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Computer System Security CSE 5339/7339
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Transcript of Computer System Security CSE 5339/7339
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Computer Science and Engineering
Computer System SecurityComputer System Security
CSE 5339/7339CSE 5339/7339
Lecture 3Lecture 3
August 26, 2004August 26, 2004
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Computer Science and Engineering
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
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Computer Science and Engineering
Algorithms -- revisited
Hashing - Why?
Hash Tables
Hash Functions
Insert
Lookup
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Computer Science and Engineering
Applications
OS -- Review
OS – a program that acts as an intermediary between a user of a computer and the computer hardware.
OS
Hardware
Users
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Computer Science and Engineering
OS -- Review
OS Services
Program Execution
I/O Operation
File System manipulation
Communications
Error detection
Resource Allocation
Accounting
Protection
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Computer Science and Engineering
OS -- Review
Process
Memory Management
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Computer Science and Engineering
Student Presentations (15 minutes)
8/31
9/2
9/7
9/9
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Computer Science and Engineering
Main Components in Sending Messages
sender receiverMedium
Intruder•Block it
•Intercept it
•Modify it
•Fabricate an authentic looking message
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Computer Science and Engineering
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)
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Computer Science and Engineering
Encryption
Encryption Decryption
plaintextOriginal
plaintextciphertext
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Computer Science and Engineering
Encryption
Encryption Decryptionplaintext
Original
plaintextciphertext
Encryption Decryptionplaintext
Original
plaintextciphertext
Symmetric CryptosystemSymmetric Cryptosystem
Asymmetric CryptosystemAsymmetric Cryptosystem
key
KE KD
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Computer Science and Engineering
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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Computer Science and Engineering
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)
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Computer Science and Engineering
Exercise
wklv phvvdjh lv qrw wrr kdug wr euhdn
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Computer Science and Engineering
The Caesar Cipher -- Substitution
Ci = pi + 3
A d
B e
C f
…
X a
Y b
Z c
Time complexity table search ??
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Computer Science and Engineering
Cryptanalysis of the Caesar Cipher
TREATY IMPOSSIBLE wuhdwb lpsrvvleoh
Break is preserved Double letters are preserved Repeated letters
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Computer Science and Engineering
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
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Computer Science and Engineering
Example
ABCDEFGHIJKLMNOPQRSTUVWXYZ wordabcefghijklmnpqstuvxyz
ABCDEFGHIJKLMNOPQRSTUVWXYZ profesinalbcdghjkmqtuvwxyz
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Computer Science and Engineering
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
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Computer Science and Engineering
Vernam Cipher
Encryption Decryptionplaintext Original
plaintext
ciphertext
Non-repeating series of numbers
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Computer Science and Engineering
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
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Computer Science and Engineering
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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Computer Science and Engineering
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
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