Test 1 Computer Security

7/31/2019 Test 1 Computer Security

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QUESTIONS AND ANSWER FOR TEST1 (COMPUTER SECURITY)

QUESTION 1

a) What is computer security?Computer security is the protection/prevention of organizational assets from unauthorized

disclosure and modification and provided he computer services is available. (P/s: insert the

three objectives of computer security CIA)

b) Explain the role of security policy and security mechanism in implementing computer security?Define security policy->Security policy is a statement of what is and what is allowed and not

allowed

Role of security policy->Policy define security

Define security mechanism->isa method, tool, or procedure for enforce security policy

Role of security mechanism->enforce security policy

QUESTION 2

Explain with example THREE instruments in computer mechanism.

Instrument Explanation Example

1. Prevention Try to avoid attack from occur Firewall2. Detection Identify the attack Antivirus3. Recovery/reaction Remove the attack /stop the

attack from happen again, then

implement mechanism to avoid

repetition.

Recovery back-up

QUESTION 3

Security risk is always being associated with threats and vulnerabilities. Explain with example what is

threat and vulnerability.

Term Definition Example

Threat Action by adversaries to exploitvulnerabilities

Information disclosure Virus attack

Vulnerability Weakness of a system that can

be exploited by attacker

Software bugs Wrong access

permission

P/s: Jus

additio

note

P/s: Justadditional n

Confidentiality

AvailabilityIntegrity


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QUESTION 4

a) Cryptanalysis is the study of methods for obtaining the meaning of encrypted information.Without access to the secret information that is normally required to do so. Typically, there are

three types of approaches:

Explain the above cryptanalysis approaches.I. Cipher-text cryptanalysis->Attacker only has cipher-text given and need to find the key

II. Known plain-text cryptanalysis ->Attacker only have plain text & cipher text given needto find key & algorithm

III. Chosen text cryptanalysis ->Attacker are given with either plain or cipher text .He knowthe algorithm and able to encrypt and decrypt the message to find the key.

b) What is the main objective of cryptanalysis?Main objective of cryptanalysis is to obtain the meaning of encrypted information.(P/s: just take

from the question)

QUESTION 5

Encryption is the conversion of data into a form, called a cipher-text that cannot be easily understood by

unauthorized people.

THE COOK WAS A GOOD COOK.

a) Encrypt the above plain-text using:I. Columnar transposition cipher with the size of 5

1 2 3 4 5

T H E C O

O K W A S

A G O O D

C O O K Z

The cipher text: TOAC HKGO EWOO CAOK OSD

II. Rail-fence transposition cipher with the size of 61 T A

2 H S G

3 E A O K

4 C W O O

5 O K D O

6 O C

The cipher text: TA HSG EAOK CWOO OKDO OC

b) Discuss how the encryption of the above techniques can be made more secure.???????????

Key

Padding

Key

WRITE IN RO

READ IN

COLUMN

(COLUMNAR


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QUESTION 6

Decryption is the process of converting encrypted data back into its original form, so it can be

understood.

a) Write a pseudo-code for Thomas Jefferson Wheels decryption.Identify key

define cipher-text

find position of key

read plain-text

b) Using Vigenere Tableau as in Appendix A, decrypt the following cipher-text withCOMPUTER as the key.

XWSYG IIGHM VEEVW GUGNP RVCD (cipher text)

X W S Y G I I G H M V E E V W G U G N P R V C D

C O M P U T E R C O M P U T E R C O M P U T E R

The plain text is: VIGJM PEPFY JPKCS PSSBA XCYM

Plaintext

Ke

1) Put COMPUTER

as key at the

bottom of the

cipher text

2)find cipher letter

along the row

Eg) letter c have

cipher letter x..so

find the plain letterwhich is letter v

3) continue until

finish

Key

Ci her text

Ci her


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QUESTION 7

In cryptanalysis, frequency analysis is the study of the frequency of letters or groups of letters in a

cipher-text. The method is used as an aid to breaking classical ciphers.

a) Explain how monograms, bigrams/digrams and trigrams being used in frequency analysis. Create frequency table Base on most used appeared single letter(monogram) Base on most used appeared couple letter(bigram/digram) Base on most used appeared triple letter(trigram)

b) Determine the plain-text for the cipher-text below: (Please refer to Appendix B)BPMKW WSEIA IOWWL KWWSI AKWWS AOWIV LIAKW WSAOW APMEM VB

In the sentence, w is the most appeared single letter.

So, compare w with the highest probability in single letters frequency

Try w= e (key=18)

Try w= t (key=3)

Try w = a (key=22)

Try w = o (key=8)

The plaintext is = THE COOK WAS A GOOD COOK AS COOKS GO AND AS COOKS GO SHE WENT

Match


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Test 1 Computer Security