Cryptography

1
Chapter 2: Cryptography with
Classical Cipher
Faculty of Physical and Basic Education
Computer Science
By: Msc. Karwan M. Kareem
2014 - 2015
© University of Sulaimani, Faculty of Physical & Basic Education, Department of Computer Science 2014 / 2015

Topics
Basic concepts of Cryptography
Secret writing with Cryptography
Cipher
Cipher Types
Substitution Cipher
Substitution Cipher Types
1945
1964
2
 Monoalphabetic Cipher
 Polyalphabetic Cipher
 Polygram Cipher
 Homophonic Cipher
 Transposition Cipher
Transposition Cipher Type
 Rail Fence Cipher
 Route Cipher
 Simple Columnar Cipher
 Keyword Columnar Cipher

Basic concepts of Cryptography
 Basic Concepts ( terminology)
 Plaintext : the original message.
 Ciphertext : the coded message.
 Cipher : algorithm for transforming plaintext to ciphertext.
 Key : info used in cipher known only to sender/receiver.
 Encipher (encrypt) : converting plaintext to ciphertext.
 Decipher (decrypt) : recovering ciphertext from plaintext.
 Cryptography : study of encryption principles/methods.
 Cryptanalysis (code breaking) : the study of principles/ methods of deciphering
ciphertext without knowing key.
 Cryptology : the field of both cryptography and cryptanalysis
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Basic concepts of Cryptography
 Cryptography
 Cryptography is the science of providing security for information.
 Process or Practice of the enciphering and deciphering of messages in secret
code.
 It has been used to obtain secure communication between individuals,
government agencies, banking, and military forces.
 The principles of cryptography are today applied to the encryption of FAX,
TELEVISION and computer Network communications
 Cryptanalysis
The procedures, processes, methods, etc., used to translate or interpret
secret writings (encrypted messages) into plaintext without known the secret
key.
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Cryptology: the scientific study of cryptography and cryptanalysis
Cryptology  Cryptography + Cryptanalysis

Secret writing with Cryptography
Polygram
Cipher
Affine Cipher Atbash Cipher ROT13 Cipher Caesar Cipher
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Homophonic
Cipher
 NOTE:
Secret Writing
Cryptography
(scrambled)
Classical
Ciphers
Substitution
Mono
alphabetic
Cipher
Polyalphabetic
Cipher
Vigenère
Cipher
Beaufort
Cipher
Rail Fence
Cipher
One Time Pad
Cipher
 Cryptography does not hide the existence of messages.
 Steganography hides even the existence of a message.
Playfair
Cipher
Transposition
Simple
Columnar
Cipher
Keyword
Columnar
Ciphe
Route Cipher
Modern
Ciphers
Steganography
(hidden)

Cipher
 Cipher
In Cryptography, a cipher is an algorithm for performing encryption or decryption. It
can be Symmetric or Asymmetric “a way of changing a message to keep it secret”.
 Symmetric (Private-key cryptography): In Symmetric Key Algorithms same key used
for encryption and decryption.
 Asymmetric (Public-key cryptography): In Asymmetric Key Algorithms different
keys used for encryption and decryption.
Stream ciphers encrypt the digits (typically bits) of a message one at a time.
Block ciphers take a number of bits and encrypt them as a single unit (Block),
Blocks of 64 or 128 bits have been commonly used.
 There are two types of ciphers:
1. Classical Ciphers
2. Modern Ciphers
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Cipher Type
1. Classical Ciphers
A classical cipher is a type of cipher that was used historically. In general, classical
ciphers operate on an alphabet of letters (such as "A-Z"), and are implemented by hand
or with simple mechanical devices.
Sender and Recipient share a same key.
All classical encryption & decryption algorithms are Symmetric.
Classical Cipher Types:
Substitution Cipher
Transposition Cipher
Substitution Cipher
Is a method of encryption by which units of plaintext are replaced
with Ciphertext, according to a regular systems.
Substitution Cipher Types
1- Monoalphabetic Cipher 2- Polyalphabetic Cipher 3- Polygram Cipher.
7 4- Homophonic Cipher.

