Week 2: Introduction to Cryptography

Basics of Cryptography

     What is cryptography?
Cryptography is the science of using mathematics to encrypt and decrypt data.
Cryptography enables you to store sensitive information or transmit it across insecure networks (like the Internet) so that it cannot be read by anyone exceptthe intended recipient.
Cryptanalysts are also called attackers.
Cryptology embraces both cryptography and cryptanalysis.

     Conventional cryptography

       In conventional cryptography, also called secret-key or symmetric-keyencryption, one key is used both for encryption and decryption. The DataEncryption Standard (DES) is an example of a conventional cryptosystem thatis widely employed by the Federal Government.

    

                 plaintext    encryption       ciphertext         decryption      plaintext

      Caesar’s Cipher

 An extremely simple example of conventional cryptography is a substitutioncipher. A substitution cipher substitutes one piece of information for another.This is most frequently done by offsetting letters of the alphabet. Two examplesare Captain Midnight’s SecretDecoder Ring,which youmay have ownedwhen you were a kid, and Julius Caesar’s cipher. In both cases, the algorithm is to offset the alphabet and the key is the number of characters to offset it.
For example, if we encode the word “SECRET” using Caesar’s key value of 3,we offset the alphabet so that the 3rd letter down (D) begins the alphabet.
So starting withABCDEFGHIJKLMNOPQRSTUVWXYZ
and sliding everything up by 3, you get DEFGHIJKLMNOPQRSTUVWXYZABC
where D=A, E=B, F=C, and so on.

Using this scheme, the plaintext, “SECRET” encrypts as “VHFUHW.” Toallow someone else to read the ciphertext, you tell them that the key is 3.
Obviously, this is exceedingly weak cryptography by today’s standards, buthey, it worked for Caesar, and it illustrates how conventional cryptographyworks.

Vigenère cipher

The Vigenère cipher is a method of encrypting alphabetic text by using a series of different Caesar ciphers based on the letters of a keyword. It is a simple form of polyalphabetic substitution.


This cipher is well known because while it is easy to understand and implement, it often appears to beginners to be unbreakable; this earned it the description le chiffre indéchiffrable (French for 'the unbreakable cipher'). Consequently, many people have tried to implement encryption schemes that are essentially Vigenère ciphers, only to have them broken.