Lecture 1 Overview

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Nov 21, 2013 (3 years and 6 months ago)

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Lecture 1 Overview

Computer
-
Based System


Hardware


Software


Data



Vulnerability


Weakness (to be exploited)


Threat


Circumstances (Potential to cause harm)


Attack


Exploits vulnerabilities

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Threats


Interception


Unauthorized party has gained access to an asset


Interruption


An asset becomes lost, unavailable, or unusable


Modification


Unauthorized party tampers with an asset


Fabrication


Unauthorized party may create objects


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Security Goals

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Availability

Confidentiality


Integrity

Assets are accessed

only by authorized people

Assets can be modified

only by authorized people

Assets are accessible to

authorized people

Lecture 2

Elementary

Cryptography

CS 450/650


Fundamentals of

Integrated Computer Security

Slides are modified from
Hesham El
-
Rewini

Objectives




Learn how cryptography works



Learn how encryption systems are broken



Understand basic ciphers:


substitution

and
transposition
codes

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Goal of Cryptography


Ensure
security

of communication over
insecure medium


Privacy (secrecy, confidentiality)


Integrity


Communicate even with possibility of
adversaries

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Main Components in Sending Messages

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sender

receiver

Medium

Intruder



Interrupt



Intercept



Modify



Fabricate


Availability


Confidentiality


Integrity

Approaches to Secure Communication


Steganography


Hide message existence


Cryptography


Hide message meaning

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Cryptography


Secret writing


Disguised data cannot be read, modified, or fabricated
easily


Encryption

: encoding (encipher)


plaintext





cipher text


P = <p1, p2, p3, .., pn>




C = <c1, c2, c3, .., cm>


C = E(c) (E = encryption rule)


Decryption

: decoding (decipher)


Cipher text





plaintext


C = <c1, c2, c3, .., cm>




P = <p1, p2, p3, .., pn>


P = D(c) (D = decryption rule)

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Cryptosystem


How does this help us accomplish our goals?


Privacy


Integrity

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Encryption

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Encryption

Decryption

plaintext

Original

plaintext

ciphertext

Keyless

Encryption

Decryption

plaintext

Original

plaintext

ciphertext

Symmetric key

Encryption

Decryption

plaintext

Original

plaintext

ciphertext

Asymmetric key

Cryptanalysis


How to break an encryption!



Cryptanalyst


Deduce the original meaning of the ciphertext


Determine the decryption algorithm that matches
the encryption one used



Breakable Encryption!



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Exercise




wklv phvvdjh lv qrw wrr kdug wr euhdn


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Ciphers


Substitution Ciphers


Substitute a character or a symbol for each
character of the original message



Transposition Ciphers


The order of letters is rearranged



Notation


UPPERCASE

PLAINTEXT


lowercase

ciphertext



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The Caesar Cipher
--

Substitution


C
i

= p
i

+ 3


A


d

B


e

C


f



X


a

Y


b

Z


c

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Cryptanalysis of the Caesar Cipher



TREATY IMPOSSIBLE


wuhdwb lpsrvvleoh



Break is preserved


Double letters are preserved


Repeated letters


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Other Substitutions: Permutation


Alphabet is scrambled, each plaintext letter
maps to a unique ciphertext letter



For example





1, 2, 3, 4, 5, 6, 7, 8, 9


p1 = 1, 3, 5, 7, 9, 8, 6, 4, 2

p1(1) = 1, p1(2) = 3, p1(3) = 5, p1(4) = 7, etc.