Substitution Cipher Type
 Substitution Cipher Types
1.Monoalphabetic Cipher
The Monoalphabetic substitution cipher is so called because each plain text letter is
substituted by the same cipher text letter throughout the entire message.
Example:
In the cipher table below, plaintext ‘ r ’ is always replaced by cipher text ‘ H ’.
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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
X D G S Z A N Y O B T M J C E V F H K W P L Q U R I
Plain Text:
Cipher Text:

1. Monoalphabetic Cipher Types
Monoalphabetic Cipher is oldest and weakest cipher method.
Monoalphabetic Cipher can be broken through the use of Frequency Analysis
method.
The most famous methods of this type are:
A.Caesar Cipher.
B.ROT13 Cipher.
C.Atbash Cipher.
D.Affine Cipher.
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1.Monoalphabetic Cipher Types
A.Caesar Cipher
Caesar Cipher is One of the simplest examples of a substitution cipher, which
have been used by Julius Caesar to communicate with his army.
Caesar is considered to be one of the first persons to have ever employed
encryption for the sake of securing messages. “ Caesar used a shift of 3”.
Encryption Method: C = P + K Key Always = 3
Decryption Method: P = C - K Key Always = 3
Example:
Plain Text: ZAGROS Key = 3
Cipher Text: CDJURV
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B. ROT13 Cipher
ROT13 is a simple encryption cipher. It is a variant of the Caesar Cipher.
 Encryption Method: C = P + K Key Always = 13
 Decryption Method: P = C - K Key Always = 13
 Example:
Plain Text: HELLO Key = 13
Cipher Text: URYYB
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C. Atbash Cipher
Atbash is a simple substitution cipher for the Hebrew alphabet. Using this method
sometimes the encrypted words has another meaning.
 Encryption Method: Reversing each letter once.
 Decryption Method: Reversing each letter once.
An Atbash cipher for Hebrew
alphabet;
Plain: אבגדהוזחטיכלמנסעפצקרשת
Cipher: תשרקצפעסנמלכיטחזוהדגבא
An Atbash cipher for English alphabet;
Plain: abcdefghijklmnopqrstuvwxyz
Cipher: zyxwvutsrqponmlkjihgfedcba
 Example:
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D. Affine Cipher
In Affine Cipher each letter in an alphabet is mapped to its numeric equivalent as
shown below;
 English alphabet is 26 character therefore ( m = 26 ).
 This method uses two keys “ K1 & K2 ”. We will call K1 (A) & K2 (B).
 K1 can have a value from “1, 3, 5, 7, 9, 11, 13, 15, ……..……. Up to 25”
 K2 can have a value from “0, 1, 2, 3, 4, 5, 6, 7, 8, 9, …………. Up to 25”
 Encryption Method C = ( A * P) + B (mod m)
Decryption Method P _=1 A * (C - B) (mod m)
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D. Affine Cipher
 Example: Plaintext: KURDISTAN K1 (A) = 5 & K2 (B) = 8 Ciphertext:
Encryption Method C = (A * P) + B (Mod m)
C = (5 * 10) + 8 (Mod m) = 58 mod 26 = 6 = G
C = (5 * 20) + 8 (Mod m) = 108 mod 26 = 4 = E
C = (5 * 17) + 8 (Mod m) = 93 mod 26 = 15 = P
C = (5 * 3) + 8 (Mod m) = 23 mod 26 = 23 = X
C = (5 * 8) + 8 (Mod m) = 48 mod 26 = 22 = W
C = (5 * 18) + 8 (Mod m) = 98 mod 26 = 20 = U
C = (5 * 19) + 8 (Mod m) = 103 mod 26 = 25 = Z
C = (5 * 0) + 8 (Mod m) = 8 mod 26 = 8 = I
C = (5 * 13) + 8 (Mod m) = 73 mod 26 = 21 = V
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D. Affine Cipher
 Example: Plaintext: GEPXWUZIV K1(A) = 5 & K2(B) = 8 Plaintext:
_1
Decryption Method P = A * (C - B) (Mod m)
P = 21 * (6 - 8) (Mod m) = -42 mod 26 = 10 = K
P = 21 * (4 - 8) (Mod m) = -84 mod 26 = 20 = U
P = 21 * (15 - 8) (Mod m) = 147 mod 26 = 17 = R
P = 21 * (23 - 8) (Mod m) = 315 mod 26 = 3 = D
P = 21 * (22 - 8) (Mod m) = 294 mod 26 = 8 = I
P = 21 * (20 - 8) (Mod m) = 252 mod 26 = 18 = S
P = 21 * (25 - 8) (Mod m) = 357 mod 26 = 19 = T
P = 21 * (8 - 8) (Mod m) = 0 mod 26 = 0 = A
P = 21 * (21 - 8) (Mod m) = 273 mod 26 = 13 = N
-42 mod 26 = -16
= 26 -16
= 10 = K
-84 mod 26 = - 6
= 26 - 6
= 20 = U
147 mod 26 = 17 = R
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2. Polyalphabetic Cipher
Leon Battista invented the Polyalphabetic Substitution Cipher in year 1568. “Poly
means Many in Greek language”. This method uses a mixed alphabet to encrypt a
message. These were thought to be unbreakable for almost 300 years.
 The Enigma machine is more complex but still fundamentally a polyalphabetic
substitution cipher.
Polyalphabetic Cipher Types
A.Vigenère Cipher
B.Beaufort Cipher
C.One Time Pad Cipher
D.Playfair Cipher
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2. Polyalphabetic Cipher Types
A.Vigenère Cipher (Using Code Wheel)
A tool that was used for the Vigenère Cipher was a code wheel. The outer ring of the
wheel represented plaintext letters and the inner wheel represented Ciphertext letters.
 Encryption Method: Go to the plaintext letter in the outer wheel and find its
corresponding Ciphertext letter.
 Encryption Method: Position the keyword letter under “A” but go from the inner
Ciphertext wheel to the outer plaintext wheel.
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A.Vigenère Cipher (Using Code Wheel)
 Example:
Plain Text: KURD
Key = S
Cipher Text:
K = C
U = M
R = J
CMJV D = V
Decrypt this; “ DGNW “ Key = S & Use Code Wheel
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A.Vigenère Cipher (Using Keyword)
 Example1:
Plain Text: “ on a plane the plane is due ” Key = milk
Cipher Text: ?
• Encode plain text.
• Write key under coded plaintext, repeating as many times as necessary.
• Encode key.
• Add key character code to corresponding plain text code (mod 26).
• Decoding of resulting integer is Ciphertext.
• Note: If (Pencoding + Kencoding) >= 26 then - 26
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 Example1: Answer
+
 Note: If (Pencoding + Kencoding) >= 26 then - 26
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 Example2: Ciphertext: “ avlzxiyofppzxiyouaoep ” Key = milk Plaintext: ?
_
0 – 12 = – 12 26 – 12 = 14
