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CMSC 414: Hw 1 Solution and Grading Guidelines
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1.
(text 3.3) In DES, how many keys, on the average, encrypt a particular plaintext block
to a particular ciphertext block.
Each key maps 2
64
plaintext blocks to 2
64
ciphertext blocks.
So it has a 1/2
64
chance of mapping a plaintext block b to a ciphertext block c.
There are 2
56
keys, so the total probability of mapping p to c is (1/2
64
)
⋅
2
56
= 1/256.
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2.
(text 3.5) Suppose the DES mangler function maps every 32bit value to zero,
regardless of the value of its input. What function would DES then compute?
DES does the following (see text figure 32):
•
Initial permutation
•
16 DES rounds
•
Swap left and right halves
•
final permutation (inverse of initial permuation)
With a mangler function that outputs 0 always, each DES round just swaps L and R.
So after 16 (even number) DES rounds, the initial 64bit word would be unchanged.
So DES would do the following:
•
Initial permutation
•
Swap left and right halves
•
final permutation
Based on the initial permutation, the net result is a permutation that interchanges
consecutive even and odd bits.
[If the swap were not there, DES would have no affect at all.]
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This note was uploaded on 12/03/2008 for the course CSC 574 taught by Professor Ning during the Spring '04 term at N.C. State.
 Spring '04
 NING

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