Lecture 4 of Computing Security 1 Course for 4th class

Lecture 4 overview:
• Basic Concepts & terms of Cryptography
• Cryptographic Algorithms and Keys.
• Types of Cryptosystems
• Symmetric Algorithms
11. Basic Concepts & terms of Cryptography
Human being from ages had two inherent needs: (a) to communicate and
share information and (b) to communicate selectively. These two needs
gave rise to the art of coding the messages in such a way that only the
intended people could have access to the information.
The art and science of keeping messages secure is cryptography. The
word ‘cryptography’ was coined by combining two Greek words, ‘Krypto’
meaning hidden and ‘graphene’ meaning writing.
However, the roots of cryptography are found in Roman and Egyptian
civilizations about 4000 years ago,
The earlier Roman method of cryptography, popularly known as the
Caesar Shift Cipher, relies on shifting the letters of a message by an
agreed number, the recipient of this message would then shift the letters
back by the same number and obtain the original message as below:
Suppose a Sender wants to send a message securely to a Receiver.
Computing Security 1 Lecture 4
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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 cipher text back into plaintext is Decryption. This is all shown in Figure (8).
Figure (8): Encryption and Decryption.
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 cipher text); 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 mathematics they have to be.