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Introduction to Cryptography Summer ’04
Homework #1
Assigned: Wednesday, 5/12/04
Due: Wednesday, 5/26/04
Note: All ciphertexts for this assignment will be included in text files linked off the
course web page.
1)
Use the playfair cipher (as it is described in the text) with the keyword
"CRYPTOGRAPHY" to encrypt the following plaintext:
PLEASE MEET ME AT THE PARK TOMORROW AT TWO PM IF IT IS
RAINING I WILL BE BEHIND THE RED BENCH
2)
A message is called a fixed point if e
k
(x) = x. Note that this depends on the key.
Consider the affine cipher for N=35. How many valid keys are there? Classify each valid
key according to how many fixed points it creates when used for encryption.
3)
Using the extended Euclidean algorithm,
find two integers x and y that satisfy the
following equation: 403x+221y = 13.
4)
Here is a matrix to use for the Hill cipher:
7
2
8
4
0
1
5
3
2
.
Is this a valid encryption key for the Hill cipher for an alphabet of size 35? If so, what is
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 Summer '09

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