Www.cs.virginia.edu



E3 Camp Cryptography!Crypto-graphy“Secret”“writing”Plaintext:the original messageCiphertext:the encrypted (scrambled) messageKey:secret needed to transform ciphertext into plaintextSimple substitution cipher: each letter is replaced with a ciphertext letterExample key: Alphabet: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 Cipher: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 yDecode this message (encrypted using the example key):dbaqkmMuch 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 CipherThe 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 paperFor each strip of paper:Cut each strip of paper to make one of the wheels. 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:Turn the wheels so the message you want to send appears in a row.Pick a random different row and write down the letters along that row. This is the ciphertext.To decrypt a message:Line up the wheels to show the received message (ciphertext).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). ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download