Cryptography and the RSA Public-Key System

Several number theoretic concepts (prime numbers, modulo arithmetic, relatively prime, etc.) were introduced in the first three sections of this chapter. Many of them are central to the creation and implementation of check digit schemes. They also can be applied in the area of cryptography to create some fairly sophisticated codes that are extremely difficult to break. The RSA Public-Key Cryptosystem, discussed in this section, is one such code. 

Consider the following situation Alice wants to send Bob a message. The only problem is that Alice and Bob do not want anyone else to be able to read the message, and they know that a third person, Cindy, is viewing all of their correspondence. The only choice 

Definition 2,8.1, Cryptography is the study of methods to send messages in disguised form so that only die intended recipient can decode and read the message. 

The message that is to be sent is called the plaintext and the message in coded form is called the ciphertext. There are two main tasks involved in sending a coded message  

Taking the ciphertext message and obtaining the plaintext message from it. 

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