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# Exam2-Sol - CIS 3362 Test#2 Symmetric Block Ciphers...

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CIS 3362 Test #2: Symmetric Block Ciphers Solution Date: 10/13/2010 1) (10 pts) In order to teach DES, Arup has come up with a new symmetric block cipher, similar to DES, with 16 bit blocks and a 16 bit key, called mini-DES. It follows a similar structure to DES and has an IP matrix as follows: 13 9 5 1 16 12 8 4 15 11 7 3 14 10 6 2 What is the corresponding matrix, IP -1 that matches this matrix? 4 16 1 2 8 3 15 1 1 7 2 14 1 0 6 1 13 9 5 Grading: ½ point for each entry, 2 pts bonus if all are correct. 2) (4 pts) If the input to S-box 6 was 110100, what is the output? Row = 10 = 2, Col = 1010 = 10, the entry in row 2, col 10 of S-box 6 is 4. 4, grading: full credit for 4, 2 pts for 0, 10, 1, 13, 1 pt for 1, 7 14. 3) (5 pts) Why are the numbers 8, 16, 24, 32, 40, 48, 56 and 64 missing from the matrix PC-1?

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These are the parity bits in the key, but PC-1 only permutes actual bits from the key. (Grading: 4 pts for mentioning parity bits, 1 point for a rational answer as to why they are missing.) 4) (10 pts) How many days, to the nearest day, would it take a computer program that can test one billion keys per second to test all possible keys for DES block? Number of seconds to search keyspace= 2 56 /10 9 . (4 pts) Number of seconds in a day = 60 x 60 x 24 = 86400 (3 pts) Number of days to find the key = 2 56 /10 9 /86400 ~ 834. (3 pts)
5) (16 pts) In DES, using the matrices PC-1, PC-2 and the schedule of key shifts, we can actually

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Exam2-Sol - CIS 3362 Test#2 Symmetric Block Ciphers...

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