23 + Interview Questions in Classical Encryption Techniques in Cryptograph & Network Security Page 1 InterviewSolution

1.	Choose from among the following cipher systems, from best to the worst, with respect to ease of decryption using frequency analysis.(a) Random Polyalphabetic, Plaintext, Playfair(b) Random Polyalphabetic, Playfair, Vignere(c) Random Polyalphabetic, Vignere, Playfair, Plaintext(d) Random Polyalphabetic, Plaintext, Beaufort, PlayfairThis question was addressed to me in an interview.This is a very interesting question from Substitution and Transposition Techniques in section Classical Encryption Techniques of Cryptograph & Network Security
Answer» RIGHT option is (c) Random Polyalphabetic, Vignere, Playfair, PLAINTEXT Explanation: Random Polyalphabetic is the most resistant to FREQUENCY analysis, FOLLOWED by Vignere, Playfair and then Plaintext.

Discussion

2.	Caesar Cipher is an example of(a) Poly-alphabetic Cipher(b) Mono-alphabetic Cipher(c) Multi-alphabetic Cipher(d) Bi-alphabetic CipherThis question was addressed to me in an online interview.This interesting question is from Substitution and Transposition Techniques topic in chapter Classical Encryption Techniques of Cryptograph & Network Security
Answer» Right answer is (b) Mono-alphabetic Cipher Easy EXPLANATION: Caesar Cipher is an EXAMPLE of Mono-alphabetic cipher, as single ALPHABETS are ENCRYPTED or decrypted at a TIME.

Discussion

3.	Monoalphabetic ciphers are stronger than Polyalphabetic ciphers because frequency analysis is tougher on the former.(a) True(b) FalseThe question was asked in an interview for internship.This intriguing question originated from Substitution and Transposition Techniques topic in section Classical Encryption Techniques of Cryptograph & Network Security
Answer» RIGHT answer is (B) False The explanation: Monoalphabetic CIPHERS are EASIER to break because they reflect the frequency of the original ALPHABET.

Discussion

4.	The Index of Coincidence for English language is approximately(a) 0.068(b) 0.038(c) 0.065(d) 0.048This question was posed to me in exam.This intriguing question originated from Substitution and Transposition Techniques topic in portion Classical Encryption Techniques of Cryptograph & Network Security
Answer» CORRECT CHOICE is (c) 0.065 Explanation: The IC for the English LANGUAGE is approximately 0.065.

Discussion

5.	Reduce the following big-O natations:O[ ax7 + 3 x3 + sin(x)] =(a) O[ax^7](b) O[sin(x)].(c) O[x^7].(d) O[x^7 + x^3].I had been asked this question in an interview for internship.My question comes from Symmetric Cipher Models and Elementary Number Theory in chapter Classical Encryption Techniques of Cryptograph & Network Security
Answer» The correct CHOICE is (c) O[x^7]. For EXPLANATION: O[ ax^7 + 3 x^3 + sin(x)] = O(ax^7) = O(x^7)

Discussion

6.	The estimated computations required to crack a password of 6 characters from the 26 letter alphabet is-(a) 308915776(b) 11881376(c) 456976(d) 8031810176This question was posed to me in quiz.My doubt is from Symmetric Cipher Models and Elementary Number Theory topic in section Classical Encryption Techniques of Cryptograph & Network Security
Answer» CORRECT CHOICE is (a) 308915776 Best EXPLANATION: The REQUIRED ANSWER is 26^6 = 308915776.

Discussion

7.	In base 26, multiplication of YES by NO gives –(a) THWOE(b) MPAHT(c) MPJNS(d) THWAEThe question was asked in exam.My query is from Symmetric Cipher Models and Elementary Number Theory topic in section Classical Encryption Techniques of Cryptograph & Network Security
Answer» Right option is (C) MPJNS To explain I WOULD SAY: Convert the alphabets into their respective values in base 26 and proceed with base 26 MULTIPLICATIONS.

Discussion

8.	If all letters have the same chance of being chosen, the IC is approximately(a) 0.065(b) 0.035(c) 0.048(d) 0.038I have been asked this question in an international level competition.Question is from Substitution and Transposition Techniques topic in section Classical Encryption Techniques of Cryptograph & Network Security
Answer» Right ANSWER is (d) 0.038 Explanation: If all letters have the same CHANCE of being CHOSEN, the IC is approximately 0.038, about half of the IC for the English LANGUAGE.

Discussion

9.	Divide (HAPPY)26 by (SAD)26. We get quotient –(a) KD(b) LD(c) JC(d) MCI had been asked this question in a job interview.My doubt stems from Symmetric Cipher Models and Elementary Number Theory in portion Classical Encryption Techniques of Cryptograph & Network Security
Answer» RIGHT CHOICE is (a) KD Explanation: Dividing (HAPPY)26 by (SAD)26gives US KD with a remainder MLP.

