# (ENCODING AND DECODING TECHNIQUES)

## Presentation on theme: "(ENCODING AND DECODING TECHNIQUES)"— Presentation transcript:

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

Introduction What is Cryptography Why we need Cryptography

Some terms associated with Cryptography
Plaintext Ciphertext Encryption Decryption Key

Methods for Encryption/Decryption
Conventional Methods Public key Methods

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

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

Publickey Encryption

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

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

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

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

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)

Thank you

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