Cryptography Cryptanalysis Classics The Evolution of Secret Writing

Cryptography & Cryptanalysis Classics

The Evolution of Secret Writing 1. Steganography

2. Cryptography “The aim of cryptography is not to hide the existence of a message, but rather to hide its meaning, a process known as encryption”

• Transposition • Substitution Cow cow, cwo, owc, ocw, wco, woc. c o w cow X G P XGP

“For example, consider this short sentence” has 50, 000, 000, 000 rearrangements.

Rail Fence Transposition

Spartan Scytale

Substitution Cipher meet at midnight CUUZ VZ CGXSGIBZ

CAESAR Shift Cipher

Kerckhoffs’ Principle: The security of a cryptosystem must not depend on keeping secret the cryptoalgorithm. The security depends only on keeping secret the key.

JULIUS CAESAR JULISCAER

Muslims Monoalphabetic Substitution Cipher a b # + Cryptanalysis The Science of Unscrambling a message without knowledge of the key.

Frequency Analysis al-Kindı’s “A Manuscript on Deciphering Cryptographic Messages”

English

P is a consonant.

• Combination OO appears twice, whereas XX does not appear at all. • X and Y appear on their own in the ciphertext. (a and I are the only English word that consisit of a single letter) • In the English language, the letter h frequently goes before the letter e (as in the, then, they, etc. ), but rarely after e. h

O=e, X=a, Y=i, B=h. the most common three-letter words in English are the and, and these are relatively easy to spot—Lhe, which appears six times, and a. PV, which appears five times. P=n, V=d, L=t.

• Every word has a vowel in it, so C in Cn is a vowel. The only vowels remaining are o and u. u does not fit so C = o. • We also have the word Khe, which implies that K represents either t or s. But we already know that L = t, so it becomes clear that K = s. • tho. Msand one ni. Dhts thousand one nights

A VOID BY GEORGES PEREC AVOIDBYGERSPC

1. Begin by counting up the frequencies of all the letters in the ciphertext. About five of the letters should have a frequency of less than 1 percent, and these probably represent j, k, q, x and z. One of the letters should have a frequency greater than 10 percent, and it probably represents e. If the cipher-text does not obey this distribution of frequencies, then consider the possibility that the original message was not written in English. 2. the most common repeated letters are ss, ee, tt, ff, ll, mm and oo. 3. The only one-letter words in English are a and I. The commonest two-letter words are of, to, in, it, is, be, as, at, so, we, he, by, or, on, do, if, me, my, up, an, go, no, us, am. The most common three-letter words are the and.

Code • One of the simplest improvements to the security of the monoalphabetic substitution cipher was the introduction of nulls, symbols or letters that were not substitutes for actual letters, merely blanks that represented nothing.

The Pigpen Cipher

New Ciphers Leon Battista Alberti hello AFPAD

L 1 -L 2 -L 3 -L 4 -L 5

EMILY

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