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CSC 574 Section 001
Spring 2011
Homework #1ANSWER KEY
Keywords:
Secret Key Cryptography, DES, Authentication, Integrity, MAC, Multiple
Encryption DES (EDE), Encrypting Large Messages (CBC, OFB, CFB, CTR).
Instructions
You can do this homework in groups of two (at most).
The total number of points is 60.
You must answer all questions for full credit.
The due date is as posted on the web page (submission through Wolfware).
In general, you can make any reasonable assumptions in your answers. Do NOT make
unnecessary assumptions.
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Question 1:[Affine Cipher] [10 points]
Decrypt the following ciphertext produced by the affine cipher
C = (9P + 4) mod 26
XERKXABTPOJEKTTUAUOOQKMACPELONOORBOEFYRGENACTTPONEFNBOEQYGA
TMOTTAFEMYWARKYFOBIMKOZXTPOZCWQYOKTIERARTPOXEWOAXTPOOEBTPYP
ELONOORYRNEZZJEBQKXABKOLORTOORMOEBKERFPELOROLOBBOWOYLOFERMT
PYRGNCTQYRFROKKERFORWACBEGOIORTXBAIMACXERK
Solution:
To decrypt this affine cipher, one needs to subtract 4 and then divide by 9
(mod 26) every ciphertext letter to get the corresponding plaintext
letter. Dividing by 9 is the same as multiplying by the multiplicative inverse of
9, which is 3 (i.e., 9*3 = 27 = 1 mod 26).
The result (with spaces and punctuation added for readability) is an excerpt of a
famous speech by Lou Gehrig (after whom Lou Gehrig's disease is named):
FANS, FOR THE PAST TWO WEEKS YOU HAVE BEEN READING ABOUT THE BAD BREAK I
GOT. YET TODAY I CONSIDER MYSELF THE LUCKIEST MAN ON THE FACE OF THE EARTH.
I HAVE BEEN IN BALLPARKS FOR SEVENTEEN YEARS, AND HAVE NEVER RECEIVED
ANYTHING BUT KINDNESS AND ENCOURAGEMENT FROM YOU FANS.
Question 2:[Monoaphabetic Cipher][10 points]
Find or write a program (I will provide one written in Java) that reads an input ciphertext, and
outputs the (sorted) letter frequencies, digram (consecutive pairs of letters) frequencies, and
trigram (consecutive triplets of letters) frequencies.
Using this program, decode the following
ciphertext.
The original plaintext that produced this ciphertext contains only upper case
letters;
all punctuation and spaces were removed before encoding.
It was encrypted with a
monoalphabetic cipher.
Explain how you decoded the ciphertext in your answer.
WMTLXJAVQNXBTECJIZMWDMEIGXXVTSXRAWHXHTNHVMTVWKVMXBTECQGXUCX
SWXNDXWYNQZQGNQZTIQGPAHRWNRQGVMXGAVASXJAVJIVMXCTKVTNHPAHRWN
RJIVMXCTKVWZWKMVQYNQZZMTVVMXSXMTKJXXNWNVMXDQNHADVQGVMXJSWVW
KMEWNWKVSIGQSVMXBTKVVXNIXTSKVQPAKVWGIVMQKXMQCXKZWVMZMWDMRXN
VBXEXNMTLXJXXNCBXTKXHVQKQBTDXVMXEKXBLXKTNHVMXMQAKXWKWVVMTVW
NKWHWQAKKEWBXZWVMZMWDMQASCXVWVWQNMTKJXXNBTVXBISXDXWLXH
Solution:
The best starting place was to find the word THE, which gives you 3 letters. From
there you have to guess a few main letters, possibly using backtracking if your guesses
were wrong, and then things start getting obvious.
The result is an excerpt of a speech by an early American patriot, Patrick Henry (with
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This note was uploaded on 04/04/2011 for the course CS 574 taught by Professor Harfoush during the Spring '11 term at N.C. State.
 Spring '11
 HARFOUSH

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