Plaintext this is the original message or data that

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Plaintext: This is the original message or data that is fed into the algorithm as input 2. Encryption algorithm: The encryption algorithm performs various substitutions and transformations on the plaintext. 3. Secret key: The secret key is input to the encryption algorithm.The exact substitutions and transformations performed by the algorithm depend on the key. 4. Ciphertext: This is the scrambled message produced as output. It depends on the plaintext and the secret key, For a given message. Two different keys will produce two different ciphertexts. 5. Decrytion algorithm: This is essentially the encryption algorithm run in reverse. I t takes the ciphertext and the secret key and produces the original plaintext. There are two requirements for secure use of conventional encryption: 1 We need a strong encryption algorithm. At a minimum, we would like the algorithm to one such that an opponent who knows the algorithm and has access to one or more cipher texts would be unable to decipher the cipher text or figure out the key. 2 Sender and receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure. There are two general approaches to attacking a conventional encryption scheme. The first attack is known as cryptanalysis . Cryptanalysis attacks rely on the nature of the algorithm plus some knowledge of general characteristics of the plaintext or even some sanmple plaintext ciphertext pairs. The second method, known as the brute-force attack , is to try every possible key on a piece of ciphertext until an intelligible translation into plaintext is obtained. On average, half of all possible keys must be tried to achieve success. Figure 18.2 Simplified Model of Conventional Encryption 15.Specify the Message Authentication code.(MAC) (10 marks) Solution Message Authentication Code One authentication technique involves the use of secret key to generate a small block of data, known as a message authentication code. This technique assumes that two communicating parties, say A and B, share a common secret key KAB . When A has a message to send to B, it calculates the message authentication code as a function of the message and the key: MACM=F(KAB,M).The message plus code are transmitted to the intended recipient. The recipient performs the same calculation on the received message, using the same secret key, to generate a new message authentication code. The received code is compared to the calculated code. Figure 18.7 Message Authentication Using a Message Authentication Code (MAC)
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