A Technical Seminar Presentation - 2004 CLASSICAL CRYPTOGRAPHY Under the guidance of Dr. Partha S. Mallick Presented by Jyotishman Baishya Roll # EC200119320

Introduction Basic Concepts Key Cryptography Plaintext Cipher text Cipher Key Encipher (encode) Decipher (decode) Cryptanalysis Cryptology Code

Types of Ciphers Substitution Transposition Mono-alphabetic Scytale Poly-alphabetic Poly-graphic Transposition Scytale Reverse Rail Fence Geometric Row/Column

Mono-alphabetic Substitution Caesar Cipher 2000 years ago Julius Caesar used a simple substitution cipher, now known as the Caesar cipher first attested use in military affairs (Gallic Wars) replace each letter by 3rd letter on, eg. L FDPH L VDZ L FRQTXHUHG is I CAME I SAW I CONQUERED can describe this mapping (or translation alphabet) as: Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ Cipher: EFGHIJKLMNOPQRSTUVWXYZABC

General Mono-alphabetic Substitution rather than just shifting the alphabet could shuffle (jumble) the letters arbitrarily each plain text letter maps to a different random cipher text letter hence key is 26 letters long have a total of 26! ~ 4 x 1026 keys! but … can be cracked through frequency analysis

Poly-alphabetic Substitution Cipher Vigenère Cipher Blaise de Vigenère is generally credited as the inventor of the “polyalphabetic substitution cipher” To improve security use many monoalphabetic substitution alphabets Hence each letter can be replaced by many others Use a key to select which alphabet is used for each letter of the message ith letter of key specifies ith alphabet to use Use each alphabet in turn Repeat from start after end of key is reached

Vigenère Example Write the plaintext out and under it write the keyword repeated Then using each key letter in turn as a Caesar cipher key Encrypt the corresponding plaintext letter Plaintext THISPROCESSCANALSOBEEXPRESSED Keyword CIPHERCIPHERCIPHERCIPHERCIPHE Ciphertext VPXZTIQKTZWTCVPSWFDMTETIGAHLH In this example the keyword is "CIPHER". Hence the following translation alphabets are mapped to the plaintext letters: C: - CDEFGHIJKLMNOPQRSTUVWXYZAB I: - IJKLMNOPQRSTUVWXYZABCDEFGH… … for ABCDEFGHIJKLMNOPQRSTUVWXYZ

Hill Cipher A multiple-letter encryption method – encrypts m letters of plaintext at each step The encryption key K is a m x m matrix of coefficients To encrypt – multiply the matrix K by a vector of m plaintext letters to receive a vector of m ciphertext letters. (Arithmetic is modulo the size of the alphabet.) The decryption key K-1 is the m x m matrix of coefficients that is the inverse of matrix K: K * K-1 = I To decrypt – multiply the matrix K-1 by the vector of m ciphertext letters to receive the vector of m plaintext letters. (Arithmetic is modulo the size of the alphabet.)

Transposition Ciphers Scytale cipher a strip of paper was wound round a staff message written along staff in rows, then paper removed leaving a strip of seemingly random letters not very secure as key was width of paper & staff Reverse cipher write the message backwards Rail Fence cipher write message with letters on alternate rows Geometric Cipher Write message following one pattern and read out with another.

Transposition Ciphers (contd…) Row Transposition ciphers in general write message in a number of columns and then use some rule to read off from these columns. key could be a series of number being the order to: read off the cipher; or write in the plaintext. Block (Columnar) Transposition ciphers the message is written in rows, but read off by columns in order given by key. for ease of recovery may insist matrix is filled.

Poly-alphabetic Cipher Machines Jefferson Wheel Rotor Based Machines

Conclusion Cryptography has undergone tremendous development since the days it had come into existence some 4000 years ago. The world of research and development continues to experiment with new concepts such as quantum cryptography, cellular automata, chaotic cryptosystems, elliptic curves based on cryptography and visual cryptography. Developments are also taking place in the area of most sophisticated methods of digital steganography. Simultaneously some challenging area of cryptanalysis, being the other side of development exercise will gain importance in view of the plethora applications demanding assured security.

Thank You.