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Published byRebecca Jacobs Modified over 2 years ago

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Seminar on (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, k p Other party uses a private key, k s 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=P k p modulo N Send C as ciphertext

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

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

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

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