E3 Camp Cryptography!
Crypto“Secret”
graphy
“writing”
Plaintext:
Ciphertext:
Key:
the original message
the encrypted (scrambled) message
secret needed to transform ciphertext into plaintext
Simple substitution cipher: each letter is replaced with a ciphertext letter
Example key:
Alphabet:
Cipher:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
u r d t g o n i p v f h w x m q l b s k j c z e a y
1. Decode this message (encrypted using the example key):
dbaqkm
2. Much harder: decode this message (encrypted using a secret key):
NQWOO DGU BOOE G KOSWON, RY NIL LY NQOD GWO POGP.
(AOVCGDRV YWGVBHRV)
Hints:
- What are the possible 1-letter words?
- What is the most common letter in English?
- What are the likely two-letter words?
Jefferson Wheel Cipher
The simple substitution cipher isn’t very good because every time a letter appears in the
plaintext, it maps to the same ciphertext letter. Thus means the patterns in the text are not
hidden. A better cipher will map letters in a more complex way.
Here’s how to make a wheel cipher similar to the one used by Thomas Jefferson. This is
much tougher to break than the simple substitution cipher.
You need: 1 bottle, Scissors, Tape, Paper sheet with each wheel printed as a strip of paper
For each strip of paper:
1. Cut each strip of paper to make one of the wheels.
2. Tape the ends of the strip together around the bottle to make one wheel.
The sender and receiver of a message must put the wheels in the same order. The more
wheels you make the longer messages you can send.
To encrypt a message:
1. Turn the wheels so the message you want to send appears in a row.
2. Pick a random different row and write down the letters along that row. This is the
ciphertext.
To decrypt a message:
1. Line up the wheels to show the received message (ciphertext).
2. Look around the wheels to find the message: one of the rows should look like the real
message; the other rows should look like random letters.
To make a much more secure cipher, instead of using the paper sheet where everyone has
the same wheels, you should make up your own random wheels (and share them with the
person who will receive the messages).