Crypto: HW 3            Fall 2001            Michael Siff      

Due Thursday, September 20
    Read: the rest of Chapter Two as well as Appendix D of The Code Book (pp. 63-99).

  1. Decrypt the pigpen cipher on the last line of Appendix D.

    Modular arithmetic

    For each of the following English sentences, write a modular congruence describing the sentence. For example, for the sentence:

    "If Monday is the first day of the week and Sunday is the seventh day of the week, then four days past Saturday (the sixth day) is Wednesday (the third day)."
    write: 
      (6 + 4) mod 7 = 3
  2. "Twenty one hundred hours in military time is what we might call nine p.m."

  3. "In a shift cipher with key 5, plaintext letter x becomes ciphertext letter C."

  4. Solve the following modular arithmetic problems: 
    23 mod  9 = 

23 mod 11 =

47 mod 11 = 

 5 mod  9 =


 (5 + 23) mod  9 = 

(47 + 23) mod 11 =

    Vigenère Cipher

  5. Encrypt the plaintext "a man a plan" using a Vigenere Cipher with keyword "STAR". (Ignore the spaces in the plaintext.)

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