Cipher techniques

Cipher Techniques
Rana Muhammad Saqib

Some Basic Terminology
• plaintext - original message
• cipherText - 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) - study of principles/methods of deciphering
cipherText without knowing key
• cryptology - field of both cryptography and cryptanalysis

Conventional Encryption Principles
• An encryption scheme has five ingredients:
– Plaintext
– Encryption algorithm
– Secret Key
– Ciphertext
– Decryption algorithm
• Security depends on the secrecy of the key, not the secrecy of the
algorithm

Types of Cryptanalytic Attacks
• ciphertext only : only know algorithm / ciphertext, statistical, can
identify plaintext, or worse: the key
• known plaintext:
know/suspect plaintext & ciphertext to attack cipher
• chosen plaintext:
select plaintext and obtain ciphertext to attack cipher
• chosen ciphertext:
select ciphertext and obtain plaintext to attack cipher
• chosen text:
select either plaintext or ciphertext to en/decrypt to attack cipher

Brute Force Search
• always possible to simply try every key
• most basic attack, proportional to key size
• assume either know / recognize plaintext

Characteristics of Cryptographic Techniques
Classified along three independent dimensions:
• – The type of operations used for transforming plaintext to ciphertext
• – The number of keys used
symmetric (single key)
asymmetric (two-keys, or public-key encryption)
• – The way in which the plaintext is processed

Cryptography
• Symmetric
private key / single key
• Asymmetric
public-key / two - key

Symmetric Encryption
• or conventional / private-key / single-key
• sender and recipient share a common key
• all classical encryption algorithms are private-key
• was only type prior to invention of publickey in 1970’s
• and by far most widely used

Requirements
• two requirements for secure use of symmetric encryption:
– a strong encryption algorithm
– a secret key known only to sender / receiver
• mathematically have:
Y = EK(X) [= E(K, X) ]
X = DK(Y) [= D(K, Y) ]
• assume encryption algorithm is known
• implies a secure channel to distribute key

Symmetric cryptography
• Substitution techniques
1. Caesar Cipher
2. Monoalphabetic Cipher
3. Playfair Cipher
4. Vigenère Cipher
5. One time pad (vernam) Cipher
• Transposition Techniques
1. Rail Fence Cipher
2. Row transposition

(1) 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

• 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
0 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
• then have Caesar cipher as:
c = E(p) = (p + k) mod (26)
p = D(c) = (c – k) mod (26)

(2)Monoalphabetic Cipher
• rather than just shifting the alphabet
• could shuffle (jumble) the letters arbitrarily
• each plaintext letter maps to a different random ciphertext letter
• hence key is 26 letters long

(3) 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

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

Encrypting and Decrypting
• plaintext is encrypted two letters at a time
1. if a pair is a repeated letter, insert filler like 'X’
(low probability of appearance in language)
2. if both letters fall in the same row, replace
each with letter to right (wrapping back to start from end)
3. if both letters fall in the same column, replace
each with the letter below it (again wrapping to top from bottom)
4. 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

(5) One time pad (vernam) Cipher
• if a truly random key as long as the message is used, the cipher will
be secure
• called a One-Time pad
• is unbreakable since ciphertext bears no statistical relationship to
the plaintext
• since for any plaintext & any ciphertext there exists a key mapping
one to other
• unconditional security! why look any further??

Example of One time pad (vernam) Cipher

Transposition Ciphers
• Consider classical transposition or permutation ciphers
• these hide the message by rearranging the letter order
• without altering the actual letters used
• can recognise these since have the same frequency distribution as the
original text

(1) Rail Fence cipher
• write message letters out diagonally over a number of rows
• then read off cipher row by row
e.g.: meet me after the toga party

(2) Row Transposition Ciphers
• a more complex scheme
• write letters of message out in rows over a specified number of columns
• then reorder the columns according to some key before reading off the
rows
• If key in alphabetic so ordering ( a to z)
e.g.:
Plaintext: attack is post poned until two am
Key: 4 3 1 2 5 6 7

Thank you
BY RANA MUHAMMAD SAQIB

Cipher techniques

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Cipher techniques