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Difficulty of Vigenère cipher
 The longer the keyword, the more difficult it will be for a third person to crack it.
 In fact, for any plain text and any cipher text of the same length, there is a key
that connects them.
 Example:
 consider the plain text: “ enter plain text here ”
 The key connecting them (ignoring spaces) is: “ arlanptewrlahlxan ”
 The cipher text will be: “ eeeee eeeee eeee eeee ”
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B. Beaufort Cipher
The Beaufort cipher, created by Sir Francis Beaufort, The Beaufort cipher is
a Reciprocal Cipher that is, Encryption and Decryption algorithms are the same.
Encryption & Decryption Methods:
 Firstly, choose the plaintext character from the top row of the tableau, call this
column P.
 Secondly, travel down column P to the corresponding keyLetter K.
 Finally, move directly left from the Key letter to the left edge of the tableau, the
Ciphertext encryption of Plaintext P with Key K will be there.
 Example: Plaintext: “ HOW STUFF WORKS ’’ Key = CIPHER
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B. Beaufort Cipher
Example: Answer
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C. One Time Pad Cipher
known as Vernam Cipher is implemented using a random set of non repeating
characters as the Key. The rest of process is same as the Vigenère cipher.
Example1: Plaintext: “ HOW ARE YOU ’’ Key = NCBTZQARX Ciphertext: ?
 Note: If (Pencoding + Kencoding) >= 26 then - 26
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
+
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C. One Time Pad Cipher
Example2: Ciphertext: “ UQXTQUYFR ’’ Key = NCBTZQARX Plaintext: ?
_
16 – 25 = – 9 26 – 9 = 17
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D. Playfair Cipher
The Playfair cipher encrypts pairs of letters, instead of single letters. This is
significantly harder to break. Each plaintext letter is replaced by a diagram in this
cipher.
 User chooses a keyword and puts it in the cells of a 5 x 5 matrix.
 Letter I and J always stay in one cell .
 Duplicate letters appear only once.
 Number of diagrams is 26 x 26 = 676.
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D. Playfair Cipher
Playfair Cipher Rules
 Group plaintext letters two at a time.
 Separate repeating letters with an x.
 Plaintext letters in the same row are replaced by letters to the right.
 Plaintext letters in the same column are replaced by letters below.
 Plaintext letters in different row and column are replaced by the letter in the
row corresponding to the column of the other letter and vice versa.
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D. Playfair Cipher
 Example1: Plaintext: CRYPTO IS TOO EASY Key = INFOSEC Ciphertext: ??
Grouped text: CR YP TO IS TO XO EA SY
Ciphertext: AQ TV YB NI YB YF CB OZ
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D. Playfair Cipher
 Example2: Ciphertext: AQTVYBNIYBYFCBOZ Key = INFOSEC Plaintext: ??
Grouped text: AQ TV YB NI YB YF CB OZ
Plaintext: CR YP TO IS TO XO EA SY
 To Decrypt: The receiver reconstructs the 5 x 5 matrix using the
keyword and then uses the same rules as for encryption.
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Substitution Cipher Types
3. Polygram Cipher
This cipher technique, replaces one block of plain text with a block of cipher text, it does
not work on a character by character basis. This cryptosystem make cryptanalysis
harder by destroying the single character frequencies.
 Example:
 HELLO HELL Could be replaced by “ YUQQW TEUI ”.
 CAUSE OF BECAUSE Could be replaced by “ XAYWQOA MH IGAYK ”
 Despite the characters of the four blocks of text being almost the same.
 Problems in PolyGram Substitution:
Different keys are required for each block of characters. The encryption and decryption
technique is applied to each and every block of character of a text differently: even
when the block is repeated in the plain text. So it requires more time for encryption.
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4. Homophonic Cipher
In this cipher technique, , plaintext letters map to more than one cipher text symbol.
Usually, the highest-frequency plaintext symbols are given more equivalents than lower
frequency letters. In this way, the frequency distribution is flattened, making analysis
more difficult.
 Since more than 26 characters will be required in the cipher text alphabet, various
solutions are employed to invent larger alphabets such as a numeric substitution,
uppercase, lowercase, upside down, and fanciful symbols etc.
 A Mantua Homophonic Cipher (15th century, Roman Empire) is an example to this
type of cipher.
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4. Homophonic Cipher
 Example:
Plaintext: HE EATS THEN SLEEPS AND DREAMS Ciphertext: ??
Key: The following table is the key.
Ciphertext:
55QP69 11JJK 77OG?? 4987PB 99101ZS XXAZBB 58ND43 DD@AA 88
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Transposition Cipher
 Transposition Cipher
Is a method of encryption by which the positions held by units of plaintext are shifted
according to a regular system.
Transposition Cipher Types
1.Rail Fence Cipher.
2.Route Cipher.
3.Simple Columnar Cipher.
4.Keyword Columnar Cipher.
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Transposition Cipher Type
 Transposition Cipher types
1.Rail Fence Cipher
The Rail Fence Cipher involves writing messages so that alternate letters are written
on separate upper and lower lines. The security of the cipher can be improved by
choosing more than two lines to encrypt your message with.
 Example1: Rail fence of depth 2
 To encrypt, write the plaintext in a zigzag pattern in two rows and form the cipher
text by reading off the letters from the first row followed by the second.
 To decrypt, write the cipher text in two rows and read off the plaintext in a zigzag
fashion.
35

