2012
CMSC Spring 198I: Homework 4
Khuller
Due at the start of class Wed, Mar 14, 2012.
Problem 1. What is the relationship of ROT13 with Caesars cipher?
Problem 2. The algorithm for encrypting data we studied today uses a secret keyword
where we used each letter of the word to come up with a shift after repeating the
secret keyword many times. Encode the following word using the secret keyword
AIM.
String to be encoded: I AM ENJOYING THIS COURSE!
Problem 3. I used Caesars cipher to encrypt the following message. Can you decode it?
I used a shift that is < 5.
String: PGZVSWKBKUQPYGFOCTHQWTVGGP.
Problem 4. We had discussed Mickeys Algorithm for verifying credit card numbers in the
exam. Suppose credit card numbers are made up using 8 digits. Example: 8342 1219.
To this we would like to add a 9th digit which will be our checksum digit X. The
checksum in Mickeys algorithm is chosen as follows. We take the all digits and add
them doubling the alternate ones. Finally we choose X so that the entire number is 0
mod 10. An example is worked out in detail to make this process clear. Computing
checksum for 8342 1219 (right to left): 9 2 + 1 + 2 2 + 1 + 2 2 + 4 + 3 2 + 8 =
18 + 1 + 4 + 1 + 4 + 4 + 6 + 8 = 46. The checksum digit miust be 4, so that the total
is 50, which is 0 mod 10. So the card number would be issued as 8342 12194.
1. Suppose Goofey uses a dierent algorithm for computing checksum. He chooses
the checksum such that the sum of all the digits plus the checksum digit is 0
mod 10. Which is a stronger scheme in terms of error detection? Why? Give
an example where the stronger scheme catches an error in the number while the
weaker one fails.
2. Give an example where the stronger scheme fails to detect an erroneous credit
number.
Problem 5. Write a short description about the enigma machine.