1. (ENCODING AND DECODING TECHNIQUES)
Seminar on
CRYPTOGRAPHY
(ENCODING AND DECODING TECHNIQUES)

2. Introduction
What is Cryptography
Why we need Cryptography

3. Some terms associated with Cryptography
Plaintext
Ciphertext
Encryption
Decryption
Key

4. Methods for Encryption/Decryption
Conventional Methods
Public key Methods

5. Conventional methods Character-level encryption Substitutional
Transpositional
Bit-level encryption

6. Data encryption standard (DES)
Bit-level encryption
Designed by IBM and adopted by the U.S government for nonmilitary use
Encrypts a 64-bit plaintext using a 56-bit key
The text is put through 19 different procedures to create a 64-bit ciphertext

8. Publickey Encryption

9. RSA Encryption Rivest, Shamir, Adleman encryption
Public key encryption technique
One party uses a public key, kp
Other party uses a private key, ks
Both use a number, N

11. Encryption algorithm (RSA)
Encode the data to be encrypted as a number to create the plaintext P
Calculate the ciphertext C=Pkp modulo N
Send C as ciphertext

12. Decryption algorithm (RSA)
Receive C, the ciphertext
Calculate plaintext P=Cks modulo N
Decode P to the original data

13. Example of RSA Let Kp=5, Ks=77, and N=119
Let the character F is encoded as 6 (sixth char in alphabet)
We calculate 6Kp modulo 119 =41
At the receiver side, we calculate 41Ks modulo 119=6
Decode 6 as F

14. Choosing Kp, Ks and N
First choose 2 prime numbers p and q (we choose 7 and 17)
Calculate N=p*q (N=7*17=119)
Select Kp such that it is not a factor of (p-1)*(q-1) (we choose 5, not factor of 96)
Select Ks such that (Kp*Ks) modulo (p-1)*(q-1)=1 (4*96+1=385 and 385/5=77)

15. Thank you