1.Rail Fence Cipher
 Example2: Rail fence of depth 3
Plain text: meet me after the toga party
Row 1: m m t h g r
Row 2: e t e f e t e o a a t
Row 3: e a r t p y
Cipher Text: MMTHGRETEFETEOAATEARTPY
Decrypt this: “ IOEUDSALVKRITN “ Key: Rail fence of depth 3
Decrypt this: “ IWTKGAAAEHNSOOMKATD “ Key: Rail fence of depth 3
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2. Route Cipher
In a route cipher, the plaintext is first written out in a grid of given dimensions, then
read off in a pattern given in the key.
 Example1:
Plain Text: meet me after the toga party at seven clock
Key = 5 Rows &
Row 1: m e r o r s c
Row 2: e a t g t e l
Row 3: e f h a y v o
Row 4: t t e p a e c
Row 5: m e t a t n k
Cipher Text: kcolcsroremeetmetatnevetgtaftepayah
(35 letters “ 35 / 5 = 7”)
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2. Route Cipher
 Example 2:
Cipher Text: meetmetaxxytroreaftepagth
Key = 5 Rows &
Plain Text: ?
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2. Route Cipher
 Example 3:
Cipher Text: MTETTEEFEATPHAMETAANKEYGROTVCOERSIC
Key = 7 Columns &
Plain Text: meet me after t?he toga party at seven clock
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3. Simple Columnar Cipher
This cipher technique simply arranges the plain text as a sequence of rows of a
rectangle that are read in columns.
 Example1:
Plain Text: HELLOWORLD Key = 2 Columnar
Column 1 Column 2
H E
L L
O W
O R
L D
Cipher Text: HLOOL ELWRD
40

