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2009 Fall Midterm 2

2009 Fall Midterm 2 - Name RUID Midterm Exam 2 CS352 Fall...

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Name: RUID: Midterm Exam 2 CS352: Fall 2009 November 23, 2009 Do not open the exam until you are told to begin. Write your name legibly on the top of each page of the exam. You have 80 minutes to complete this exam. There is no leaving the room while taking the exam. If you think you might need to leave the room during the exam, you must do so before starting the exam. The exam has 7 questions, 100 points total. Not all points are created equal. Go for the easy points first! Make sure you have all 10 pages of the exam before you begin. You may only use a pencil or a black- or blue-colored pen. You may not use any notes, books, or electronic devices. Cell phones must be turned off or in silent mode. The Rutgers policy on Academic Integrity applies to this exam. Show ALL work. Grading Use Only I (10 pts) II (15 pts) III (15 pts) IV (15 pts) V (15 pts) VI (20 pts) VII (10 pts) Total (100) Page 1 of 10
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Name: RUID: I. Short Answer (10 points) A. In the BitTorrent (BT) protocol, after the handshake is negotiated, how many Interested messages can a client send to a remote peer before receiving an Unchoke message? Justify your answer for full credit. A client may send as many Interested messages as it wants before receiving an Unchoke message. Any message after the first will be discarded by the remote peer. B. Why do cryptographic hash functions ( e.g. , SHA-1) need to be one-way? Justify your answer for full credit. Assuming that cryptographic hash functions were not 1-way ( i.e., two inputs mapped to the same output), then it would be possible to replace a hashed message with another message without the receiver detecting the change. This would mean the hash function was no longer cryptographically secure, and would be useless in that context.
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