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: evans@cs.virginia.edu
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.