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Francesco Romeo added section_Introduction_of_Cryptography_From__.tex
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\section*{Introduction of Cryptography}
From the beginning of the Empires and Wars, Emperors and Generals needed to "secure" their communications.\\
So the \textbf{Cryptography} is the way to hide messages, (\textit{krypt\`{o}s} in Greek means \textit{hidden}) in order that no one couldt read them, without a way to decrypt them.
It could be absurd to think, but in the Ancient Greece to exchange a message, this one was written on the shaved head of a courier; when his hair had grown back, he was sent to the recipient and shaved again to show the message. The art of hiding physically the message is called \textit{Steganography}. But our attention will be focused on the method to encrypt messages by using other letters or characters.\\
We present our "protagonist" that are the \textbf{plaintext}, the real message without any encryption,in lower case, and the \textbf{ciphertext}, the text that is encrypted and exchanged between people,in upper case. These both with the \textit{keys} set up a \textbf{Cryptosystem}. The other part of the cryptography regards the attacks to the cryptosystems: a cryptosystem is used and safe until someone finds an attack to break it, he could intercept messages and decrypt it easily.
To describe the main cryptosystems we will consider a situation in which there are two people Alice and Bob that have to exchange encrypted messages and there is a third person Eve that tries to intercept and decrypt Alice and Bob's messages.
