Criptography

1. Department of Computer Application
Kalinga Institute of Industrial Technology,
Bhubneshwar
By:
Amrita Mohapatra
Reg. No.: 0305203034
E-mail : amrita_19832003@yahoo.co.in

2.  How the concept of cryptography arises
 Technical Meaning

3. Plain text
Cipher text
Encryption Algorithm
Decryption Algorithm

4. Mathematically
C = E (M ,K )
M = D (M , K )

5. 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.

6. Shift Ciphers
Caesar Cipher  Shift Cipher
Pick 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

7. Cryptanalysis of shift Ciphers
Pivot

8. Substitution CCiipphheerr ((TTaakkiinngg ppeerrmmuuttaattiioonn ooff
lleetttteerrss ))
SSeeccrreett MMeessssaaggee :: --
TTKK IILL KKQQ JJKKTT TTKK IILL TTBBSSTT CCRR TTBBLL OOUULLRRTTCCKKJJ
OOrriiggiinnaall MMeessssaaggee ::--
TTOO BBEE OORR NNOOTT TTOO BBEE TTHHAATT IISS TTHHEE QQUUEESSTTIIOONN

9. TRANSPOSITION TECHNIQUE
Transposition ciphers are rarely encountered nowadays. They
differ from both code systems and substitution ciphers; in a
transposition cipher the letters of the plaintext are shifted
about to form the cryptogram.

10. Types of cryptography
• symmetric key cryptography
•Asymmetric key cryptography

11. Symmetric Key Cryptography
In 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)

12. Asymmetric Key Cryptography
In 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).

13. Cryptographic services
• User Authentication

14. Data authentication
• Data integrity
Data origin authentication

15. Non-repudiation of origin
Data confidentiality

16. Cryptography Today
DES 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
military communications.
With the help of digital signature, digital certificate (not
discussed due to limitation of the scope) the authenticity
and truth of a particular digital piece is well defined.
Hash functions and Message Authentication Codes are
there to make sure that what message goes is not
modified.
In case of money transaction in Banks through network
cryptography is a must, otherwise their would be a havoc
in the economy.