Key can be used to control the permutation
used to


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Substitution Cipher Example


ABCDEFGHIJKLMNOPQRSTUVWXYZ


word
abcefghijklmnpqstuvxyz




ABCDEFGHIJKLMNOPQRSTUVWXYZ


profesinal
bcdghjkmqtuvwxyz


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Cryptanalysis of substitution ciphers


Brute force attack


26! possibilities


Clues


Short words,


Words with repeated patterns,


Common initial and final letters, …


Knowledge of language may simplify it


English E, T, O, A occur far more than J, Q, X, Z


Context

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Cryptanalysis


Example:

wklv phvvdjh lv qrw
wrr

kdug
wr

euhdn




wrr

--
> see,
too
, add, odd,
off
...


wr

--
> to, of



Best guess:
w = T, r = O

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Cryptanalysis

wk
lv

phvvdjh
lv

qrw
wrr

kdug
wr

euhdn



wrr

--
> see,
too
, add, odd,
off
...


wr

--
> to, of


Best guess:
w = T, r = O


lv

--
>
so, is, in, ...


T_SO

very unlikely...

T_IS

likely

Best guess:
l = I, v = S

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Cryptanalysis

wk
lv

phvvdjh
lv

qrw
wrr

kdug
wr

euhdn



wrr

--
> see,
too
, add, odd,
off
...


wr

--
> to, of


Best guess:
w = T, r = O


lv

--
>
so, is, in, ...

Best guess:
l = I, v = S


w
k
lv

ph
vv
djh
lv

q
rw

wrr

kdug
wr

euhdn

T
-
IS
--
SS
---

IS
-
OT TOO
----

TO
-----

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Avoid Regularity

Encryption

Decryption

plaintext

Original

plaintext

ciphertext

Non
-
repeating series of numbers

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One
-
Time Pads


Name


set of sheets of paper with keys,
glued into a pad



The sender would tear off enough number of
pages



The receiver needs a pad identical to the one
used by the sender


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One
-
Time Pads (cont.)


The sender would write the keys one at a time above
the letters of the plaintext.

K
1

k
2

k
3

k
4

... K
n

p
1

p
2

p
3

p
4

... p
n



The plaintext is enciphered using a pre
-
arranged chart


Vignere Tableau


all 26 letters in each column in some scrambled order


select the substitution in row pi, column Ki


Problems
:


Unlimited number of keys & Absolute
synchronization between sender and receiver


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Vernam Cipher Example

Plaintext

V E R N A M C I P H E R

21 4 17 13 0 12 2 8 15 7 4 17

Random numbers

76 48 16 82 44 3 58 11 60 5 48 88

Sum

97 52 33 95 44 15 60 19 75 12 52 105

Sum mod 26

19 0 7 17 18 15 8 19 23 12 0 1

Ciphertext

t a h r s p i t x m a b

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Book Ciphers


Both sender and receiver need access to identical
objects


Example: telephone book


xxx
-
xxx
-
xxxx


use xx mod 26 as a key


Problem


High frequency letters


A, E, O, T


40% of all letters used in Standard English text


A, E, O, T, N, I


50% of all letters used in Standard English
text


The probability that the key letter and plain text letter is in
these 6 letters is 0.25

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Transposition


The letters of the message are rearranged



Columnar transposition



Example:

THIS IS A MESSAGE TO SHOW HOW A

COLMUNAR TRANSPOSITION WORKS


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Transposition Example


T H I S I


S A M E S


S A G E T


O S H O W


H O W A C


O L M U N


A R T R A


N S P O S


I T I O N


W O R K S



tssoh oaniw haaso lrsto imghw utpir seeoa
mrook istwc nasna

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Summary


Maintain privacy and integrity despite
adversaries



Cryptanalysis



Cryptosystems


Substitution


Transposition


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Review Questions


What is the process for going from
plaintext

to
ciphertext
back to
plaintext
?




What is a substitution code?




Why are there 26! possible
substitution

codes
for simple English messages?




What is a
frequency distribution

analysis?




What is a
transposition

code?




How do
secret key

and
public key

cryptography differ?



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Much More To Discuss...


What makes a "good" encryption algorithm



Data Encryption Standard (DES)


Double and Triple DES



Advanced Encryption Standard (AES)



Public Key Encryption


Rivest
-
Shamir
-
Adelman (RSA)

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