Dan+Mack+ComputerTime

=Caesar Cipher - Playing the Spy=

Cipher Assignment and Rubric - [|Link]


 * Review Clock Function:**

Clock time can be thought of as a type of modular arithmetic. Modular arithmetic is used when we only want to consider the first N natural numbers, for some finite number N. There are two cases to consider when we add or multiply numbers in this situation:
 * 1) When we add or multiply two numbers and the result is less than N, the final answer is just the number resulting from the addition or multiplication.
 * 2) When we add or multiply two numbers and the result is more than N, the final answer is found by thinking of the numbers as looping back to the beginning, i.e., N = 0, N + 1 = 1, N + 2 = 2, etc.

We can think of clocks this way. For our usual clock, we think of the typical dial, except with 12 = 0. So 13 hours after 0 o'clock is 1 o'clock (13 = 12+1 which is 1 on the dial). A clock with 5 hours works the same way: 17 hours after 0 o'clock is 2 o'clock (17 = 5 + 12 = 10 + 7 = 15 + 2 which is 2 on the dial).


 * Review of Caesar Cipher:**
 * A || B || C || D || E || F || G || H || I || J || K || L || M ||
 * 0 || 1 || 2 || 3 || 4 || 5 || 6 || 7 || 8 || 9 || 10 || 11 || 12 ||


 * N || O || P || Q || R || S || T || U || V || W || X || Y || Z ||
 * 13 || 14 || 15 || 16 || 17 || 18 || 19 || 20 || 21 || 22 || 23 || 24 || 25 ||

The numbers are changed via an affine (linear) transformation to new, coded numbers. The coding function has the form: Y = A * X + B where X is the uncoded number, A is the multiplier, B is the constant (both A and B known to allies, but unknown to enemies) and Y is the calculated, coded number. The arithmetic is done mod 26 to ensure that we get numbers back that can be translated back to letters before sending the coded message. This method of coding messages is ancient. In fact, Caesar is said to have liked to use the cipher with A = 1 and B = 3. This method of coding is fun to play with, but easy to crack, as can be seen from this activity. Modern coding techniques still make use of modular arithmetic, but in a highly sophisticated way.


 * Review of ASCII: **



====The American Standard Code for Information Interchange is acharacter-encoding scheme originally based on the English alphabet. ASCII codes represent text in computers,communications equipment, and other devices that use text. Most modern character-encoding schemes are based on ASCII, though they support many additional characters.====

**A Short video about Caesar Ciphers:**
media type="youtube" key="36xNpbosfTY" height="315" width="560" align="center"

media type="youtube" key="Aj5Q1tw8go4" height="315" width="560" align="center"
 * A Short video about ASCII:**


 * Helpful Website Links:**

A Wiki page about Caesar Cipher - [|Link] A Sample Java Caesar Cipher - [|Link] Another Sample Java Caesar Cipher - [|Link] Encryption vs. Encoding - [|Link] Learning about ASCII - [|Link]


 * Reflections:**

Word Document - [|Link]


 * Audio/Visual:**

You may want to full screen if possible, and super sorry its really long. I didn't know what to take out of the reflection so I just went with everything. The second video is a little better then the first, image wise. Warning: Really Dull... again sorry but full of information.


 * **Part One:** || **Part Two:** ||
 * media type="youtube" key="RcWhDzK0Ad4" height="315" width="420" || media type="youtube" key="kIxVpDOhQtg" height="315" width="420" ||

Copyright in the Classroom - [|Link]
 * Issues in Education:**