E-CES, 212-81, Module 1, History of Cryptography

Cryptography

Science of altering communication so that it cannot be understood without a key

Mono-Alphabet Substitution Cipher

Algorithms that simply substitute one character of cipher text for one character of plain text, these are the most primitive algorithms

Caesar Cipher

Every letter is simply shifted a fixed number of places to the left or to the right

ROT13 Cipher

A single alphabet substitution cipher where all characters are rotated 13 characters through the alphabet.

Single substitution weaknesses

Literacy rates have risen since ancient times, all languages have certain word and letter frequencies, underlying word and letter frequencies lead to vulnerability to cryptanalysis

Examples of Multi-Alphabet Substitution

Cipher Disk, Vigenere Cipher

Vigenere Cipher

Once considered very secure, invented by Giovan Battista Bellaso in 1553. Used until early 1900's. Encrypts text by using a series of different Caesar cipher based on a keyword.

Friedrich Kasiski

First person to carry out a successful attack on a Vigenere cipher

Playfair Cipher

Invented by Charles Wheatstone in 1854. Encrypts two letters instead of one, this makes it more complex. Uses a 5x5 table containing a keyword. No more secure than any other older ciphers.

Homophonic Substitution

Early attempt to make substitution ciphers more robust, masks letter frequencies, plain text letters map to multiple cipher text symbols

Rail Fence Cipher

Most widely known transposition cipher, encrypts the message by altering each letter on a different row, message must then be written down left to right and put into rows