Department of Computer Application

Kalinga Institute of Industrial Technology, Bhubneshwar

By: Amrita Mohapatra Reg. No.: 0305203034 E-mail : amrita_19832003@yahoo.co.in

How the concept of cryptography arises

Technical Meaning

Plain text

Cipher text

Encryption Algorithm

Decryption Algorithm

Mathematically

C = E (M ,K )

M = D (M , K )

Traditional methods of cryptography

Substitution Technique

Caesar Cipher

Plain Text M consists of the letter A, B… Z.When computing a cipher text “add 3” modulo 26 to each letter.i.e. A->D, B->E… X->A, Y->B, Z->C.Do it for every letter.e.g. CAESAR -> FDHVDU.The security remains if you hide the fact that you are applying Caesar cipher to your message.

Shift Ciphers

Caesar Cipher Shift CipherPick a secret key K from 0-25.Add K modulo 26 to all letters.C = M+K modulo 26

For Example :- M = HAL and K = 1 we increment each character of HAL by one and find C = IBM

Cryptanalysis of shift Ciphers

Pivot

Substitut ion Cipher Substitut ion Cipher (Taking permutation of (Taking permutation of letters )letters )

Secret Message : - Secret Message : - TK IL KQ JKT TK IL TBST CR TBL OULRTCKJ TK IL KQ JKT TK IL TBST CR TBL OULRTCKJ

Original Message :- Original Message :- TO BE OR NOT TO BE THAT IS THE QUESTION TO BE OR NOT TO BE THAT IS THE QUESTION

TRANSPOSITION TECHNIQUE Transposit ion ciphers are rarely encountered nowadays. They dif fer from both code systems and substitut ion ciphers; in a transposit ion cipher the letters of the plaintext are shifted

about to form the cryptogram.

Types of cryptography

• symmetric key cryptography

•Asymmetric key cryptography

Symmetric Key CryptographyIn this kind of encryption the key used for encryption and the key used for decryption are the same.It is also called as secret key cryptography.i.e. Alice and Bob share a key and use that both for encryption and decryption.We can think this as Alice and Bob together by a lock which has two keys of similar kind within it . Alice and Bob both can lock and unlock the message as they have the same key.Examples: DES, 3-DES, RC4, RC5 etc. (these are the cryptographic algorithms currently in use)

Asymmetric Key CryptographyIn this kind of encryption technique the key used for encryption and the key used for

decryption are different.This is also called as Public Key cryptography.

Each user has two keys – one public key, which is revealed to all users, and one private key, which remains a secret. The private key and the public key are mathematically linked.

Encryption is performed with the public key and decryption is performed with the private key.

It is something like Alice wants to receive a message from Bob, and then she will give her private key to Bob. Bob can lock the message with the private key and send to Alice. No one other than Alice can unlock the message as unlocking needs private key and only

Alice will be knowing her private key. (Let us forget about the detail for now as they are big mathematical calculations).

Cryptographic services

• User Authentication

Data authentication• Data integrity

Data origin authentication

Non-repudiation of origin

Data confidentiality

Cryptography TodayDES algorithm (a symmetric key) is a famous algorithm and widely used. But it was proved that it can be broken in just 24 hours with the help of a super computer and a huge number of computers working in parallel.That is why public key cryptography came into picture and RSA algorithm was designed which is widely used now-a-days.Cryptography is mostly used in network security and secret mil i tary communications.With the help of digital signature, digital cert if icate (not discussed due to l imitat ion of the scope) the authenticity and truth of a part icular digital piece is well defined.Hash functions and Message Authentication Codes are there to make sure that what message goes is not modif ied.In case of money transaction in Banks through network cryptography is a must, otherwise their would be a havoc in the economy.