A High Speed TRNG Based on SRAM for Resource Constrained Devices

Chunhu Zhang and Yu YaoDec 1st, 2009

Motivation

Foundation of Hardware Secuirty Mifare Classic RFID was cracked RNG is fundamental to cryptographic applications

Background

Traditional ways to generate RNG 1. Pseudo-random Number Generators based on

mathematical algorithm

2. direct/indirect applification of thermal noise on large resistors

Our approach: Hardware-based True RNG based on Highly Phase-noise-sensitive Bitcell

Key Ideas

Start from the metastable point of bitcell

Release the bitcell, the final state will be determined by noise/jitter

Capture the clock jitter by taking advantage of positive feedback and fast response of the bitcell

Approach

1. principles of the bitcell structure

2. our efforts in improving the bitcell performance

3. robustness to manufacturing variation

4. phase noise response and phase noise simulation

5. summary and future work

Jitter Sensitive Bitcell

Initially, the looped INV is clamped close to Vm point by short-circuiting Q, QB, then noise is introduced through the jitter

When CLK1, CLK2 rising edge comes, pos spikes on Q,QB

When CLK1B, CLK2B falling edge comes, neg spikes on Q,QB

Jitter Sensitive Bitcell

So if CLK1(CLK1B) comes earlier/later than CLK2(CLK2B), the bitcell will eventually resolve to 1/0

Jitter Sensitive BitcellWhen jitter is larger than a threshold value

CLK1 is earlier CLK1 is lagged behind

jittering is 500ps

Jitter Sensitive BitcellWhen jitter is smaller than a threshold value

CLK1 is earlier CLK1 is lagged behind

jitter is 10ps

Jitter Sensitive Bitcell

Efforts to improve the bitcell's sensitivity to jittering

1. make the working point closer to the Vm point of butterfly curve

2. make the voltage spikes on Q, QB nodes higher when the MOSFETs at the pass gate is turned off

Jitter Sensitive Bitcell

1. scale down Vdd

Jitter Sensitive Bitcell

2. sizing effecta.

the greater the

size of the pass

gate, the greater

the voltage

spike

Jitter Sensitive Bitcell

2. sizing effect

b. size the inverters bigger

→ mitigate the pass gate current influence on the inverters

→ working point closer to the Vm point

→ higher sensitivity

In all, achieve a balanced size ratio for inverters and pass gate FETs

MC simulation Manufacture Variation → biased inverter →

biased bitstream output

Evaluation

How do we generate the clock signals we need?

Evaluation

Mathematical model for jittering in inverter chainThe time delay of a signal passing through an inverter chain

is

t = t0 + δt

where t0 is the mean time delay along the chain and δt is a random variable following the distribution N(0, σ2).

Note that δt is essentially caused by thermal noise in the MOSFETs and is varying with time.

Evaluation

Matlab SimulationWe used Matlab to generate a set of N(0, σ2) random numbers,

used them as jittering for rising/falling edges to construct the clock waveforms, directed the waveform into bitcell, simulated and obtained an output of bitstream.

We can also calculate the theoretical ouput bitstream from the set of random numbers, keeping in mind the property of the bitcell that, if the relative jittering is greater than 25ps, earlier CLK rising edge gives Q=0; while if the relative jittering is less than 25ps, earlier CLK rising edge gives Q=1.

Evaluation

Evaluation

δThe cumulative probability that jittering is greater than 25ps

The cumulative probability that jittering is less than 25ps

Rate of correct bits

500ps 0.972 0.949

100ps 0.860 0.802

20ps 0.6232 0.805

10ps 0.923 0.990

Summary and Future Work

Advantages of our circuits:1. low power requirement, widely applicable to low power devices

such as RFID

2. on-chip generation of random bitstream, immune to hacking

3. simple and easily integrable to digital systems

4. secure and truly random HRNG

5. high speed

Summary and Future Work

Future Work1. Use SpectreRF to simulate the jittering in inverter chains;

2. generate a long enough bitstream for randomness test using NIST randomness test toolkit;

3. and more...

Summary and Future Work

Thanks!