CRYPTOGRAPHY

CRYPTOGRAPHY Greek Word - kryptos : hidden - graphein : to write

It is an art or science to transform a

message into non-readable form

CipherThe encoding & decoding

Algorithm

KeyA secret value used by the cipher

Plain TextThe original message

Cipher TextThe encoded message

SYMMETRIC KEY CRYPTOGRAPHY The encryption and decryption keys

are same. It is of the form : P = D(K, E(K,P))

ASYMMETRIC KEY CRYPTOGRAPHY In this method, we have two

different keys : 1. Encryption Key (Public) 2. Decryption Key (Private)

It is of the form : P = D( KD , E(KE ,P))

SYMMETRIC KEY CRYPTOGRAPHY

ASYMMETRIC KEY CRYPTOGRAPHY

An encryption scheme that does not require the use of a

key is called a KEYLESS CIPHER

CIPHERSCipher

Substitution Transposition

Polyalphabetic

Monoalphabetic

SUBSTITUTION CIPHER A substitution cipher replaces one symbol with

another. Monoalphabetic Cipher : The relationship

between a symbol in the plain text to a symbol in the cipher text is always one-to-one.

Polyalphabetic Cipher : The relationship between a symbol in the plain text to a symbol in the cipher text is always one-to-many.

EXAMPLES

- Caesar Cipher- Vigenere Cipher- One Time Pad

CAESAR CIPHER Earliest known Substitution Cipher Invented by Julius Caesar Monoalphabetic If each letter is represented by integers 0,1,...,25

Then, C = ( P + K ) mod 26 P = ( C – K ) mod 26 where the key is K = 3

VIGENERE CIPHER

Simplest Polyalphabetic Cipher The Vigenere Cipher chooses a sequence of keys i.e. Key = K1 K2 K3 ……… Kn When the end of the key sequence is reached, the

key starts over again Ci = ( Pi + Ki%n ) mod 26 Pi = ( Ci – Ki%n ) mod 26

ONE TIME PAD

Also called Vernam Cipher Invented by Vernam Polyalphabetic Cipher The key is chosen at random The length of the key is at least as long as that

of the message, and so it does not repeat. Ci = ( Pi + Ki ) mod 26 Pi = ( Ci – Ki ) mod 26

TRANSPOSITION CIPHER

A transposition cipher does not substitute one symbol for another, instead it changes the location of the symbols.

A transposition cipher reorders symbols

TYPES- Keyless 1. Rail Fence Cipher 2. Columnar Cipher- Keyed- Combination of Keyless & Keyed

KEYLESS

Rail Fence Cipher

Plain Text : Meet me at the park

Cipher Text : MEMATEAKETETHPR

Columnar Cipher

Plain Text : Meet me at the park

Cipher Text : MMTAEEHREAEKTTP

KEYEDPlain Text : Meet me at the park

Key (A permutation Key)

Cipher Text : EMTME TETHA AERKP

MEETM EATTH EPARK

COMBINATION OF KEYED & KEYLESS

DES Data Encryption Standard Developed for the U.S. government in 1976 Accepted as a cryptographic standard both

in the United States and abroad Many hardware and software systems have

been designed with the DES

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