Assignment 5

               cns06  revised: 9/13/06

Assigned: 9/19/06
Due: 6:00pm 9/30/06   (use 594cns_submit to send program/answers to TA)

Objective:  classical crypto

Points: 25

Description:

 1. You are a lucky cryptanalyst -- you have access to the
    encrypting program so you can perform chosen-plaintext
    (and in two cases, chosen ciphertext) attacks on the encryptor.
    See if you can determine the encryption algorithm and key
    for the following.  In your answer, also describe what
    techniques (text) were most useful in your mission and
    whether the encryptor is also the decryptor -- that is, if you type
    in the cipher text it produces do you get back the plaintext you
    originally typed in?
    Things to try the alphabet, strings of the same character, a's, z's, etc.
    If you can't figure out the key and algorithm, describe
    any interesting patterns you discover.

    a) telnet whisper.cs.utk.edu 7777
	 type in a line and it will encrypt it

    b) telnet whisper.cs.utk.edu 7778
	 type in a line and it will encrypt it

      (use ctrl-] quit  to get out of telnet)

    c) run the program ~dunigan/cns06/blkbox on a linux box (e.g., cetus1).
       It will encrypt lines from stdin.  
       Try various inputs to determine the algorithm and key.
       It is very similar to an algorithm in the book, what are
       any differences?

       Sometimes the cryptanalyst can find helpful information
       in other places. If you must :-), use 
	 clue 1: try   nm ~dunigan/cns06/blkbox
	 clue 2: try   strings ~dunigan/cns06/blkbox
       In your ANSWERS, indicate if you had to use the clues.
       Even if you didn't use the clues, describe what info they provide.

 2. View the strange text at the following URL

      http://www.cs.utk.edu/~dunigan/cns06/marstxt.gif
    
    Decrypt it.


 3. Develop a C program to implement the 26x26 vigenere cipher.
    Your program should take two arguments, (1) whether to
    encrypt or decrypt and (2) a key. Your program will
    read lines from stdin and encrypt or decrypt them, writing
    the result to stdout.  For example,

       vigenere e thisisakey
	  will encrypt strings from stdin to stdout using the key
       vigenere d thisisakey
	  will decrypt strings from stdin to stdout using the key

       Key and lines from stdin will not be longer than 127 characters.
       Convert lower case to upper case for both input and the key.
       Drop any non-alphabetic characters (including spaces) from input.
       Key should only be alphabetics. (A-Z, no spaces)

       Test your program to confirm that it decrypts what it
       encrypts. Confirm that the following test case works:
	  key:bigdogs
	  plaintext:computer science
	  ciphertext:DWSSIZWSAILSTUF

       Try it with a key of all A's or all Z's
       and with input of various strings, including the alphabet.
       "paste" some of your tests into the ANSWERS file.

       Encrypt "roses are red" using the key "table" and include
       the encrypted output (cipher text) in your ANSWERS file.

 4. Use your vigenere program to answer the following:
    Given the following ciphertext:

    COGMEIKQGRDAQXNBFFGCWCVDDNDBQEVTBDVULGHIEAB

    find the key that yields the following plaintext:   (throw away the blanks)

    MR MUSTARD WITH THE CANDLESTICK IN THE HALL

    and find another key that yields the following plaintext:

    MISS SCARLET WITH THE KNIFE IN THE LIBRARY

    How does this problem relate to the unbreakability of the one-time pad?

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  Place the answers to the questions in a file named ANSWERS
  in a directory with your C source file and makefile (if any)
  and use   594cns_submit to submit your answers to the TA.
  See the class policy page for internal documentation requirements
  and such.