Computer Science/ Cryptology Please Label Each Answers Alphabeticaly A. Explain
ID: 3828565 • Letter: C
Question
Computer Science/ Cryptology
Please Label Each Answers Alphabeticaly
A. Explain precisely how and where Alice is authenticated. What prevents a replay attack? B. If Trudy is a passive attacker (i.e., she can only observe messages), she cannot determine the key K Why? C.Show that if Trudy is an active attacker (i.e., she can actively send messages and she can impersonate Bob, then she can determine the key K that Alice uses in the last message. Explain why this does not break the protocol. d. What is the purpose of the encrypting the final message with the key K?Explanation / Answer
Answer
1. Bob and Alice are said to be autheticated when they send their signatures (Sa and Sb) to eachother. As, the signatures are intended to provide or provide mutual authentication
The nonce RA is Alice's challenge to Bob and Sb is Bob's answer. That is, the nonce RA provides protection to a replication attack so only Bob can give the correct answer, since a signature is required (assuming, of course, that his private key has not been compromised.
2. When using a rlogin command for a remote login, ie to log on to a remote machine over a network. A login usually requires a password and rlogin simply sends the password, passwords that could be observed by a Trudy. But by setting up an SSH session first, any inherently insecure command like rlogin will be safe. That is, an SSH session provides confidentiality and integrity protection, thereby eliminating Trudy's ability to obtain passwords or K and other sensitive information that would otherwise be sent without protection.
3. In this attack, Trudyintercepts Alice's public value and sends her own public value to Bob. When Bob transmits his public value, Trudy substitutes it with her own and sends it to Alice. Trudy and Alice thus agree on one shared key and Trudy and Bob agree on another shared key. After this exchange, Trudy simply decrypts any messages sent out by Alice or Bob, and then reads and possibly modifies them before re-encrypting with the appropriate key and transmitting them to the other party. This vulnerability is present because key exchange does not authenticate the participants. The best way to protect this is to include the use of digital signatures and other protocol variants.
4. To provide the authentication of Alice in such a way that only Alice can give the correct answers to bob's Rb. Hence, it also helps in preventing replication attacks.
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