1. Computer and Network Security
Rabie A. Ramadan

2. CIA Triad
Security Goals
Confidentiality,
Integrity , and
Availability

3. Confidentiality
The property of preventing disclosure of information to unauthorized individuals or systems.
Real Scenario
a credit card transaction on the Internet requires the credit card number to be transmitted from the buyer to the merchant and from the merchant to a transaction processing network.
The system attempts to enforce confidentiality by encrypting the card number during transmission, by limiting the places where it might appear (in databases, log files, backups, printed receipts, and so on), and by restricting access to the places where it is stored.
If an unauthorized party obtains the card number in any way, a breach of confidentiality has occurred.
To ensure confidentiality
To ensure confidentiality

4. Integrity Data cannot be modified without authorization.
Real scenarios:
Integrity is violated when an employee (accidentally or with malicious intent) deletes important data files,
When a computer virus infects a computer,
When an employee is able to modify his own salary in a payroll database,
When an unauthorized user vandalizes a web site,
When someone is able to cast a very large number of votes in an online poll, and so on.
Preventing by Access Control and Encryption

5. Availability The information must be available when it is needed.
Highly available systems aim to remain available at all times.
Real Scenarios
Power outages,
Hardware failures,
DoS attacks (denial-of-service attacks).
Preventions by fault tolerance , access control, and attack prevention mechanisms.

6. Security Goals (Summary)
Confidentiality
Ensures that computer-related assets are accessed only by authorized parties.
Sometimes called secrecy or privacy.
Integrity
Assets can be modified only by authorized parties or only in authorized ways.
Availability
Assets are accessible to authorized parties at appropriate times.
The opposite is denial of service.

7. Security Goals
Strong protection is based on Goals relations

8. Goals are Applied to
Computer Security - generic name for the collection of tools designed to protect data and to thwart hackers
Network Security - measures to protect data during their transmission
Internet Security - measures to protect data during their transmission over a collection of interconnected networks

9. Threats , vulnerability, and Attacks
Crossing the water to the right is a Threat to the man.
Ex. (Computer) software failures
Crossing the water through the wall crack is a Vulnerability.
Ex. (Computer) Open ports
Somebody or another system destroyed the wall is an Attack
Ex. (Computer) sending an overwhelming set of messages to another system to block it.

10. Attacks Passive Attacks Active Attacks
Attempts to learn or make use of information from the system but does not affect system resources.
Eavesdropping or monitoring of transmissions
Active Attacks
Attempts to alter system resources or affect their operation.

11. Passive Attacks
Release of message contents / snooping

12. Passive Attacks (Cont.)
Traffic Analysis/ spoofing
Passive Attacks are hard to be detected

13. Active Attacks

14. Active Attacks
Masquerade
One entity pretends to be a different entity

15. Active Attacks (Cont.) Replay Attack
Passive capture of a data unit and its subsequent retransmission to produce an unauthorized effect.

16. Active Attacks (Cont.) Modification Attack
Some portion of a legitimate message is altered, or that messages are reordered, to produce an unauthorized effect

17. Active Attacks (Cont.) Denial of Service
Prevents or inhibits the normal use or management of communications facilities

18. Group Activities
Which of the following attacks is a threat to which of the security goals?
Attacks
Security Goals
Modification
Confidentiality
Masquerading
Integrity
Traffic Analysis
Availability
Denial of service
Replaying
Snooping

19. Answer Security Attacks Snooping Traffic Analysis Modification
Masquerading
Replaying
Denial of Service
Confidentiality
Integrity
Availability
Replaying might change the message sequence.. That is why it is under the integrity .

20. Security Services
Authentication - assurance that the communicating entity is the one claimed
Access Control - prevention of the unauthorized use of a resource
Data Confidentiality –protection of data from unauthorized disclosure
Data Integrity - assurance that data received is as sent by an authorized entity
Non-Repudiation - protection against denial by one of the parties in a communication

21. Security Mechanisms Specific security mechanisms:
Implemented on specific layer (OSI model)
Encipherment, digital signatures, access controls, data integrity, authentication exchange, routing control, notarization
Pervasive security mechanisms:
Not related to a specific layer
Trusted functionality, security labels, event detection

22. Model for Network Security

23. Model for Network Security
Using this model requires us to:
Design a suitable algorithm for the security transformation.
Generate the secret information (keys) used by the algorithm.
Develop methods to distribute and share the secret information.
Specify a protocol enabling the principals to use the transformation and secret information for a security service.

