Assignment 3: Three problems of classical cryptanalysis
Deadline: November 22, 23:59
Points: 10 points basic award (problems 1 and 2), 7-point bonus for solutions to problem 3.
Submissions: Please post your submissions as ZIP files where files for each problem are in a
separate subdirectory. The programs shall be either in Java or C or C++ and must be reasonably
well commented and documented.
Problem 1 ­ 4 points to be awarded ­ Mono-alphabetical substitution
Introduction: One historical cryptography method is the mono-alphabetical substitution that
substitutes each letter of the alphabet with a different one.
e.g.
Plaintext 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
Cryptext Q W E R T Y U I O P L K J H G F D S A Z X C V B N M
e.g.
The message MY NAME IS BOB will become JN HQJT OA WGW
Write a (documented!) program that is able to decrypt the input (ciphertext message) without
knowledge of the key. It should read a mono-alphabetic ciphertext from a file and write the
corresponding plaintext(s) to a file. Consider only capital letters, and use any dictionary (of words)
you see fit to this purpose (indicate the source of this dictionary in the program documentation).
Problem 2 ­ 6 points to be awarded ­ A partial bi-gram substitution cipher
In this case:
ˇ The 27 letters of the alphabet (26+_) are substituted with a letter;
ˇ Four pairs of letters and five triples are substituted each one with a special different symbol.
e.g.
Plain Crypto
ZI 1
ZO 2
CE 3
GI 4
OZZ 5
EZZ 6
CCI 7
GGI 8
SCI 9
A Q
B W
C E
D R
E T
F Y
G U
H I
I O
J P
K L
L K
M J
N H
O G
P _
Q D
R S
S A
T Z
U X
V C
W V
X B
Y N
Z M
_ F
Write a (documented!) program that is able to decrypt message without knowing the key. It should
read a mono-alphabetic ciphertext from a file and write the corresponding plaintext(s) to a file.
Consider only capital letters, and use any dictionary (of words) you see fit to this purpose (indicate
the source of this dictionary in the program documentation).
Problem 3 ­ BONUS ­ 7 points to be awarded
In this case each group of three letters is substituted with another group of letters, they could be 1,
2, 3 or 4. It is allowed to substitute two different trigrams with the same N-gram.
e.g.
THE ABC
ELP ABC
PLAIN CRYPTO
THE ABC
ELP ABC
HEL LKI
ING HJU
AGE HTR
CRY DEW
ITH QVG
PAG OKY
RYP FRT
THI SDR
YOU CVG
YPT VGHI
ALL ADFS
ART A
LIN AB
RTI PL
STA K
TAR KIY
TIN KIYR
... ...
WIT
CAN PTY
ENU JUY
HIN LQY
INE JT
MEN M
NLI P
Write a (documented!) program that is able to decrypt message without knowing the key. It should
read a mono-alphabetic ciphertext from a file and write the corresponding plaintext(s) to a file.
Consider only capital letters, and use any dictionary (of words) you see fit to this purpose (indicate
the source of this dictionary in the program documentation).
Bibliography:
Elementary cryptanalysis : a mathematical approach by Abraham Sinkov
Basic Cryptanalysis: HTTP://WWW.UMICH.EDU/~UMICH/FM-34-40-2/
Lecture notes in Cryptanalysis: Dr. Alex Biryukov
http://www.wisdom.weizmann.ac.il/~albi/cryptanalysis/lectures.htm
http://www.gtoal.com/wordgames/cryptograms.html
Software:
http://secretcodebreaker.com/download.html
HTTP://WWW.ICS.UCI.EDU/~EPPSTEIN/SOFTWARE.HTML
www.cryptool.com