1. Cryptography and Network Security
CLASSICAL ENCRYPTION TECHNIQUES

2. Classical Substitution Ciphers
letters of plaintext are replaced by other letters or by numbers or symbols Or
if plaintext is viewed as a sequence of bits, then substitution involves replacing plaintext bit patterns with ciphertext bit patterns
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3. Caesar Cipher earliest known substitution cipher by Julius Caesar
first attested use in military affairs
replaces each letter by 3rd letter after
example:
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB
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4. Caesar Cipher can define transformation as:
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
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
mathematically give each letter a number
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
then have Caesar cipher as:
c = E(p) = (p + k) mod (26)
p = D(c) = (c – k) mod (26)
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5. Cryptanalysis of Caesar Cipher
• only have 26 possible ciphers
A maps to A,B...Z
• could simply try each in turn
• a brute force search
• given ciphertext, just try all shifts of letters
• do need to recognize when have plaintext
• eg.. break ciphertext "GCUA VQ DTGCM"
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6. Monoalphabetic Cipher
rather than just shifting the alphabet just shuffle (jumble) the letters arbitrarily
each plaintext letter maps to a different random ciphertext letter
hence key is 26 letters long
Plain: abcdefghijklmnopqrstuvwxyz
Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN
Plaintext: ifwewishtoreplaceletters
Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA
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7. Monoalphabetic Cipher Security
now we have a total of
26! = 4 x 10^26 keys
• with so many keys, one might think it is secure
• problem is with the language characteristics
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8. English Letter Frequencies
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9. Playfair Cipher
not even the large number of keys in a monoalphabetic cipher provides security
one approach to improving security was to encrypt multiple letters
the Playfair Cipher is an example
invented by Charles Wheatstone in 1854, but named after his friend Baron Playfair
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10. Playfair Key Matrix a 5X5 matrix of letters based on a keyword
fill in letters of keyword (minus duplicates)
fill rest of matrix with other letters
eg. using the keyword MONARCHY M O N A R C H Y B D E F G
I/J K L P Q S T U V W X Z
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11. Encrypting and Decrypting
plaintext is encrypted two letters at a time
if a pair is a repeated letter, insert filler like 'X’ (low probability of appearance in language)
if both letters fall in the same row, replace each with letter to right (wrapping back to start from end)
if both letters fall in the same column, replace each with the letter below it (again wrapping to top from bottom)
otherwise each letter is replaced by the letter in the same row and in the column of the other letter of the pair
Wireless Wi re le sx sz XG MK UL XA XT
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12. Security of Playfair Cipher
• security much improved over monoalphabetic since have 26 x 26 = 676 digrams
• would need a 676 entry frequency table to analyze (verses 26 for a monoalphabetic) and correspondingly more ciphertext
• was widely used for many years by US & British military in WW1
• it can be broken, given a few hundred letters since still it has much of plaintext structure
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13. Hill Cipher Developed by Lester Hill in 1929
The encryption algorithm takes m successive plaintext letters and substitutes for them m ciphertext letters
The substitution is determined by m linear equations and is assigned a numerical value (a=0, b=1,…..,z=25) or . Or
C=PK mod 26 and P= CK^-1 mod 26
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14. Hill Cipher
C and P are row vectors of length 3 representing the plaintext and ciphertext.K is a 3 X 3 matrix representing the encryption key. Operations are performed on mod 26.
For example, consider the plaintext: paymoremoney and
The first three letters of the plaintext are represented by the vector ( ). Then( )K = ( ) mod 26 = ( ) = RRL. Continuing in this fashion, the ciphertext for the entire plaintext is RRLMWBKASPDH. .
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15. Polyalphabetic Ciphers
A set of related monoalphabetic substitution rules is used
use a key to select which alphabet is used for each letter of the message
use each alphabet in turn
repeat from start after end of key is reached
make cryptanalysis harder with more alphabets to guess and flatter frequency distribution
Key: deceptive
plaintext: wireless
ciphertext: zmtiaxao
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16. Vigenère Cipher
In this scheme, the set of related monoalphabetic substitution rules consists of the 26 Caesar ciphers with shifts of 0 through 25.
Each cipher is denoted by a key letter, which is the ciphertext letter that substitutes for the plaintext letter a.
Thus, a Caesar cipher with a shift of 3 is denoted by the key value 3
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17. Example of Vigenère Cipher
Let the key be “deceptive”
Plaintext be “we are discovered save yourself”
key: deceptivedeceptivedeceptive
plaintext: wearediscoveredsaveyourself
ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ
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18. When expressed numerically
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19. Vernam Cipher and One-time Pad
Introduced by an AT&T engineer named Gilbert Vernam in 1918.
The scheme uses Keyword as long as the plaintext and has no statistical relationship to it.
Vernam system works on binary data with ith bit of text exclusive ored with ith bit of key to produce ith bit of cipher
In one one-time pad key is used only once
This scheme is unbreakable
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20. Vernam Cipher Ci = Pi XOR Ki and Pi = Ci XOR Ki
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21. Transposition Cipher
Mapping is performed by some sort of permutation on the plaintext letters.
Example: Rail fence of depth 2
text : meet me after the toga party
m e m a t r h t g p r y
e t e f e t e o a t
cipher: MEMATRHTGPRYETEFETEOAAT
Rail fence of depth 2
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22. Summary Caesar Cipher Monoalphabetic Cipher Playfair Cipher
Polyphabetic Cipher
Vigenère Cipher
Vernam Cipher and One-time Pad
Transposition Cipher
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23. Reference
William Stallings, Cryptography and Network Security, 6th Edition, Pearson Education, March 2013
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24. THANK YOU
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