Section 2.4
The RSA cryptosystem


The RSA cryptosystem (RSA stands for Rivest, Shamir en Adleman, the three mathematicians who designed the system) is a modern cryptosystem based on modular arithmetic. This cryptosystem can be used, for example, for the protection of payments over the internet. The basis for the RSA cryptosystem is the following theorem, known as Fermat's little theorem.
 Fermat


Fermat's little theorem

Let p be a prime. For every integer a we have

ap = a mod p .

In particular, if a is not 0 mod p then

ap - 1 = 1 mod p .