Classical Cryptography The Jefferson cylinder The Enigma Rotor machine Scytale Hieroglyphics
Dec 20, 2015
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
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
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 ONCEIKOB YT KJIB
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 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
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