perating Systems and Networks cs159342

Question 1 Break Monoalphabetic Substitution Ciphers (2 marks).

zbty cu wsibjsw jb zbgujnmzjcge q gmnjmncge sgicnbgtsgj qgw znsqjcge sozsvvsgj vsqngcge soysncsgzsu xbn ujmwsgju cguycncge jdst jb mgvsqud jdscn ybjsgjcqvu cgjb aszbtcge xmjmns’u cggbiqjbnu cj wsvcisnu dced hmqvcjl nsusqnzd ybujenqwmqjs ynbenqttsu jqmedj tqujsn ynbenqttsu qgw mgwsnenqwmqjs ynbenqttsu cg zbtymjsn uzcsgzs qgw tmvjc wcuzcyvcgqnl qnsqu bx cj qgw sgjsnyncus zbtymjcge cju ynbenqttsu cgjsenqjs jdsbncsu ulujstu qgw qyyvczqjcbgu tssjcge jds gsswu bx ujmwsgju kdb kcud jb ymnums jdscn zqnssn cg zbtymjcge nsvqjsw jszdgbvbel

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1．What is the original message?

Use online tools to help: Frequency Analysis - 101 Computing

2. Write down the detailed steps and attach the screenshot (if you use the frequency analysis tool).

Question 2 RSA Encryption and Signature Scheme (2 marks).

Alice tries to establish her own RSA public key and private key. She chooses two prime numbers p = 13 and q = 5 and sets n = pq = 65.

1. If Alice chooses her public exponent e=11, what is her private exponent d? what is her RSA public key?
2. Encrypt a message M = [the eighth digit of your student id], e.g. for 12345678d, encrypt M=8, using Alice’s public key.
3. Sign a message M = [the eighth digit of your student id], e.g. for 12345678d, sign M=8, using Alice’s key. (Plain RSA signature without hashing the message)

Use online tools to help

Inverse Modular Calculator: Online calculator: Modular Multiplicative Inverse Calculator 

Modular Exponentiation Calculator : Online calculator: Modular exponentiation 

Question 3 Confidentiality and Integrity (1 marks)

We learnt in the lecture that encryption and message authentication code can be used to provide confidentiality and integrity respectively. Let

 be a file, be two secret keys, which of the following method is better to if we wish to ensure both confidentiality and integrity oif and  represent the algorithm of encryption and message authentication respectively?Why?

(a) Compute ,  Store . (b) Compute . Store .