JNTU
JNTU B.Tech IV NETWORK SECURITY AND CRYPTOGRAPHY Previous Question Paper (July - 2021 - R16)
Answer any Five Questions. All Questions Carry Equal Marks.
Main
Explain the types of attacks on data being transmitted over an insecure channel? Also describe the services on which these attacks are performed.
Answer
Attacks on data transmitted over an insecure channel are generally classified into two categories: Passive attacks and Active attacks.
Passive attacks involve monitoring transmissions to obtain information without altering the data. These include Release of message contents (e.g., eavesdropping on sensitive data) and Traffic analysis (observing the frequency and length of messages to infer patterns even if the content is encrypted).
Active attacks involve modifying the data stream or creating a false stream. These include Masquerade (pretending to be an authorized entity), Replay (capturing data and retransmitting it to produce an unauthorized effect), Modification of messages (altering, delaying, or reordering parts of a message), and Denial of service (disrupting or preventing the normal use of communication facilities).
These attacks target fundamental Security Services, which include Confidentiality (protecting data from unauthorized disclosure), Authentication (verifying the identity of the sender), Integrity (ensuring data has not been altered), Non-repudiation (preventing a sender from denying a sent message), and Access Control (limiting access to resources).
Alice has a long message to send. She is using mono alphabetic substitution cipher. She thinks that if she compresses the message it may protect the text from single letter frequency attack by Eve. Does the compression help? Should she compress the message before the encryption or after the encryption? Defend.
Answer
Compression does not effectively protect against a frequency attack. While it changes the frequency distribution of characters, it does not eliminate the underlying statistical patterns inherent in the language. A sophisticated attacker can still perform frequency analysis on the compressed output, as the mapping remains mono-alphabetic.
Alice should compress the message before encryption. Compression reduces redundancy in the plaintext, which makes the subsequent encryption more robust by removing predictable patterns that cryptanalysts exploit. If she compresses after encryption, the compression algorithm will fail because high-quality encryption produces ciphertext that appears random and possesses maximum entropy, leaving no patterns for the compression algorithm to reduce.
How is steganography different from Cryptography? Will it result in better security combining the two? If so, how? If not, why?
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Differentiate IDEA and DES algorithm.
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List the characteristics of modern block cipher algorithms.
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With the linear congruential algorithm, a choice of parameters that provides a full period does not necessarily provide a good randomization. For example, consider the following two generators:
Write out the two sequences to show that both are full period. Which one appears more random to you?
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Why is gcd(n, n + 1) = 1 for two consecutive integers n and n + 1?
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If n is composite and passes the Miller-Rabin test for the base a, then n is called a strong pseudo prime to the base a. Show that 2047 is a strong pseudo prime to the base 2.
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In a public-key system using RSA, you intercept the ciphertext C = 10 sent to a user whose public key is e = 5, n = 35. What is the plaintext M?
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In what ways can a hash value be secured so as to provide message authentication?
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List two disputes that can arise in the context of message authentication. In what order should the signature function and the confidentiality function be applied to a message, and why?
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What drawbacks of Kerberos version 4 are addressed in Kerberos Version 5? Explain how?
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Is Oakley protocol vulnerable to clogging attack? If so, how? If not, why?
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Explain in detail an open encryption and security specification designed to protect credit card transactions on the internet?
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What are two default policies that can be taken in a packet filter if there is no match to any rule? Which is more conservative? Explain with example rule sets both the policies?
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Explain in detail virus structure? Explain how a compression virus propagates?
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