Copyright © University of Cambridge. All rights reserved.
'The Public Key' printed from https://nrich.maths.org/
The idea of Public Key Cryptography is to send messages in such a
way that only the person who receives the messages can understand
them. Even if everyone knows the method of encryption nobody except
the receiver has all the information needed in order to decypher
the message. To solve this problem you are in the position of Bob
receiving a secret number from Alice who has sent him the coded
message 180. Bob is the only person in the world who knows that he
has to use 59 in order to find the secret number $M$ which Alice
has sent to him, and which he knows to be between 0 and 390.
Bob has to find the number equivalent to $180^{59}\pmod {391}$ so
can you do this for him?
NOTES AND BACKGROUND
So that you can decipher the secret number using only a calculator
the example given involves only small numbers like 59 and 391
whereas very big numbers are involved when the method is used in
the real world. Instead of 391, the product of two very large prime
numbers would be used in real world applications of this method.
The reason that Bob, and nobody else, knows to use the number 59 to
decode the message is that it is derived from one of the factors of
391 and in real world applications it is impossible to find the
factors of the very large numbers that are used.
The article '
Public Key
Cryptography ' gives a detailed explanation of how the method
works and gives you help in working with modulus arithmetic.
This wikipedia page might also be of interest
http://en.wikipedia.org/wiki/Cobalt-59