Classical Cryptography The Jefferson cylinder The Enigma Rotor machine Scytale Hieroglyphics.

Classical CryptographyThe Jefferson cylinder The Enigma Rotor machine

Scytale

Hieroglyphics

Symmetric Cryptography

• Uses a single key for both encryption and decryption

• The encryption and decryption algorithms are inverses of each other

Simple Substitution Ciphers (Monoalphabetic Ciphers)

A B C D E F G H I J K L M N O P Q R S T U V W X Y ZY M I H B A W C X V D N O J K U Q P R T F E L G Z S



\\COME AT ONCE



\\COME AT ONCEIKOB YT KJIB



\\COME AT ONCEIKOB YT KJIB• GIVE TO INGE• HAVE TO ACHE• SECT IN EAST

Methods for decrypting a simple substitution cipher:

• Brute force attack– key space of the substitution cipher = 26! ≈ 288

• Letter frequency analysis– determine the frequency of every ciphertext

letter– look at pairs , triples, or quadruples of

ciphertext symbols

Letter Frequency

The most common digrams (in descending order): th, he, in, en, nt, re, er, an, ti, es, on, at, se, nd, or, ar, al, te, co, de, to, raThe most common trigrams (in descending order): the, and, tha, ent, ing, ion, tio, for, nde, has, nce, edt, tis, oft, sth, men

Polyalphabetic Ciphers

• Encrypt multiple characters at a time• Relationship from plain to ciphertext is one-to-

many• Thwart statistical attacks

Polyalphabetic Ciphers

• Vigenère • Autokey• Playfair• Hill• One-time pad• Rotor• Enigma

Vigenère Cipher

• Let m be a positive integer (the key length)

• P = C = K = Z26 x ... x Z26 = (Z26) m

• For k = (k1, ..., km):

– ek(x1, ..., xm) = (x1 + k1 (mod 26), ..., xm

+ km (mod m))

– dk(y1, ..., ym) = (y1 - k1 (mod 26), ..., ym -

km (mod m))

• AES• RSA

•The Future of Cryptography and Quantum Computing

Rivest, Shamir, and Adleman

Modern Cryptography

Average Time Required for Exhaustive Key Search

Classical Cryptography The Jefferson cylinder The Enigma Rotor machine Scytale Hieroglyphics.

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Classical Cryptography The Jefferson cylinder The Enigma Rotor machine Scytale Hieroglyphics.