An NCTM Illuminations Lesson
Please cut out the two circles on
the Caesar Shifter Activity
Sheet in your packet.
Enigma Machine
Used 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 proportional
to 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.
U.S. Navy Signal Flags
Man
Overboard
I am on fire
and have
dangerous
cargo on board
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?
‘V’ occurred 13 times
Which letter occurred the second
most times?
‘K’ occurred 9 times
Which letter occurred the third most
times?
‘R’ occurred 7 times
Notice the similarity of the word
before the semi-colon and the first
word on the last line.
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 cipher
as opposed to a substitute cipher
Rome is the Greatest Empire
math ma thm athmathm athmat
Encode 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 Caesar Cipher?
25
How many different ways can a message
be encoded using the Vigenere Cipher?
n
26
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