1 Montgomery Modular Multiplication in Hard- ware
1 Montgomery Modular Multiplication in Hard- ware
1 Montgomery Modular Multiplication in Hard- ware
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FEI KEMT<br />
over time. For this reason, the peak-peak value should be used <strong>in</strong> conjunction with<br />
the population size and some knowledge of the type of distribution.<br />
5.2.2 Survey of Designs Based on Jitter<br />
In this section we summarise currently most known concepts and designs of genera-<br />
tors based on extraction of randomness from clock jitter. The jitter appears <strong>in</strong> clock<br />
signals generated by free-runn<strong>in</strong>g oscillators or PLL circuitry implemented <strong>in</strong>side a<br />
digital device.<br />
The Tkacik TRNG Design The generator <strong>in</strong>vented by Tkacik [111] <strong>in</strong>cludes<br />
comb<strong>in</strong>ation of two determ<strong>in</strong>istic circuits – a l<strong>in</strong>ear feedback shift register (LFSR)<br />
and cellular automation shift register (CASR). The registers are clocked by two <strong>in</strong>de-<br />
pendent r<strong>in</strong>gs whose clock frequency is <strong>in</strong>fluenced by external impacts and <strong>in</strong>cludes<br />
jitter. In addition, the selected outputs of CASR and LFSR are XORed together<br />
provid<strong>in</strong>g the f<strong>in</strong>al random signal. The harvest<strong>in</strong>g technique of the generator is very<br />
complex and no verification of its effectiveness is provided.<br />
The design was evaluated by Dichtl [43] who po<strong>in</strong>ted out an issue with unclear<br />
source of randomness <strong>in</strong> the generator. Under certa<strong>in</strong> conditions and with partial<br />
knowledge of some <strong>in</strong>ternal values an attacker is able to predict the generated value<br />
due to low level of entropy.<br />
The Fischer and Drutarovsk´y Design In design from Fischer and Drutarovsk´y<br />
[60] the idea is to extract random values by sampl<strong>in</strong>g a clock signal <strong>in</strong>fluenced by<br />
track<strong>in</strong>g jitter caused by analogue PLL <strong>in</strong> FPGAs from Altera. The jitter can be<br />
sampled only under def<strong>in</strong>ed condition when frequencies of sampled and sampl<strong>in</strong>g<br />
clock signals are <strong>in</strong> a certa<strong>in</strong> ratio.<br />
Sampl<strong>in</strong>g of clock signal is executed periodically with period given by PLL di-<br />
viders. Samples taken <strong>in</strong> transition zones have nonzero probability to result <strong>in</strong><br />
logical one or zero and are called critical samples. The position of critical samples is<br />
stabilised dur<strong>in</strong>g operation of the generator as far as the work<strong>in</strong>g conditions of the<br />
generator do not change.<br />
More details on the TRNG implementation and features of the generator are<br />
described <strong>in</strong> the next section. This design provide us a reference for theoretical<br />
test<strong>in</strong>g and theories which are presented <strong>in</strong> the thesis.<br />
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