We will use three characters: Alice and Bob
who want to communicate secretly,
and Eve who wants to see what they are talking about.
Alice and Bob do not want Eve to be able
to decode their messages.

The Plaintext is the message you want to send.
For example Discrete Math Plus Plus is the nickname for this course.
The Ciphertext is the message after it is encoded.
If Alice and Bob want to exhange messages then they need to both know
SOMETHING ahead of time. What they know is called a key.
This will be clearer with examples.

In the above example of coding 
I used several conventions:
I wrote the message in all capitols. This is a standard convention for
Eduation. the Plaintext is in normal font, the ciphertext is all capitols.
I wrote the message in blocks of five. This is also a standard convention.
then it would be MUCH easier for Eve to decode.
Using the blocks of five does NOT make it much harder for Alice and Bob.
In the above I shifted the letters by one. This is a standard cipher
discussed below. I also shifted the numbers by one. This is not standard.

Note that this is a version of the beginning of the notes.
I took out various non characters and numbers. Also amusing is that
I took out what Discrete Math Plus Plus is the nickname for this course
coded into since that may have messed you up. Enjoy.