Discussion

10.	Division of(131B6C3) base 16 by (lA2F) base 16 yeilds –(a) 1AD(b) DAD(c) BAD(d) 9ADThis question was posed to me in a national level competition.Query is from Symmetric Cipher Models and Elementary Number Theory in division Classical Encryption Techniques of Cryptograph & Network Security
Answer» Right OPTION is (d) 9AD To elaborate: BASE 16 division to be followed where A-F STAND for 10-15.

Discussion

11.	Reduce the following big-O natations:O[ en + an10] =(a) O[ an^10 ](b) O[ n^10 ].(c) O[ e^n ].(d) O[ e^n + n^10 ].This question was addressed to me during an internship interview.Asked question is from Symmetric Cipher Models and Elementary Number Theory topic in chapter Classical Encryption Techniques of Cryptograph & Network Security
Answer» CORRECT OPTION is (C) O[ e^n ]. The BEST I can EXPLAIN: O[ e^n + an^10] = O[ e^n ].

Discussion

12.	Reduce the following big-O natations:O [ n! + n^50 ] =(a) O [ n! + n^50 ](b) O [ n! ].(c) O [ n^50].(d) None of the MentionedThis question was posed to me in unit test.My enquiry is from Symmetric Cipher Models and Elementary Number Theory topic in section Classical Encryption Techniques of Cryptograph & Network Security
Answer» CORRECT OPTION is (B) O [ N! ]. EXPLANATION: O [ n! + n^50 ] = O [ n! ].

Discussion

13.	The time required to convert a k-bit integer to its representation in the base 10 in terms of big-O notation is(a) O(log2 n)(b) O(log n)(c) O(log2 2n)(d) O(2log n)This question was posed to me in an online interview.This question is from Symmetric Cipher Models and Elementary Number Theory topic in portion Classical Encryption Techniques of Cryptograph & Network Security
Answer» Correct answer is (a) O(log2 n) The BEST explanation: Let n be a k-bit integer in BINARY. The conversion algorithm is as follows. Divide 10 = (1010) into n. The remainder – which will be one of the integers 0, 1, 10, 11, 100, 101, 110, 11 1, 1000, or 1001 – will be the ones digit d0. Now replace n by the quotient and repeat the process, dividing that quotient by (1010), using the remainder as d1 and the quotient as the next number into which to divide (1010). This process must be repeated a number of times equal to the number of decimal digits in n, which is [log n/log 10] +1 = O(k). We have O(k) divisions, each requiring O(4k) operations (dividing a number with at most k bitsby the 4 bit number (1010)). But O(4k) is the same as O(k) (CONSTANT factors don’t matter in thebig-0 notation, so we CONCLUDE that the total number of bit operations is O(k). O(k) = 0(k2). If wewant to express this in terms of n rather than k, then since k = O(1og n), we can write Time(convert n to decimal) = 0(log2n).

Discussion

14.	If the sender and receiver use different keys, the system is referred to as conventional cipher system.(a) True(b) FalseThis question was addressed to me in examination.This key question is from Symmetric Cipher Models and Elementary Number Theory in section Classical Encryption Techniques of Cryptograph & Network Security
Answer» The correct OPTION is (b) False For explanation I would say: Such a system is called ASYMMETRIC, two-key, or public-key CIPHER system.

Discussion

15.	The Index of Coincidence is –(a) 0.065(b) 0.048(c) 0.067(d) 0.044This question was posed to me in examination.My doubt is from Substitution and Transposition Techniques in section Classical Encryption Techniques of Cryptograph & Network Security
Answer» Correct CHOICE is (d) 0.044 Easy EXPLANATION: Number of LETTERS = 145.From this, IC=0.0438697 .This is very strong evidence that the MESSAGE CAME from a polyalphabetic ciphering scheme.

Discussion

16.	An encryption scheme is unconditionally secure if the ciphertext generated does not contain enough information to determine uniquely the corresponding plaintext, no matter how much cipher text is available.(a) True(b) FalseThis question was posed to me during an interview for a job.Asked question is from Symmetric Cipher Models and Elementary Number Theory in section Classical Encryption Techniques of Cryptograph & Network Security
Answer» The correct option is (a) True To EXPLAIN: The above statement is the definition for UNCONDITIONALLY SECURE cipher SYSTEMS.

Discussion

17.	On Encrypting “thepepsiisintherefrigerator” using Vignere Cipher System using the keyword “HUMOR” we get cipher text-(a) abqdnwewuwjphfvrrtrfznsdokvl(b) abqdvmwuwjphfvvyyrfznydokvl(c) tbqyrvmwuwjphfvvyyrfznydokvl(d) baiuvmwuwjphfoeiyrfznydokvlThe question was posed to me in class test.My doubt is from Substitution and Transposition Techniques topic in portion Classical Encryption Techniques of Cryptograph & Network Security
Answer» Correct OPTION is (B) abqdvmwuwjphfvvyyrfznydokvl The EXPLANATION is: Cipher text:=Ci = Pi + KI MOD m (mod 26).

