An NCTM Illuminations Lesson

Please cut out the two

circles on the Caesar Shifter Activity Sheet in your packet.

Enigma MachineUsed to encode messages during WWII.With substantial aid from Poland, the Allies were able to break the code and read German messages.

Morse Code: Designed for coding efficiency.The length of each coded letter is inversely proportionalto its frequency of occurrence in English.

Secret Decoder Ring

These rings became popular in the 1930’s as premiums for children who were

fans of the radio program

Little Orphan Annie.

I am on fire and have dangerous cargo on board

ManOverboard

U.S. Navy Signal Flags

Deciphering a Simple Coded Message

20-15-4-1-25 25-15-21

23-9-12-12 12-5-1-18-14

1-2-15-21-20 3-15-4-5-19

Today you will learn about codes.

Use Letter Frequency Analysis

TFNRIUJ UZV DREP

KZDVJ SVWFIV KYVZI

UVRKYJ;

KYV MRCZREK EVMVI

KRJKV FW UVRKY SLK

FETV.

Cowards die many times before their deaths;The valiant never taste of death but once.

Julius Caesar by William Shakespeare

Which letter occurred most often?

Which letter occurred the second most times?

Which letter occurred the third most times?

Notice the similarity of the word before the semi-colon and the first word on the last line.

‘V’ occurred 13 times

‘K’ occurred 9 times

‘R’ occurred 7 times

The Caesar Cipher

Julius Caesar is thought to have used this method to communicate with officers in the Roman army.Caesar would inform his generals what the shift was, so they would be the only ones who could read the encrypted message.

Pretend you are Julius Caesar. You are conducting a meeting about sending encrypted messages.

You have decided to use a shift of 7 units. What letter replaces ‘A’?

Use your Caesar Shifter to encrypt this message:

ROME IS THE GREATEST EMPIRE

YVTL PZ AOL NYLHALZA LTWPYL

Your generals receive this message:

YVTL DHZ UVA IBPSA PU H KHF.

What does the message say?Rome was not built in a day.

The Caesar Cipher was eventually discarded.

Why do you think it was discarded?

A coded message included in a short story titled The Adventure of the Dancing Men by Sir Arthur Conan Doyle.

Connect to Literature: Edgar Allen Poe was also fond of using secret messages in his stories.

a polyalphabetic cipheras opposed to a substitute

cipher Rome is the Greatest Empire math ma thm athmathm athmatEncode this message using your copy of the Vigenere Grid.

DOFL US MOQ GKLMTXZF EFWURX

What makes the Vigenere Cipher more difficult for code breakers?

What letters were used to replace ‘E’?Perform a Frequency Analysis to see which letters occur most often.

How many different ways can a message be encoded using the Vigenere Cipher?

How many different ways can a message be encoded using the Caesar Cipher?25

26n

This message is written using the Vigenere Code. The keyword is ‘code’.

OOWL KG HZGFBAJSUI

Decode the message.

Encode a message using the Vigenere Cipher.

algebra