One of the uses of prime numbers is the RSA cryptosystem.

In this we can establish secure communications, even though the method of communicating is insecure.

This relies of the fact that multiplication of numbers is easy, but factoring numbers can be very hard.

For example, what prime numbers mulitply together to give 1,111,111?

N.B. Numbers of the form 1111 ... 111 are called repunits.

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