Teachers As Scholars (Steven J Miller: sjm1
AT williams.edu)
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(Introduction to Cryptography)
Wednesday, March 20th and
Wednesday, March 27th, 2019
Quick links to subpages:
Cryptography
Math Riddles
My Homepage
Cryptography
The ability to encode information so that only certain recipients can read it
(or, conversely, to read information you are not supposed to have!) contains
some of the most exciting applications of pure and applied mathematics. Since at
least the time of Julius Ceasar (the title to this course is encoded with the
cipher he made famous), codes and ciphers have been used to protect important
information. We'll discuss various cryptosystems used over the centuries, mixing
history and theory. In the course of our studies we'll discuss results from
number theory, group theory, graph theory and combinatorics. This seminar is
most appropriate for middle school and high school math teachers, but anyone who
enjoys numbers and problem solving is welcome.
DAY ONE: March 20, 2019:
 Lecture One: Introduction to Cryptography: Alphabet Codes
 Lecture Two: German Tank Problem
 Lecture Three: Introduction to RSA
 Video:
https://youtu.be/D_0fKq4LjFE

Public key encryption: In this
chapter we discuss some modern cryptosystems. These are believed to provide a
high level of security through the use of difficult math problems. The danger,
of course, is just because no one has found a simple way to do a problem
doesn't mean that there isn't a simple way. We discuss two systems, one based
on graph theory (which we won't cover due to time constraints) and the other (RSA) based on primes.
 Babylonian multiplication:
http://math.widulski.net/worksheets/BabylonianMultiplication.html
(this leads to the great concept of a lookup table); more generally see
https://en.wikipedia.org/wiki/Babylonian_mathematics
 Lecture I gave in my number theory class on the Euclidean algorithm:
https://www.youtube.com/watch?v=BZwacoCYU4Y&feature=youtu.be
 Two great papers on calculus and dogs (sometimes what is locally
best is not globally best):
 Some great algorithms that run faster than you might expect:
 TAKEAWAYS / ITEMS FOR CLASSES:
 Just because you've always done something one way doesn't mean
there isn't a better (think of Horner's algorithm), that special
cases can often be done far better than the general case (think of
fast multiplication / repeated squaring), the power of lookup
tables and recording things you know and will need often (think of
Babylonian multiplication), the most abstract mathematics can turn
out to be useful (look what RSA is built on!), estimating the size
of quantities.
 Riddles to think about for Day II
 General code:
http://mathriddles.williams.edu/?p=107 : Consider an army with 10
generals. One wants a security system such that any three of them can
determine the code to launch nuclear missiles, but no two of them can.
It is possible to devise such a system by using a quadratic polynomial,
such as a x^2 + bx + c; to launch the missiles, one must input (a,b,c).
One cannot just tell each general one of a, b, or c (as then it is
possible that some subset of three generals won't know a, b and c);
however, if a general knows two of (a,b,c), then a set of two generals
can launch the missiles! What information should be given to the
generals so that any three can find (a,b,c) but no two can? What about
the general situation with N generals and any M can launch (but no set
of M1) can?
 Three hats and a strange probability:
http://mathriddles.williams.edu/?p=113 : Three players enter a room
and a red or blue hat is placed on each person's head. The color of each
hat is determined by a coin toss, with the outcome of one coin toss
having no effect on the others. Each person can see the other players'
hats but not his own. No communication of any sort is allowed, except
for an initial strategy session before the game begins. Once they have
had a chance to look at the other hats, the players must simultaneously
guess the color of their own hats or pass. The group shares a
hypothetical $3 million prize if at least one player guesses correctly
and no players guess incorrectly.The same game can be played with any
number of players. The general problem is to find a strategy for the
group that maximizes its chances of winning the prize.
 Pirates of the cryptobean: Imagine Alice and Bob are stranded on two
different desert islands. Neither can travel to the other's island, but
the pirate ship Revenge constantly sails between the two. Her captain
isn't a bad guy and he's happy to carry any cargo between Alice and Bob.
Unfortunately his sailors are easily tempted, and will steal anything
from an unlocked box. Bob wants to send a ring to Alice and ask her to
marry him. Bob and Alice each have their own lock and a key that opens
only their lock. There's a box on the pirate ship that can be locked by
either (or both) of their locks. How can Bob get the ring to Alice
without the pirates taking it? Remember, so long as either Alice or Bob
have their lock on the box, the pirates cannot open it and they'll
transfer the item. The ring represents the information we want to send,
and the pirates are the insecure channel.
DAY TWO: March 27, 2019:
 Lecture One: Why RSA Works
 Lecture Two: Error Detecting and Correcting
 Lecture Three: Error Correcting Codes (cont)
 Video:
https://youtu.be/DZeKfI_4M0g
 Hamming Codes:
https://en.wikipedia.org/wiki/Hamming_code
 Summary: We saw how basic geometry and algebra can work together.
Similar to many problems, if you look at things the right way the problem
can be easy, the wrong way and it's a hair puller. If we start with the tell
me three times the natural generation (writing the bits on a line) is tell
me four times, tell me five times, .... This leads to a terrible percentage
of data being transmitted. If, however, we write it as one bit in the upper
left and then a check digit in the first row and another in the first
column, then we can start playing with different geometry arrangements. For
example, we saw triangles with 3 data bits, squares with 4 data bits, and
then different patterns did differently for 6 points. We then extended to
three dimensions  what would we gain by going to 4?
 Lecture Four: Pythagorean WonLoss Theorem
Reading list: the following files are from an earlier version of a book I
wrote, `The Mathematics of Encryption',
with Midge Cozzens, for a general audience. Feel free to download these files, but
please
do not distribute further. If you want to use variants
of these in your schools, just let me know. Comments on choice of topics and exposition are
especially welcome; email me at sjm1 AT williams.edu. I've also posted some
links to related material for those who want to read more.