24. Symmetric Cipher Model

25. Symmetric Cipher Model
Known as:
Conventional Encryption
Single-Key Encryption
Plaintext
Original text/msg
Ciphertext
Coded msg
Enciphering/Encryption
The process of converting the plaintext to ciphertext
Deciphering/Decryption
The process of converting the ciphertext to plaintext

26. Symmetric Cipher Model (Cont.)
Cryptography
The developed encryption schemes
Cryptanalysis
Techniques used to get the plaintext out of the ciphertext without prior knowledge to the encryption scheme (breaking the code)
Cryptology
Both the cryptography and cryptanalysis

27. More Definitions Unconditional Security Computational Security
The ciphertext provides insufficient information to uniquely determine the corresponding plaintext.
Computational Security
The time needed for calculations is greater than age of universe

28. Symmetric Cipher Model (Cont.)

29. Symmetric Cipher Model
Requirements
Strong Key  the opponent can not figure it out even if he/she has a number of ciphertexts
The key must be exchanged through a secure channel
Y = E(K,X) ~ Y = EK(X)
X =D(K,Y) ~ X = DK(Y)

30. Brute Force Search Always possible to simply try every key
Most basic attack, proportional to key size

31. Substitution Ciphers

32. “Ygjcxgvqmnnvjgrgumfgpv” Can you decrypt this message?
Lets have Fun
You are spying on your friend Ahmed while he is chatting with John, you received the following message:
“Ygjcxgvqmnnvjgrgumfgpv”
Can you decrypt this message?

33. “Ygjcxgvqmnnvjgrgumfgpv” “We have to kill the president”
Answer
Ahmed is telling John:
“Ygjcxgvqmnnvjgrgumfgpv”
“We have to kill the president”
Encryption Key:
Replacement Table
Plaintext ABCDEFGHIJKLMNOPQRSTUVWXYZ
Ciphertext CDEFGHIJKLMNOPQRSTUVWXYZAB
Encryption Technique
Each letter is replaced by the second one after it
Remove blanks

34. Caesar Cipher Earliest known substitution cipher by Julius Caesar
first attested use in military affairs
replaces each letter by 3rd one after it
E.g.
meet me after the party
PHHW PH DIWHU WKH SDUWB

35. Caesar Cipher (Cont.) Transformation :
Mathematically give each letter a number
a b c d e f g h i j k l m
n o p q r s t u v w x y Z
Then have Caesar cipher as:
C = E(p) = (p + k) mod (26)
p = D(C) = (C – k) mod (26)

36. Caesar Cipher (Cont.) Cryptanalysis Only have 26 possible ciphers
A maps to A,B,..Z
Could simply try each in turn

37. Monoalphabetic Cipher
Rather than just shifting the alphabet
Could shuffle (jumble) the letters arbitrarily
Each plaintext letter maps to a different random ciphertext letter
The key is 26 letters long
Plain: abcdefghijklmnopqrstuvwxyz
Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN
Plaintext: ifwewishtoreplaceletters
Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA

38. Monoalphabetic Cipher Security
now have a total of 26! = 4 x 1026 keys
with so many keys, might think is secure
but would be !!!WRONG!!!
Language Characteristics Problem
Using the occurrence frequency of each letter , we can deduce the letters in the ciphertext

39. English Letter Frequencies

40. Playfair Cipher
Invented by Charles Wheatstone in 1854, but named after his friend Baron Playfair.
Encrypts multiple letters
Uses Playfair Matrix
Uses some of the rules to interpret the matrix

