All posts by profstange

Your friendly professor.

Friday, September 23rd

Groupwork homework will be due today, no other homework except to keep on with your studying of Chapter 3 as described in recent daily posts.

Note: please bring a laptop/tablet to class if it is fairly easy for you, I may do an activity that uses them (not sure yet)

Wednesday, September 21st

To do for class today:

  • Read Chapter 6, up to end of 6.1.
  • Keep working on exercises of Chapter 3 (as described in last daily post), to solidify your number theory understanding.
  • Keep reading The Code Book Chapter 3 as time permits.
  • FYI:  in class we’ll continue with primality testing & factoring algorithms.

Monday September 19th

To do for today:

  • Heads-up:  Check out Mission #4 and consider how to break up the work appropriately.
  • Read Trappe & Washington Chapter 3 up through the end of Section 3.6, filling in gaps in your learning (and learning about Euler’s Theorem, which we haven’t done in class).  Take notes of anything that is unclear/incomplete for you; discuss these issues with your group, since this week’s mission is about Euler’s Theorem.
  • Begin/continue doing exercises from Chapter 3 as needed to solidify your understanding.  Good ones include: 3.13 exercises 1, 2, 3, 4, 5, 7, 8, 9, 10, 11, 12, 13, 14, for now.  Not for turning in, and you may want to spread these out through the week.
  • Get back to reading The Code Book, i.e. Chapter 3, as time permits.

Wednesday, September 7th

Today is your first quiz! 

It will be a written quiz taking the whole period.  It will be closed-book.  I will provide such things as the Vigenere square, multiplication table mod 26, english letter frequencies, and alphabet-to-number table, if needed.


  • Use the Cryptosystem organizer to study the following cryptosystems (so this means being able to describe things like key space, how to encrypt/decrypt, being able to apply various cryptanalysis techniques).  I may ask you to encrypt, decrypt, or cryptanalyse.
    1. Caesar cipher
    2. Scytale (no need for any cryptanalysis techniques beyond exhaustive search, but do know the system)
    3. Substitution cipher (alphabetic substitution)
    4. Vigenere cipher
    5. Affine cipher
    6. Hill cipher
  • You should be adept at:
    1. modular arithmetic
    2. the notion of invertible element in modular arithmetic, finding inverses by inspection
    3. the notion of an invertible matrix in modular arithmetic
    4. notions of divisibility, primes, prime factorization, gcd, lcm
    5. performing the Euclidean algorithm and using it to find inverses in modular arithmetic (to be covered Friday Sept 2)
  • Terminology you should be able to explain:
    1. cryptography
    2. coding theory
    3. steganography
    4. plaintext and plaintext space
    5. ciphertext and ciphertext space
    6. key and keyspace
    7. types of cryptanalysis: ciphertext only, known plaintext, chosen plaintext, chosen ciphertext
    8. confusion (property of cryptosystem)
    9. diffusion (property of cryptosystem)
  • On this quiz we won’t have any proofs, but in future ones, we will.
  • You should know the broad outlines of the history, but I won’t test you on it.

Friday, September 2nd

Reminder that Mission #1 is due today.  See you in class.

Note:  Today we will cover the Euclidean algorithm and its use in finding inverses in modular arithmetic.  This is to be found in Sections 3.1, 3.2, and 3.3 of your text.  They are assigned for reading (with no particular due date, but note:  you have a quiz on Wednesday after the holiday weekend).

Wednesday, August 31

To do for today’s class:

  • Read Chapter 2.6 and 2.7 (Playfair, ADFGX and Block/Hill)
  • Make sure you can encrypt and decrypt successfully with all types of ciphers by hand: Caesar, Scytale, Substitution, Vigenere, Affine.
  • Make sure you have thought through each of the possibilities for cryptanalysis (ciphertext only, known plaintext, chosen plaintext, chosen ciphertext) for these.
  • In general, you may find some exercises in 2.13 helpful and fun in accomplishing the goals above, and testing your ability.  You should begin working through them systematically, using your judgment to skip only something you are very sure you know well.  The first 12 are on material we’ve covered so far.