CSE 100 Lab

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Hiding Your InformationSimple Ciphers
Matt HuenerfauthCSE 100Lab Session 4October
2003U. of Pennsylvania
Image theimaginaryworld.com
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Wiretapping and Ciphers

• Throughout history, people have looked for ways
  to send information without it being overheard by
  a third party.
• Some people have responded to new wiretapping
  laws and technology by trying to encode or
  scramble the information they send over wires.
• That way, even if someone is listening, they
  wont be able to understand.

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The Code Book

• The Code Book discusses some historical ciphers
• Spartan Scytale (Transposition)
• The Caesar Cipher (Substitution)
• Kama-Sutra Secret Writing (Substitution)
• Some codes rearranged the letters in a message
  (transposition) and others replaced the letters
  with other ones (substitution).

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Transposition Ciphers

• We take out original message, and we mix up all
  the letters (in a specific way) so that other
  people cant understand our message.
• The receiver knows the secret way to unscramble
  the letters in order to get the original
  message.

Hang ten!
Ghna net!
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Spartan Scytale /sit-uh-lee/

• Wrap a strip of paper around a tube of specific
  size, then write your message sideways (generally
  one letter per strip). Only someone with same
  size tube can read your message.

Image Waggener Website
Image Museum of Unnatural History
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Substitution Ciphers

• Make a table for all the letters of the alphabet.
  Pick a new code letter to stand for each one.
  Go through your message, and replace each letter
  with its code letter from the table. Only
  someone with the table could decode your message.

bed
Original a b c d e f g h i jCode Letter D F I Q
K X M Z R P
FKQ
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Caesar Cipher aka Decoder Rings

• Caesar used a simple substitution cipher. He
  just shifted the alphabet. But since theres
  only 26 ways to shift, these codes are easy to
  break (just try all 26 ways).

Image Old Time Radio Premiums
Original a b c d e f g h i j ...Code Letter D E
F G H I J K L M ...
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Kama-Sutra Secret Writing

• A harder-to-break cipher can be designed by
  instead of just shifting the letters of the
  alphabet, you assign each letter a totally random
  code letter.
• This form of secret-writing is one of the 64 arts
  explained in the Kama-Sutra.

Original a b c d e f g h i jCode Letter D F I Q
K X M Z R P
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Newspaper Cryptograms

• Why dont we all just use this approach to hide
  our information? Because people can figure out
  how to decode it!
• In fact, substitution ciphers are behind the
  cryptogram puzzles you see in the newspaper.
  People solve these in an afternoon
• Computers make them even easier to solve.

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Todays Lab Activity I

• To show why modern cryptographers have had to
  find more sophisticated ways to encode
  information, we will see how easy it is to break
  a substitution cipher with the help of a
  computer.
• We will use some computer tools to look at the
  frequency of letters in the coded text, and well
  compare this to the frequency of letters in
  standard English.

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Todays Lab Activity II

• What are the most common letters in common
  English text?
• What do people usually guess in the bonus round
  of Wheel of Fortune? R S T L N E
• In fact, e is the most common letter.
• So if theres a lot of Ws in the coded text,
  then maybe W is really standing for the letter
  e

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Todays Lab Activity III

• We will count the frequency of code letters in a
  piece of text that has been encoded using a
  substitution cipher.
• We will count the frequency of letters in a a big
  random sample of English text.
• We will try to line up the most common letters
  in each text to figure out the cipher.
• Computer tools make this really fast.

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Step 1

• http//www.seas.upenn.edu/cse100/labs/04
• Get a printout of letter-chart.html.
• Open the cipher text and look at it.
• Copy the cipher text.
• Open letter frequency program.
• Paste the cipher text into the box at the top of
  the letter frequency program.

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Step 2

• Press the Do Letters button.
• Record results onto the letter chart printout.
• Based on frequencies, fill out an initial guess
  of the letter alignment at the bottom of the
  letter chart printout.
• Alphabetize your alignment according to the
  lowercase original letters.

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Step 3

• Open the cryptogram applet program.
• Paste the cipher text into the top box.
• Type in your alphabetized alignment into the
  box at the bottom of the window.
• Press the set code button.
• Press the word DECRYPT.
• Inspect your results? Can you read this?

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References

• The Imaginary Worldhttp//theimaginaryworld.com/
  ralston.html
• Museum of Unnatural Historyhttp//www.unmuseum.o
  rg/excoded.htm
• Waggener Websitehttp//jwilson.coe.uga.edu/EMT66
  8/EMT668.Folders.F97/Waggener/Jessica.html
• Old Time Radio Premiumshttp//www.teleport.com/j
  rolsen/premiums/premiums.html