Spy Science (cryptography): Objective: Learn the basics of CRYPTOGRAPHY (Secret Writing), through the use of simple cryptographic devices.

Materials • Pencils, Pens and Markers of different diameters • Copy of Master A: Secret Decoder Badge • Paper • Scissors • Tape Introduction Cryptography is a way of keeping messages secret, even if someone else can see your message. When you put a mes- sage into a secret code, you encode it. When you receive a coded message, you have to decode it to read it. To do that, you need a key, an extra bit of information, to decode the message. People have used cryptography for thou- sands of years. In this lesson, we’ll look at ancient science of cryptography and make our own secret messages.

Encode a message (make a secret message) 1. Cut the following items out from the last page of the instructions: • Secret Decoder Badge • The two small rectangles near the bottom of the Badge. Throw rectangles away • The strip labeled “Alphabet A” 2. Run the alphabet strip through the small holes on the decoder so that the center of the strip is displayed above the alphabet on the bottom of the decoder. The sides of the strip will be behind the decoder. 3. Let’s try encoding a message—Write, “How are you?” on a piece of paper 4. Before we can encode this message, we need to line up our decoders the same way. The key tells us which two letters to line up. Let’s pick CH for our key. Find the first letter of the key, “C” in the alphabet on the strip. Find the second letter of the key, “H” in the alphabet at the bottom of the decoder badge. Slide the strip so that these two letters are lined up. You’ll need to use the sec- ond “C” on the strip to keep the strip in the badge 5. Start encoding the message! The secret message uses letters on the moveable alphabet strip. The first letter of the message is “H”. To encode this letter, find the letter H in the bottom alphabet, and write the letter you find above it, on the strip. In this case, it’s the letter“ C”. 6. Continue encoding the message, letter-by-letter, making sure not to move the alphabet strip. (If the strip does move, just make the key line up again.) [Your final encrypted message is: CJR VMZ TJP?]

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Decode a message (Read a secret message) 1. To decode the message, we will reverse how we use the strips. But first, line up the key. • Write “Key: JA. R’V ORWN, CQJWTB.” on a piece of paper 2. Just like when we encoded the message, we need to line up our devices with the key. Line up the J on the moveable strip with the A on the lower alphabet 3. The secret message letters are on the moveable strip. We move down off the strip to decode the message. The first letter of the coded message is “R”. Find it on the strip. What letter is under- neath? The first letter of the message is “I”. [Write I below R on the board.] 4. What should we write below the letter “V”? [Continue prompting the students, and writing the letters below the coded message, until you have the entire message written on the board. The correct message is “I’M FINE, THANKS.”] 5. Congratulations, you’ve decoded the message! 6. Now try writing your own message, and encoding it.

The Scytale (Side-uh-lee) The scytale is a different kind of decoder. The decoder badge is a SUBSTITUTION CIPHER - it substi- tutes one letter for another. The scytale is a TRANSPOSITION CIPHER. Rather than substituting letters for other letters, it moves letters around, or transposes them. 1. Tape one end of a strip of paper to the top of the pen or pencil 2. Wrap the paper around and down a pen so that the strip wraps from the top of the pen to the bottom. 3. Tape the bottom of the strip to the pen so that it does not move 4. Write a message on the paper, down the length of the pencil. Write one letter per overlap in paper 5. Unwrap the strip from the pencil, and fill in the blank spaces with random letters 6. Your message is now unreadable unless someone has the same diameter pen or pencil as you. 7. You can also use other cylinders (paper towel rolls, cans, etc…) 8. Make several different SCYTALE codes and see if your family can decode them!

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