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Instructions
Required Reading
Questions
Grading
Record your answers to the questions below in a plain text file. Your plain text file must be named "<username>.txt", where <username> is the user name of your Computer Science Department account. I will not accept anything other than a plain text file.
Important: Unless otherwise specified, to receive full credit, the complete answer to every question must appear in your plain text file.
Show your work. If your answer is incorrect and you did not show your work, the question will get 0 points. If your answer is incorrect but you showed your work, the question might receive partial credit.
Send your plain text file to me by email at ark@cs.rit.edu. Include your full name in the email message, and include the plain text file as an attachment.
When I receive your email message, I will:
The submission deadline is Tuesday, 26-Mar-2013, at 11:59pm. The date/time when your email message arrives in my inbox (not when you sent the message) will determine whether your project meets the deadline.
You may submit your quiz multiple times before the deadline. I will keep and grade only your most recent submission that arrived before the deadline. There is no penalty for multiple submissions.
If you submit your quiz before the deadline, but I do not accept it, and you cannot or do not submit it again before the deadline, the quiz will be late (see below). I strongly advise you to submit the quiz several days before the deadline, so there will be time to deal with any problems that might arise in the submission process.
Late quizzes: I will not accept a late quiz unless you arrange with me for an extension. See the Course Policies for my policy on extensions. Late quizzes will receive a grade of zero.
Plagiarism: The quiz must be entirely your own work. See the Course Policies for my policy on plagiarism.
Questions 1-4. The following ciphertext was encrypted with an affine cipher:VZANAKNAFAOFPAQXVZAUONPJEOZMXINCVZKVIOJSKXXOVKFFAKNVOVZAGALSAFVQXVZARONGORBNAKJ
Question 1 (6 points). What is the plaintext?
Question 2 (4 points). Describe how you found the plaintext.
Question 3 (2 points). What is the affine cipher key that was used to encrypt this plaintext?
Question 4 (2 points). Who said this plaintext?
Question 5 (4 points). A classical permutation cipher divides the plaintext into blocks of 20 letters and rearranges the letters in each block according to a specified permutation, yielding the ciphertext. (If the final block has fewer than 20 letters, the block is padded out with Xs.) As an example, suppose the permutation is (8, 10, 19, 17, 12, 18, 15, 1, 4, 11, 3, 9, 14, 7, 13, 5, 20, 16, 6, 2); that is, the eighth letter of the plaintext block becomes the first letter of the ciphertext block, the tenth letter of the plaintext block becomes the second letter of the ciphertext block, and so on. Here is a message encrypted with this permutation:
Plaintext: FRIENDSROMANSCOUNTRY MENLENDMEYOUREARSXXX Ciphertext: RMRNNTOFEAIOCSSNYUDR MYXSUXAMLONEEDREXRNEWhich is more secure against a generic brute force key search attack, the above permutation cipher, or DES? Prove your answer.
Questions 6-8. Alice and Bob are encrypting messages using Trivium. You, Harry the Hacker, are eavesdropping on their communications. Each message is a sequence of characters; each character is represented as an 8-bit binary number using the ASCII character encoding. You happen to know that Alice and Bob are using the same key and the same nonce to encrypt every message. You also happen to know that when Alice sent the plaintext message JOHNPAUL, the ciphertext was:10011011 00000110 00101101 10110000 11101100 00000001 01001110 10110010You now observe Bob send the following ciphertext:10010011 00001100 00101011 10111011 11111000 00001001 01011000 10101010
Question 6 (6 points). What is the plaintext of Bob's message in binary?
Question 7 (2 points). What is the plaintext of Bob's message in ASCII characters?
Question 8 (4 points). Explain how you found the plaintext.
Questions 9-10. A linear feedback shift register (LFSR) is used to generate a keystream. The LFSR has five bits (s4 s3 s2 s1 s0), the feedback bit is given by the formula s2 + s0 (mod 2), the LFSR is initialized with the state (1 0 1 0 1), and the sequence of s0 values forms the keystream.
Question 9 (6 points). Give the first 40 bits of the keystream.
Question 10 (2 points). Is this a maximum length LFSR? Explain why or why not.
Question 11 (6 points). We use DES to encrypt plaintext P1 with key K1 yielding ciphertext C1, and we use DES to encrypt plaintext P2 with key K2 yielding ciphertext C2. The symbol ⊕ represents bitwise exclusive-or. Suppose P1 ⊕ P2 = all 1s; that is, the bits of P1 and P2 are opposites of each other. Suppose K1 ⊕ K2 = all 1s; that is, the bits of K1 and K2 are opposites of each other as well. What can we deduce about C1 and C2? Prove your answer. (Hint: Think about what comes out of the F function in each round in the two cases.)
Question 12 (6 points). What is the output of the first round of the PRESENT algorithm when the plaintext is 79CE27716AB314DF hexadecimal and the key is C5729BCDD60FA863A305 hexadecimal? (The first round is defined to consist of the three operations addRoundKey, sBoxLayer, pLayer.)
The quiz is worth a total of 50 points as listed above for each question.
Important: Unless otherwise specified, to receive full credit, the complete answer to every question must appear in your plain text file. When grading your quiz, I will look only at your plain text file unless otherwise specified.
Show your work. If your answer is incorrect and you did not show your work, the question will get 0 points. If your answer is incorrect but you showed your work, the question might receive partial credit.
After grading your quiz I will put your grade and any comments I have in your encrypted grade file. For further information, see the Course Grading and Policies and the Encrypted Grades.
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