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

2. Symmetric Cryptography
Uses a single key for both encryption and decryption
The encryption and decryption algorithms are inverses of each other

3. 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 Z Y 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

4. 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 Z Y 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

5. 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 Z Y 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 IKOB YT KJIB

6. 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 Z
Y 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
IKOB YT KJIB
GIVE TO INGE
HAVE TO ACHE
SECT IN EAST

7. 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

8. 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, ra
The most common trigrams (in descending order):  the, and, tha, ent, ing, ion, tio, for, nde, has, nce, edt, tis, oft, sth, men

9. Polyalphabetic Ciphers
Encrypt multiple characters at a time
Relationship from plain to ciphertext is one-to-many
Thwart statistical attacks

10. Polyalphabetic Ciphers
Vigenère
Autokey
Playfair
Hill
One-time pad
Rotor
Enigma

11. 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))

12. Modern Cryptography AES RSA
Rivest, Shamir, and Adleman
The Future of Cryptography and Quantum Computing

13. Average Time Required for Exhaustive Key Search