Classical cryptography: This
is the core chapter. It begins with one of the oldest ciphers, and continues
through subsequent improvements. You should download and read all of this
before class.

Enigma and Ultra: The Germans believed their Enigma code was secure;
fortunately for us the Allies were able to crack it. In this short chapter
we'll discuss some of the mathematics behind Enigma, and see why the Germans
reasonably felt that it would be impossible to break.

Public key encryption: In this
chapter we discuss some modern cryptosystems. These are believed to provide a
high level of security through the use of difficult math problems. The danger,
of course, is just because no one has found a simple way to do a problem
doesn't mean that there isn't a simple way. We discuss two systems, one based
on graph theory (which we won't cover due to time constraints) and the other (RSA) based on primes. You should download
and read this chapter before class.

Mod p arithmetic,
group theory and cryptography: The public key encryption notes need some
knowledge of primes and group theory. This chapter (from my book `An
invitation to modern number theory', joint with Ramin TaklooBighash), goes
through much of the number theory, though at a very high level. We probably
won't discuss much of this, but I've included this for completeness. This is
mostly for reference  don't worry about reading this before class.

Errror detecting
and correcting: This chapter here deals with transmitting information in
such a way that, not only can we detect when we've made certain errors, but we
can also correct them! The mathematics is motivated through some riddles.
These riddles can be used to excite and interest students, and naturally lead
to the more advanced material.
You should skim, but only skim, the part on error detection and correction before class.
 You've seen these ideas all your life with
UPC codes.
 Here are some notes I have written with Jim Kupetz, which are being used
to develop computational modules for high school courses.
 Some cryptography links (no need to read these before the class):
Math Riddles
I also maintain a math riddles page.
Please feel free to share these riddles with your colleagues and your students,
and let me know if there is anything I can do to make the site more useful for
you and your classes. The goal is to add a student / teacher's corner sometime
in the spring to facilitate using these in classrooms. If you're interested in
helping, or want updates on the progress, let me know.
Notes from 2012:
lecture notes taken by
David Strasburger,
RSA
implementation by Tom Chiari