COMP6521 Cryptography and Blockchain Assignment 1

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COMP6521 Cryptography and Blockchain

Individual Assignment 1 (10%)

[Due: 23:59:00 29^th Jun 2024 (Sun)]

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

1．What is the original message?

Use online tools to help: https://www.101computing.net/frequency-analysis/

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: https://planetcalc.com/3311/ 

Modular Exponentiation Calculator : https://planetcalc.com/8979/ 

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 F be a file, k1, k2 be two secret keys, which of the following method is better to if we wish to ensure both confidentiality and integrity of F if ENC and MAC represent the algorithm of encryption and message authentication respectively? Why?

(a) Compute C = ENC(k1; F), T = MAC(k2; F). Store (C,T).

(b) Compute C = ENC(k1; F), T = MAC(k2; C). Store (C,T).

Question 4 Diffie-Hellman Key Exchange (2 marks).

Assume two users, A and B, have agreed to use DHKE with prime p = 23 and generator g = 8. Assuming A randomly chose private a = 7 and B randomly chose private b = 6,

(1) What is the message sent from A to B?

(2) What is the message sent from B to A?

(3) What is the final shared secret key?

Modular Exponentiation Calculator : https://planetcalc.com/8979/ 

Question 5 Certificate (2 marks).

Go to the website “https://github.com/”, check the website certificate and answer following questions.

(1) Who is the issuer of the certificate (give the CN)?

(2) What is the serial number of the certificate?

(3) Which signature scheme does the issuer used to sign this certificate?

Please also provide a screenshot of the certificate.

Question 6 Advanced Signature Scheme (1 mark)

Both aggregate signature scheme and threshold signature scheme are multi-signature scheme. Please use your word to describe what is aggregate signature scheme and what is threshold signature scheme. Especially the differences between them.