www.fakengineer.com Seminar on Seminar on (ENCODING AND DECODING TECHNIQUES)
Mar 26, 2015
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Seminar onSeminar 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, 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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