Introduction (Seeing Cryptography From Bird eye's view)

• Cryptography , in layman terms could be defined as the ancient art of secret communications, or securing information that is not intended for general use and is to be accessed by only a small group of selected individuals.
• Since the ancient times , the need for securing the communication messages was of utmost need , in the following PDFs , we'll explore in greater detail about how Cryptography developed through the ages .
• Need Of Cryptography : In earlier days , rulers or kings were required to send confidential information , for which they entrusted the information only amongst the trustworthy people , it laid the foundation of whole idea on which cryptography was based .
• Some basic terms and definitions :
  1. Plain Text : The original message or data that is to be encrypted.
  2. Cipher Text : The encrypted/modified message
  3. Key : unique key , used by sender and reciever to authorise the communication , and encrypt/decrypt the data.
  4. Encryption Algorithm : Algorithmic operations on the plain text to convert it into Cipher Text
  5. Decryption Algorithm : Reverse of the Encryption Algorithm involves operations on the Cipher Text to convert it into plain text .

Symmetric Encryption

• It is one of the fundamental technique applied in cryptography , and finds it's applications in many ways.

• Let us understand the situation through an example , say there are 2 people Alice and Bob , who live far away and are required to communicate amongst themselves , for which they use a messenger to communicate Alice's message to Bob , however it is needed that the message to be transported should only be available to Bob , and no one else could have access to it ( not even the messenger ).

  

The Problem

if the message is carried in its original form from , Alice to Bob , the message could have been leaked or accessed by another person , who shouldn't have had access to the data

The Solution

• To solve this problem , we introduce the idea of symmetric encryption , in which both sender and receiver have a common shared key , through which they could have an access to the message , and no one else without the key could access it . The same could be written as :

  • Encrypt : Alice writes the message , then using the key , fills the message with noise data ( and generates random sequences of the encrypted data ) , which is sent through the messenger.
  • Decrypt : To read the message , bob uses the key to make sense out of the random message transported to him by the messenger , and finds the original message.

  

• Therefore , all through the process of communication , the messenger only had the series of random integers, and the message with noisy data , which doesn't make any sense unless , one has key to access the secret information , which is made available only to the entrusted recipients of the information

• The key could be generated either at the transmission end and then transmitted to the destination through some secure channel or could be delivered to the source and destination , by some third party source.

• Basic assumption is that the encryption algorithm E and the decryption algorithm D is known. Therefore for a strong encryption scheme it is essential that the key isn't accessible.

• The basic steps involved in symmetric encryption could be depicted as :

  

The Evolution and the Kerckhoff's Principle

When is the cryptographic Algorithm considered to be broken