An Expert System for Solving
Simple Cryptograms
Carolyn Rucci
Dr. Ralph Morelli
1 year project
Goal
Cryptanalysis
Techniques
Encrypted text
Advantage: Perform a few quick, simple
tests to determine cipher
instead of having to try all
ciphers.
Background Information
The Art of
Secret
Writing.
Analysis
Decryption
Analysis
and
decipherm
ent of
encrypted
messages
What is an Expert System?
CipherExpert
knowledge
Reasons
about
ciphers
WWI Cryptographer
Makes
informed,
intelligent
decisions
Cipher Expert

Prototype that uses a limited set of historical
ciphers: Transposition, Caesar, Affine, Vigenere and
Playfair
Jess Expert System
Main Java Class
Expert System
Core
Hcrypto Decryption
Classes (Java)
Hcrypto Analyzer
classes (Java)
Computer Science department libraries
How Does the Expert System Work?
Creates
decision tree
with ciphers
Ciphers
Substitution
Transposition
Simple-
Caesar
Poly-
Affine
Playfair
Distribution of Letter Frequencies
70
Vigenere
Index of Coincidence
60
60
50
Used to
determine
where to
go in tree
50
40
30
20
10
0
40
30
20
10
0
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
Distribution based on an English text
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
Measure of how flat the distribution is
Easily Extensible
Want to add new cipher to system:
Create new rules
distinguishing it from other
ciphers
Letter frequency
distribution of
new cipher
Ciphers
Substitution
SimpleCaesar
Transposition
PolyAffine
Playfair
Rail Fence
Vigenere
New Cipher
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
11 / 12
10 / 12
12 / 12
12 / 12
33 / 36
32 / 36
(500 chars)
Long Text
(1500 chars)
Overall
Flow Chart
Cipher Text

false
false
IC<=.0455
Cipher is
Playfair
false
Cipher is
Vigenere
false
Error
true
IC >= .058
true
Same freqs
present
IC = Index of coincidence
Shift freq.
= normal
true
Cipher is
Affine
Cipher dist =
normal dist
true
Cipher is
Caesar
true
Cipher is
transposition
Goal


Use cryptanalysis techniques to determine
which historical cipher was used to encrypt a
text
Use that cipher to decrypt the text
Advantage: More efficient and accurate
than other methods.
Easily Extensible
What is an Expert System




Collaboration between a domain expert and an artificial
intelligence specialist
Rules are based on experts’ knowledge
Inference Engine uses rules in knowledge base to
“reason”
Example: Doctor diagnosing a patient.
Patient
response
User
Sends answer
User
Interface
asks question
about symptoms
Sends characteristics
Inference Engine
Sends question
or diagnosis
Knowledge
Base
Sends possible result
or asks for more data
Summary of Findings
Index of
Coincidence
Frequency Distribution (compared
to normal)
Transposition
~6.68%
Same
Caesar
~6.66%
Will match if shifted 3 to right
Affine
~6.66%
Same frequencies present but in
different orders
Vigenère
~4.38%
Flat distribution
Playfair
~4.72%
Flat distribution
NOTE: These values are for texts of ~2000 characters.
For Vigenère and Playfair ciphers, keyword must be at least 4
letters, to ensure these values.
Cryptanalysis
Uses
analytical techniques to decrypt a message
when the key is unknown.
–
Letter Frequencies
abcdef ghijklmnopqrstuvwxyz
monographic frequency distribution of normal alphabet
–
Index of Coincidence
thisisatestoftheindexofcoincidenceinmessages
thisisatestoftheindexofcoincidenceinmessagesthi
5matches / 44letters ~ 11%
Cipher Set
Simple Substitution Ciphers  One cipher alphabet
1.
2.
Transposition—Flips the message

hello  olleh
Caesar—Shifts each letter by 3


3.
Plain 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
Caesar alphabet d e f g h i j k l m n o p q r s t u v w x y z a b c
Affine

C = (aP + b) mod 26

a & b integer contants, C = cipher letter, P = plain text
letter
 one-to-one relationship between letters
Ciphers (cont.)

Vigenère
–
–
–

Many cipher alphabets created based on keyword
Length of keyword = # of alphabets
Cycle through alphabets when encrypting a message
Playfair – two letter pairs
D
B
M W I
C
O X
G E
Q Y
R
F
S
Z
A
K
T
P
L
U
H
M V
Case1: 2corners  replace
w/other corners
Case2: same row  replace
w/letters to right
Case3: same column  replace
w/letters below