learn.mq.edu.au You have been given the job of designing a secure tape backup sy
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learn.mq.edu.au You have been given the job of designing a secure tape backup system for a bank, to use AES 256 symmetric encryption. Realising that much of the data to be written to tape wil contain repeating sequences of bits, you reject ECB mode as inappropriate and settle on CBC mode. Each tape block is 1024 bits in size, so will contain eight 128-bit blocks of ciphertext * a. (3 marks) Suggest a scheme for generating an initialization vector for each tape block. * b. (2 marks) What will be the effect on each of the ciphertext blocks in a tape block if there is a 1-bit read error in the first ciphertext block? Question 14. 5 marks Windows NT domains authenticated users by passing a keyed hash of a known constant value with the user's password as the key, across the network.Explanation / Answer
A cryptographic algorithm, or cipher, is the mathematical function used for encryption/decryption. If the security of an algorithm is based on keeping it secret, it is a restricted cipher. Restricted ciphers are historically interesting but not adequate today. With a changing user community, all’s lost if thee wrong party discovers the cipher. Moreover, there is no ability to have quality control on the algorithm since it must be kept hidden. Far more preferable are ciphers that rely on a publicly-known algorithm that accepts a secret parameter, or key, for encryption and decryption. If the encryption and decryption keys are the same (or mathematically derivable from each other), the algorithm is known as a symmetric algorithm (DES is an example): C = EK(M) M = DK(C) If the key used for encryption is different from the key used for decryption, then the algorithm is a public-key algorithm (RSA is an example). The decryption key cannot be Security: Cryptographic communication and authentication Rutgers University – CS 417: Distributed Systems ©1997-2009 Paul Krzyzanowski, All Rights Reserved 2 calculated from the encryption key in a reasonable amount of time (and vice versa). The reason it is called a public-key algorithm is because the encryption key can be made public. A stranger can thus encrypt a message with this public key but only the holder of the decryption key (private key) can decrypt the message. A message can also be encrypted with the private key and decrypted with the public key.
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