# HOMEOWRK 2INFA 640

INFA 640 Homework 2

Choose the best answer (one) and give reason in a few sentences for your choice or not choosing others. Please give a reference. To get full credit the reason should be in your own words, not a copy from any reference. Without reason in your own words you will not get full credit. The questions 1 thru 7 weigh 5 pts each. The eighth Question carries 15 pts. Please name your file as [lastname firstname INFA640_ HW2] Due last day of the session Sunday 11:59 PM . pg. 1

1) If an attacker has a copy of the cipher text and its content yields no information at all with regard to the plain text message, the text is considered _____________.

a) cipher perfect

b) third-eye blind

c) perfectly secure

d) third-party secure

Reason:

2) Which is not a weakness of a shift cipher? _________________________

a) Once you have the code book you can decode the message.

b) Natural language letter frequency makes them easy to decode.

c) The number of letters in the alphabet makes them easy to decode.

d) Once the shift is determined the message is decoded almost instantly.

Reason:

3) Quantum cryptography is based on the physics of light. True/False,

Reason with details:

4) Claude Shannon presented the encryption design principles of _________________.

a) multiplication and factoring

b) exponentiation and Logarithms

c) confusion and diffusion

d) perplexion and reflection

Reason with how and why it helps encryption:

5) Two numbers are said to be congruent if _____________________.

a) one is a multiple of the other

b) they produce the same remainder after modulo division

c) they have the same prime number as a factor

d) they are quotients of the same number

Reason:

6) A good hash function creates _________ mapping between the source string and the output string.

a) complex

b) as many as needed

c) divisional

d) one to one

Reason why it is good:

7) A number is relatively prime to another if they _____________.

a) have only each other as factors

b) have no prime factors in common

c) only have one prime factor in common

d) are both divisible by 7

Reason :

8) (15 pts) The following is a ciphertext:

a) U GIRJ CFJ SWAY. SWAY UR JQW GUCB-TUDDWY. SWAY UR JQW DUJJDW-BWAJQ JQAJ LYUCPR JFJAD FLDUJWYAJUFC. U NUDD SAKW GM SWAY. U NUDD EWYGUJ UJ JF EARR FOWY GW ACB JQYFIPQ GW. ACB NQWC UJ QAR PFCW EARJ U NUDD JIYC JQW UCCWY WMW JF RWW UJR EAJQ. NQWYW JQW SWAY QAR PFCW JQWYW NUDD LW CFJQUCP. FCDM U NUDD YWGAUC – SYACT QWYLWY

b) What kind of cipher text is this? Mono- or Poly alphabetic; Hint: assume one and when that does not work look for the other

c) Describe your cryptanalysis process. Show all the steps you went through to decrypt the message. The steps should be in sufficient details so that a reader would be able to decrypt the encrypted text without needing any help from you.

d) State the plain text message.

e) List features of the cipher- text that hindered and helped your decryption process. mention of helpful and hindering features

Note: Only a decrypted message even if it is correct without the methodology and the detailed description of the self-explanatory steps used to decrypt, would not get points.

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