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Classical Cryptography

Classical Cryptography. The Enigma Rotor machine. The Jefferson cylinder. Scytale. Hieroglyphics. Symmetric Cryptography. Uses a single key for both encryption and decryption The encryption and decryption algorithms are inverses of each other.

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Classical Cryptography

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  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 Rivest, Shamir, and Adleman • The Future of Cryptography and Quantum Computing • AES • RSA

  13. Average Time Required for Exhaustive Key Search

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