Cryptographyx

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 X

o

G

w P


cow XGP

“For example, consider this short sentence”

has

50,000,000,000,000,000,000,000,000,000,000

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
, 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












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






EMILY




THANK YOU!!