# Eve's Cryptanalysis Challenge

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16COC140 Cryptography and Network SecurityCourseworkAlice represents a company AAA that is considering buying Bob’s company, BBB. Eve knows about the proposed transaction and she will be able to gain financial advantage if she can find out as quickly as possible the details of the deal. Eve can intercept the messages between Alice to Bob, but has no access to their computers. She knows that they will communicate using the RSA public-keyencryption algorithm in its basic form, as described in the lectures. Eve has Bob’s public key (e,n) where e=7 and n is a 256-bit integer. She does not have Bob’s private key. Eve also knows that the messages will be constructed as follows:1.An initial e-mail is sent by Alice to Bob to let him know of AAA’s decision about trying or not to buy BBB. For a “no” decision, the message is 0. For “yes”, the message is 1. The number 0 or 1, respectively, is then encrypted using the RSA algorithm.2.The next day, Alice sends Bob an e-mail to inform him of the proposed price offered. The message consists of an integer number representing the price in pounds. This number is encrypted using RSA. Eve estimates the price to be between 50 million and 60 million pounds.3.A third message is sent on the third day from Alice, consisting of a longer text (say 5,000 characters) discussing some initial details of the deal. For this e-mail, she uses the following method of encryption: each individual character of the text is viewed as the integer corresponding to the ASCII code of that character. The number thus obtained for each character is thenencrypted using the RSA algorithm, and the resulting list of numbers is sent.4.The following day Alice again writes a long e-mail (say 5,000 characters) discussing further details of the deal. For this e-mail, she uses the following method of encryption: each single word of the text is viewed as an integer number in binary, obtained by concatenating the 8-bit binary values of the ASCII code of each letter (including leading zeros). The number thus obtained for each word is then encrypted using the RSA algorithm and the resulting list of numbers is sent.i.Discuss whether Eve can find efficient methods of retrieving each of the four original messages. [10 marks]ii.For each attack you identified above, give a rough estimate of the time Eve would need to succeed. [3 marks]iii.For each of the attacks you identified above, propose changes to the encryption scheme to prevent such an attack.[7 marks]PTO

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