Computer System Security CSE 5339/7339

Computer Science and Engineering

Computer System SecurityComputer System Security

CSE 5339/7339CSE 5339/7339

Lecture 3Lecture 3

August 26, 2004August 26, 2004


ContentsContents

Algorithms (Revisited)Algorithms (Revisited)

Operating Systems ReviewOperating Systems Review

Students Topics for PresentationStudents Topics for Presentation

EncryptionEncryption

Substitution and Transposition CiphersSubstitution and Transposition Ciphers


Algorithms -- revisited

Hashing - Why?

Hash Tables

Hash Functions

Insert

Lookup


Applications

OS -- Review

OS – a program that acts as an intermediary between a user of a computer and the computer hardware.

OS

Hardware

Users


OS -- Review

OS Services

Program Execution

I/O Operation

File System manipulation

Communications

Error detection

Resource Allocation

Accounting

Protection


OS -- Review

Process

Memory Management


Student Presentations (15 minutes)

8/31

9/2

9/7

9/9


Main Components in Sending Messages

sender receiverMedium

Intruder•Block it

•Intercept it

•Modify it

•Fabricate an authentic looking message


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(P) (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)


Encryption

Encryption Decryption

plaintextOriginal

plaintextciphertext


Encryption

Encryption Decryptionplaintext

Original

plaintextciphertext

Encryption Decryptionplaintext

Original

plaintextciphertext

Symmetric CryptosystemSymmetric Cryptosystem

Asymmetric CryptosystemAsymmetric Cryptosystem

key

KE KD


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!


Ciphers

Substitution Ciphers

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

Transposition Ciphers

The order of letters is rearranged

(Uppercase – plaintext, lowercase – ciphertext)


Exercise

wklv phvvdjh lv qrw wrr kdug wr euhdn


The Caesar Cipher -- Substitution

Ci = pi + 3

A d

B e

C f

…

X a

Y b

Z c

Time complexity table search ??


Cryptanalysis of the Caesar Cipher

TREATY IMPOSSIBLE wuhdwb lpsrvvleoh

Break is preserved Double letters are preserved Repeated letters


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, 10

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

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

Key can be used to control the permutation used to


Example

ABCDEFGHIJKLMNOPQRSTUVWXYZ wordabcefghijklmnpqstuvxyz

ABCDEFGHIJKLMNOPQRSTUVWXYZ profesinalbcdghjkmqtuvwxyz


Cryptanalysis of substitution ciphers

Clues Short words Words with repeated patterns Common initial and final letters ….

Brute force attack (could be impossible – more than 1000 years)

Knowledge of language may simplify it English E, T, O, A occur far more than J, Q, X, Z Context


Vernam Cipher

Encryption Decryptionplaintext Original

plaintext

ciphertext

Non-repeating series of numbers


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 17Random numbers 76 48 16 82 44 3 58 11 60 5 48 88Sum 97 52 33 95 44 15 60 19 75 12 52 105Sum mod 26 19 0 7 17 18 15 8 19 23 12 0 1Ciphertext t a h r s p i t x m a b


Transposition

The letters of the message are rearranged

Columnar transposition

Example:

THIS IS A MESSAGE TO SHOW HOW A COLMUNAR TRANSPOSITION WORKS


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

Computer System Security CSE 5339/7339

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