Encryption Algorithms

Overview of Ascii

Ascii is a way to map numbers to letters. This lets computers easily translate from binary (0s and 1s) to numbers and then to the letter that number is mapped to. Here is a visual table showing the ordinal and binary representation of common english characters:

Shift Cipher

Since all letters can be mapped to a number, you can come up with a way to obfuscate english sentences so you could pass notes without others being able to easily read them. The easiest way to do this is using a shift cipher. The shift cipher is a simple letter obfuscation technique where you shift the ascii ordinal by a number then convert it back to a character. So if you choose 13 as your shift then an ‘A’ (ordinal 65) gets converted to 78, which is an ‘N’.

Exercise

Write an encryption and decryption engine! To avoid having to worry about encrypt versus decrypt you can chose 13 as the shift, so it is symmetrical (i.e. A->N and N->A). If you choose another shift you would need to know which way you were going which is a good logical next step. For example, using 10: A->K but K->U.

In the class have them pass notes to each other encrypted - they have fun with this.

Advanced

If this is too simple you can start to teach true encryption. That is not covered here but is on my list for 300 series classes in the future.