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1 Montgomery Modular Multiplication in Hard- ware

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FEI KEMT<br />

and a harvest<strong>in</strong>g mechanism which extracts the randomness and generates truly<br />

random values.<br />

Security level of PRNG depends on complexity of the generat<strong>in</strong>g function, the<br />

period length of the generated sequence, and the amount of entropy <strong>in</strong> the seed. As<br />

a result, the pseudo-random sequences may achieve a high level of unpredictability<br />

<strong>in</strong> case of sufficient complexity of the generat<strong>in</strong>g function. However, the pseudo-<br />

random sequence has always a f<strong>in</strong>ite period and rema<strong>in</strong>s reproducible as far as<br />

<strong>in</strong>itial conditions are susta<strong>in</strong>ed.<br />

The PRNG is the only choice for soft<strong>ware</strong> implementations and thanks to de-<br />

term<strong>in</strong>istic components it attracts also the designers of electronic digital systems.<br />

Note that also pseudo-random sequence can be unpredictable when produced by<br />

cryptographically secure PRNG e.g. based on hash (one-way) functions, stream ci-<br />

phers or Blum Blum Shub pr<strong>in</strong>ciple [28]. The PRNG requires a random seed (from<br />

a TRNG or other reliable source of entropy, if available) to obta<strong>in</strong> the start<strong>in</strong>g level<br />

of entropy. As the system is determ<strong>in</strong>istic, for identical seeds the PRNG generates<br />

identical output pseudo-random sequences, too. No more entropy is added dur<strong>in</strong>g<br />

exploitation of the seed, therefore the seed’s entropy designates the unpredictability<br />

of the generated sequence.<br />

The term generator is not completely correct <strong>in</strong> case of TRNG as the randomness<br />

is not generated but rather extracted from a source of randomness (see Figure 5 –<br />

1). In TRNG the occurrence of random events is sampled by an extractor and<br />

transformed <strong>in</strong>to a sequence of numerical values usually expressed as a b<strong>in</strong>ary stream.<br />

Source of<br />

randomness<br />

A/D conversion<br />

analogue part digital part<br />

noise<br />

signal<br />

Postprocess<strong>in</strong>g<br />

digitised<br />

noise<br />

signal<br />

<strong>in</strong>ternal<br />

random<br />

sequence<br />

Output<br />

buffer<br />

external <strong>in</strong>terface<br />

random<br />

number<br />

sequence<br />

Figure 5 – 1 Schematic diagram of a TRNG with designation of <strong>in</strong>ternal signals and <strong>in</strong>terfaces<br />

The Figure 5 – 1 represents a typical design of TRNG based on a physical phe-<br />

nomenon. Us<strong>in</strong>g a proper harvest mechanism the analogue signal is converted <strong>in</strong>to<br />

its digitised form. Accord<strong>in</strong>g to statistical properties of the signal it may be required<br />

to apply a post-process<strong>in</strong>g <strong>in</strong> order to produce an <strong>in</strong>ternal random sequence. The<br />

generated sequence can be further accumulated <strong>in</strong> output buffer before leav<strong>in</strong>g the<br />

74

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