Discussion

18.	In brute force attack, on average half of all possible keys must be tried to achieve success.(a) True(b) FalseThis question was posed to me during an interview.My doubt is from Symmetric Cipher Models and Elementary Number Theory topic in portion Classical Encryption Techniques of Cryptograph & Network Security
Answer» Correct answer is (a) True Easiest explanation: In brute FORCE attack the ATTACKER TRIES every possible KEY on a piece of cipher-text until an intelligible translation into PLAINTEXT is obtained.

Discussion

19.	pi in terms of base 26 is(a) C.DRS(b) D.SQR(c) D.DRS(d) D.DSSI had been asked this question during a job interview.This key question is from Symmetric Cipher Models and Elementary Number Theory topic in chapter Classical Encryption Techniques of Cryptograph & Network Security
Answer» RIGHT ANSWER is (c) D.DRS Easy explanation: On converting using base conversions we GET 3.1415926as D.DRS.

Discussion

20.	Dividing (11001001) by (100111) gives remainder –(a) 11(b) 111(c) 101(d) 110The question was posed to me by my college director while I was bunking the class.The query is from Symmetric Cipher Models and Elementary Number Theory in portion Classical Encryption Techniques of Cryptograph & Network Security
Answer» RIGHT choice is (d) 110 The explanation is: Dividing (11001001) by (100111) GIVES US (110).

Discussion

21.	A symmetric cipher system has an IC of 0.041. What is the length of the key ‘m’?(a) 1(b) 3(c) 2(d) 5The question was posed to me during an interview.Question is taken from Substitution and Transposition Techniques in division Classical Encryption Techniques of Cryptograph & Network Security
Answer» CORRECT CHOICE is (d) 5 For EXPLANATION I would say: USING the formula for calculating ‘m’ we GET m=5, where m≈0.027n/(I_c (n-1)-0.038n+0.065).

Discussion

22.	On Encrypting “cryptography” using Vignere Cipher System using the keyword “LUCKY” we get cipher text(a) nlazeiibljji(b) nlazeiibljii(c) olaaeiibljki(d) mlaaeiibljkiThe question was asked in an online interview.The above asked question is from Substitution and Transposition Techniques in section Classical Encryption Techniques of Cryptograph & Network Security
Answer» The CORRECT OPTION is (a) nlazeiibljji Explanation: CIPHER TEXT:=Ci = Pi + ki mod m (mod 26).

Discussion

23.	Which are the most frequently found letters in the English language ?(a) e,a(b) e,o(c) e,t(d) e,iThis question was posed to me during an interview.The question is from Substitution and Transposition Techniques in section Classical Encryption Techniques of Cryptograph & Network Security
Answer» Right answer is (c) e,t To EXPLAIN I would SAY: The RELATIVITY frequency of these LETTERS in percent : e-12.702, a-8.167, t-9.056, i-6.996, o-7.507.

Discussion

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In base 26, multiplication of YES by NO gives –(a) THWOE(b) MPAHT(c) MPJNS(d) THWAEThe question was asked in exam.My query is from Symmetric Cipher Models and Elementary Number Theory topic in section Classical Encryption Techniques of Cryptograph & Network Security

Divide (HAPPY)26 by (SAD)26. We get quotient –(a) KD(b) LD(c) JC(d) MCI had been asked this question in a job interview.My doubt stems from Symmetric Cipher Models and Elementary Number Theory in portion Classical Encryption Techniques of Cryptograph & Network Security

Division of(131B6C3) base 16 by (lA2F) base 16 yeilds –(a) 1AD(b) DAD(c) BAD(d) 9ADThis question was posed to me in a national level competition.Query is from Symmetric Cipher Models and Elementary Number Theory in division Classical Encryption Techniques of Cryptograph & Network Security

The Index of Coincidence is –(a) 0.065(b) 0.048(c) 0.067(d) 0.044This question was posed to me in examination.My doubt is from Substitution and Transposition Techniques in section Classical Encryption Techniques of Cryptograph & Network Security

pi in terms of base 26 is(a) C.DRS(b) D.SQR(c) D.DRS(d) D.DSSI had been asked this question during a job interview.This key question is from Symmetric Cipher Models and Elementary Number Theory topic in chapter Classical Encryption Techniques of Cryptograph & Network Security

A symmetric cipher system has an IC of 0.041. What is the length of the key ‘m’?(a) 1(b) 3(c) 2(d) 5The question was posed to me during an interview.Question is taken from Substitution and Transposition Techniques in division Classical Encryption Techniques of Cryptograph & Network Security

Which are the most frequently found letters in the English language ?(a) e,a(b) e,o(c) e,t(d) e,iThis question was posed to me during an interview.The question is from Substitution and Transposition Techniques in section Classical Encryption Techniques of Cryptograph & Network Security