41. Playfair Key Matrix A 5X5 matrix of letters based on a keyword
Fill in letters of keyword (Avoid repetition)
Fill rest of matrix with other letters
E.g. using the keyword MONARCHY M O N A R C H Y B D E F G
I/J K L P Q S T U V W X Z
Why I and j  look at the word frequency . J frequency is very small

42. Playfair Rules Plaintext encrypted two letters at a time:
if a pair is a repeated letter, insert a filler like 'X',
eg. "balloon" encrypts as "ba lx lo on"
If both letters fall in the same row, replace each with letter to right (wrapping back to start from end),
eg. “ar" encrypts as "RM"
If both letters fall in the same column, replace each with the letter below it (again wrapping to top from bottom),
eg. “mu" encrypts to "CM"
Otherwise each letter is replaced by the one in its row in the column of the other letter of the pair,
eg. “hs" encrypts to "BP", and “ea" to "IM" or "JM" (as desired)

43. Note: The key is an arrangement of all of the alphabetic letters
Group Activity
Based on Playfair encryption, encrypt the word
“Hello”
Key :
Note: The key is an arrangement of all of the alphabetic letters L G D B A Q M H E C U R N
I/J F X V S O K Z Y W T P

44. Answer Step 1: Group the letters
He ll o
1st rule  repeated letters ll
He lx lo
Step 2: find the corresponding text in the key
He  EC - rule 2 H and e on the same row (replace each with letter to right)  EC
Lx  QZ -- rule 3 L and x at the same column (replace each with the letter below it)  QZ
loBX -- rule 4 l and o at different rows and columns (replaced by the one in its row in the column of the other letter of the pair)
E (Hello) “ECQZBX”

45. Security of the Playfair Cipher
Security much improved over monoalphabetic
Since have 26 x 26 = 676 diagrams
Was widely used for many years (eg. US & British military in WW1)
It can be broken, given a few hundred letters since still has much of plaintext structure

46. Polyalphabetic Ciphers
Another approach to improving security is to use multiple cipher alphabets
Makes cryptanalysis harder with more alphabets to guess and flatter frequency distribution
Use a key to select which alphabet is used for each letter of the message
Use each alphabet in turn
Repeat from start after end of key is reached

47. Vigenère Cipher Simplest polyalphabetic substitution cipher
effectively multiple Caesar ciphers
key is multiple letters long K = k1 k2 ... kd
ith letter specifies ith alphabet to use
use each alphabet in turn
repeat from start after d letters in message
decryption simply works in reverse

49. Example eg using repeated keyword deceptive
key: deceptivedeceptivedeceptive
plaintext: wearediscoveredsaveyourself
ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ
From the previous table lookup the key letter then the plain text letter.
The cipher letter is the intersection letter

50. Security of Vigenère Ciphers
have multiple ciphertext letters for each plaintext letter
Letter frequencies are obscured
But not totally lost

51. Autokey Cipher Ideally want a key as long as the message
Vigenère proposed the autokey cipher
The keyword is prefixed to message as key
Still have frequency characteristics to attack
Eg. given key deceptive
key: deceptivewearediscoveredsav
plaintext: wearediscoveredsaveyourself
ciphertext: ZICVTWQNGKZEIIGASXSTSLVVWLA

52. One-Time Pad Select a random key that is equal to the message length.
Use a table structure such as Vigenère table
Problems:
Generating long random keys
Bandwidth problem  sending the key as long as the Msg

53. Transposition/Permutation Ciphers

54. Transposition (Cont.) The letters of the message are rearranged
Columnar transposition
The number of columns is required
Example:
THIS IS A MESSAGE TO SHOW HOW A COLMUNAR TRANSPOSITION WORKS

55. Transposition (Cont.) 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

56. “ This is the second lecture”
Group Activity
Given the following message
“ This is the second lecture”
Divide the message onto a block of 5 letters block
Transpose the message
Use Autokey cipher to encrypt the result
Key : “ NetworkSecurity”

57. Stream Vs. Block Ciphers
Stream  converts one symbol of plaintext into a symbol of ciphertext
Block  encrypts a group of plaintext symbols as one block.

58. Reading materials
Stallings
Chapter 1
Chapter 2