Cavalier Crypto: Day 1 Thursday, 17 July 2008 What is cryptography? An encryption algorithm is a way of scrambling a message so someone who intercepts it cannot understand what it means, but the person (or machine) it is intended for can decrypt the message and understand it. Substitution Ciphers A monoalphabetic substitution cipher replaces every letter in the plaintext message with a substitute letter in the ciphertext. The key is the mapping between plaintext letters and substitute letter. Example key: Alphabet Replacement 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 Example 1: Encrypt CRYPTO using the example key: d b _ _ _ _ Example 2: Decrypt this message that was encrypted with the example key: xmk cgba sgdbgk _ _ _ _ _ _ _ _ _ _ _ _ _ 1 Substitution Cipher Puzzle Of course, it is much harder to decrypt a message if you do not know the key! Puzzle 1: Decrypt this message that was encrypted with an unknown key. NQWOO DGU BOOE G KOSWON, RY NIL LY NQOD GWO POGP. (AOVCGDRV YWGVBHRV) How many possible keys are there? How long would it take to try all of them? How can you decrypt the message without trying them all? 2 Challenge Puzzle Decrypt this message that was encrypted with an unknown key. Send answers to: email@example.com XBWHG GJCDW CSHFY DQRDS QWXSA ACDBH CDACF OZWAQ CFPSU YJDDJ GSHFW WOCXB WGFWP XHGW; ALVXB BWZAI 3 GWXFC WUWDX WKGSY GSYSX FAWWK ZVCDQ BWZWJ FXPHG CFWSI FBJGC BWGF.