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**(ENCODING AND DECODING TECHNIQUES)**

Seminar on CRYPTOGRAPHY (ENCODING AND DECODING TECHNIQUES)

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Introduction What is Cryptography Why we need Cryptography

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**Some terms associated with Cryptography**

Plaintext Ciphertext Encryption Decryption Key

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**Methods for Encryption/Decryption**

Conventional Methods Public key Methods

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**Conventional methods Character-level encryption Substitutional**

Transpositional Bit-level encryption

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**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

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Publickey Encryption

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**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

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**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

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**Decryption algorithm (RSA)**

Receive C, the ciphertext Calculate plaintext P=Cks modulo N Decode P to the original data

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**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

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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)

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Thank you

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