                 In the name of allah
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:idea:Introduction :
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Encryption is the field of computer security that used to encrypt any private message like for example.

A-Private important message .  (Like messages related to governement for example or any very important message like what was used during the first global war by the usa when it is held the most common used encryption technique which is called Data Encryption Standard [DES]. We will discuss that technique briefly)

b-Password .

Encryption is common between any secure system over the global network . it provides facilities and abilities for websites administrators and moderators to encrypt their private messages between one sender and reciever or encrypt their passwords  over secure network .

Consider the following :
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There is one guest who want to register and open an account at one specific website alright and that user choose his password make it [[snouto]] then this password enters the encryption function and transformed to [[jdfhdfhfgdhfg9484djkfhk]]And then it was stored inside the database and one hacker comes and know the encrypted password so he can't login to the account unless he knows the perfect plaintext of the password because the encrypted password doesn't make any sense even for the scripting of the website .When the user enters again to his account so he puts his plaintext real one which is snouto so then it enters the encryption algorithm to transform to the encrypted one then comparison is held by the entire website scripting . and when it was found cookies are injected inside the cookie folder and then the website is recognizing that specific user with his all privilages to that specific website this is not the only use of encryption but there are alot of uses that encryption provide :
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1-steganography.
2-nonreputation.
3-integrity.
4-confedentiality.
and more ...................
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Some constant or common words that will be used alot during our courses we initialize almost all of them
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1.1 Terminology
Sender and Receiver
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Suppose a sender wants to send a message to a receiver. Moreover, this sender wants to send the message securely: She wants to make sure an eavesdropper cannot read the message. 

Messages and Encryption
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A message is plaintext (sometimes called cleartext). The process of disguising a message in such a way as to hide its substance is encryption. An encrypted message is ciphertext. The process of turning ciphertext back into plaintext is decryption. This is all shown in Figure 1.1.

(If you want to follow the ISO 7498-2 standard, use the terms encipher and decipher. It seems that some cultures find the terms encrypt and decrypt offensive, as they refer to dead bodies.)

The art and science of keeping messages secure is cryptography, and it is practiced by cryptographers. Cryptanalysts are practitioners of cryptanalysis, the art and science of breaking ciphertext; that is, seeing through the disguise. The branch of mathematics encompassing both cryptography and cryptanalysis is cryptology and its practitioners are cryptologists. Modern cryptologists are generally trained in theoretical mathematicsthey have to be.