3. Simple Columnar Cipher
 Example 2:
Plain Text: HELLOWORLD Key = 3 Columnar
Column 1 Column 2 Column 3
H E L
L O W
O R L
D X X
Cipher Text: HLODEORXLWLX
Decrypt this; “ ICVTNTOALEHKODMERASGX “ Key = 3 Columnar
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4. Keyword Columnar Cipher
This cipher technique simply arranges the plain text as a sequence of rows of a
rectangle that are read in columns according to the keyword.
 If the keyword has six letters in it. That means that there are six columns, and
each column must have the same number of letters.
 The columns are ordered alphabetically according to the letters in the keyword.
 Example1:
Plain Text: defend the east wall of the castle Keyword = GERMAN
1st Step 2nd Step
Cipher Text: nalcxehwttdttfseeleedsoaxfeahl
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Transposition Cipher types
4. Keyword Columnar Cipher
 Example 2:
Cipher Text: “ADDSHBGSAROLGNNVCAIISFWDIAOTRNLSAUFRLLWLOENWEHIC”
Keyword = GILLIGAN
Plain Text: ?
1st Step 2nd Step
Plain Text: “ GILLIGANS ISLAND WAS A WONDERFUL TV SHOW FOR CHILDREN ”
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END
END…
Any questions..?
Hope you have been happy
Thank you
By: Msc. Karwan M. Kareem
2014 - 2015
44

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