An Expert System for Solving Simple Cryptograms

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An Expert System for Solving Simple Cryptograms

• Carolyn Rucci
• Dr. Ralph Morelli
• 1 year project

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Overview

• Goal
• Background information
• What is an Expert System
• My Expert System - CipherExpert
• How does CipherExpert Work?
• Extensibility
• Results from testing
• Caesar Cipher

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Goal

• What?
• How?

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Background Information
Cryptography
Cryptanalysis
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What is an Expert System?
Reasons!
CipherExpert
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Cipher Expert

• Prototype that uses a limited set of historical
  ciphers Transposition, Caesar, Affine, Vigenere
  and Playfair

Main Java Class
Computer Science department libraries
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How Does the Expert System Work?
Distribution of Letter Frequencies
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Easily Extensible
New Rules
New Cipher
Add new classifications to family tree
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Testing

• Tested CipherExpert 36 times, using 3 texts of
  varying length

Determined Cipher Decrypted Text
Short Text (36 chars) 10 / 12 10 / 12
Medium Text (500 chars) 11 / 12 10 / 12
Long Text (1500 chars) 12 / 12 12 / 12
Overall 33 / 36 32 / 36
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Caesar Cipher
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
Key shift by 5
Ocdn dn v ozno oj nzz da ncjmo hznnvbzn rjmf.
v w x y z a b c d e f g h i j k l m n o p q r s t u
This is a test to see if short messages work.
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Advantages of an Expert System

• More Efficient and accurate than other methods
• Do not have to try all ciphers to find a
  decryption
• No false positive decryptions
• Analysis based on the frequencies of letters in
  the message
• Distribution of letter frequencies
• Index of coincidence

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Java Main Class
Java Main Class
Text file
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Jess Getting Started

• Fetch IndexOf Coincidence Object from rete

Search depth - 1 parent - no-parent ic - call
getIOC() on object shift - call getShift() on
object type - unknown last-move - no-move

• Create a template for initial position

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Jess Example of Move to Substitution Rule
Test shift ltgt 0
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Jess The Jess Path

• After each new node is created

1) If type known
2) Else type unknown