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

Cow cow, cwo, owc, ocw, wco, woc.

• Substitution

c Xo Gw P

cow XGP

“For example, consider this short sentence”has

50,000,000,000,000,000,000,000,000,000,000rearrangements.

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, and these are relatively easy to spot—Lhe, which appears six times, and aPV, 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.

• thoMsand and one niDhts thousand and 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 con-sider 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 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

L1-L2-L3-L4-L5

EMILY

THANK YOU!!