Intro to Encryption

Slide 2 What is encryption?

Encryption is sending private messages. Usually this means taking a message and scrambling the letters so that it’s unreadable to anybody who wants to see it, except for the person it’s intended for.

The first known type of cipher (encryption technique) is known as the Caesar Cipher, invented by Julius Caesar in the first century BC, who used it for his private mail.

Slide 3

Let’s make a Caesar Cipher!

Slide 4

Scrambling with the Caesar Cipher. Using the cipher wheel

Slide 5

What is the scrambled text? 

The sly fox jumped over the lazy dog

Slide 6: You try

Encrypt with a shift of 4.

I want some ice cream

Slide 7: Decryption

You can decrypt a message by doing the same thing in reverse!

Decrypt the message with a shift of 8.


Slide 8: Breaking the Caesar Cipher

Breaking the Caesar Cipher is easy (with a little data).


What does it say? How did you break it?

Slide 9: Brute Force Attack

One way to break the Caesar Cipher is to do a brute force attack. This mean trying every combination to figure out which one works. In English, where we have 26 letters, this would mean trying a maximum of 26 times.

Other types of ciphers can also be susceptible to brute force attacks, it’s just a matter of how long it takes to break.

Slide 10: Data Analysis on the Caesar Cipher

Another way to break the Caesar Cipher is to use data analysis– specifically on the letters used in the English language. Use the following excerpt from the English language to see which letters are most common.

As it turns out this excerpt is from your English class!

Slide 11: English Usage by Letter

Determined by (Letter occurrence)/(total letters)

Slide 12: Common Strings of 3 Numbers

the, and, tha, ent, ing, ion, tio, for, nde, has, nce, edt, tis, oft, sth, men

Slide 13: Break the code

Break the following codes! Determine the Key!

  1. Kf sv fi efk kf sv, Kyrk zj kyv hlvjkzfe
  2. kw dytmpgrc afccqc gq kslqrcp
  3. uchn hi lofym musm nby xia wuhn jfus vumeynvuff
  4. divdl opssjt dbvhiu bmm uif qplfnpo gspn b mboemjof
  5. nooj xydc grkd iye drsxu



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