLE PUF
Physically unclonable function (PUF) is hardware-based security module uses inherited properties of the integrated circuit to generate an unpredictable response.In the proposed MetaPuf a CMOS based latch circuit is the basic element of PUF, which frequently enters into metastability region by maintaining equal potential at Q and QB node, depending upon circuit internal nose latch will come out from metastable state and settle to logic '1' or logic '0' which is completely unpredictable.MetaPuf uses metastability associated with CMOS latch to generate 1-bit response shown in fig2.Q and QB node of the latch is controlled by 2:1 multiplexer applies the same potential on each node and forces the latch to enter in a metastable state.A high EQ and high SEL input enable transmission gate (TG) to apply charge pump output on each node of the latch.Latch enters in metastable state there is a race to Q and QB terminal, minute potential difference on nodes senses by the sense amplifier and produces response bit.When EQ=0 and SEL=0 transmission gate disables it cut off the latch from the charge pump and creates a loop.

Fig2 Metastable PUF
The proposed circuit is verified for 8 different PUF fig3, each latch unit is operated by the charge pump.Challenge input is applied to decoder input; the output of decoder increase or decrease the charge pump output for logic '1' and '0' respectively.A high input applies increasing potential and low input applies to decrease potential to latch node.Charge pump potential assures that latch operates near metastable region.During metastable state potential difference between Q and QB cannot be guessed, they form a race and try to reverse.Sense amplifier generates high response bit if the potential at Q > QB else low.Response bit can store in parallel in parallel out shift register.To have more response number of PUF units are parallelized and require the large size of the decoder.

Fig3 Challenge Response Pair generator 3 .
Fig3 Challenge Response Pair generator

Table of
In this paper, a unique property of CMOS latch i.e. metastability is selected to generate response bit which qualifies for acts as response bit.MetaPuf contains two parallel feedback path contains up-down charge pump control movement of Q and QB terminal towards metastable point afterward latch settle to unpredictable high or low state.Inter and intra hamming distance measure that response value is comparable with existing PUF and ideal value.It is open research question to identify new CMOS properties to have the new architecture of PUF.Kumar, J guajardo, R maes, GJ schrijen and P tuyls,"The Butterfly PUF: Protecting IP on Every FPGA" Proceedings of the IEEE international workshop on hardware-oriented security and trust, pp.67-70, (2008) 3. Erdinc Ozturk, Ghaith Hammouri, and Berk Sunar, "Physically Unclonable Function with Tristate Buffers" IEEE International Symposium on Circuits and Systems, pp 3194 -3197, (2008) 4. Srinivas Devadas, Edward Suh, Sid Paral, Richard Sowell, Tom Ziola and Vivek Khandelwal, "Design and Implementation of PUF-Based Unclonable RFID ICs for Anti-Counterfeiting and Security Applications" IEEE International Conference on RFID pp 58-64, (2008) Ref  [1] [12] [15] [16] [17] [18] Proposed MetaPuf Uniqueness 46.14 45.24 50.