Date: 11/7/08
1) (5 pts) If the input to S-box 5 is 011011 in DES, what is the corresponding output?
row = 01 (bits 1,6)
col = 1101 (bits 2,3,4,5)
The corresponding entry is 9 (1001). (Grading: -2 if wrong s-box, -2 if wrong bits)
2) (5 pts) Why do the numbers 8, 16, 24, 32, 40, 48, 56 and 64 not appear in the matrix
PC-1 in DES?
They are the parity bits and not part of the actual key. (All or nothing)
3) (10 pts) If we can check 100,000,000 keys a second, how long (in years, to the nearest
year) would it take try all possible keys in DES?
years
days
year
hrs
day
hr
keys
keys
8
.
22
25
.
365
1
24
1
min
60
1
sec
60
min
1
sec
/
100000000
2
56
≈
×
×
×
×
Since the question asks to round, the answer desired is 23 years. (3 pts for the key
space, 2 pts for dividing by 10
8
, 1 pt for each of the other factors and 1 pt for
rounding.)
4) (8 pts) If the input block of text for DES is
11001011
01101010
00100111
11110000
11101000
10010110
11011100 01010011
what are the first eight bits of the output from the initial permutation (IP)?
The first 8 bits are bits 58, 50, 42, 34, 26, 18, 10 and 2. Taking these bits from the