Chapter 7
Security in computer networks
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杨菁
1. What is the difference between an active and a passive intruder?
Answer: A passive intruder can listen to and record the
control and data messages on the channel; an active
intruder can remove messages from the channel and/or add
messages into the channel.
2. What is an important difference between a symmetric key system
and a public key system?
Answer: In the symmetric key systems, the sender’s and the
receiver’s keys are identical and secret. But in the public key
systems, a pair of keys is used. One key is known to the
public, while the other is known only by either the sender or
the receiver.
3. Suppose that an intruder has an encrypted messages as well as the
decrypted version of that message. Can the intruder mount a
cipher-text only attack, a known-plaintext, or a chosen-plaintext
attack?
Answer: The intruder can mount a known-plaintext attack.
4. What is the purpose of a nonce in an authentication protocol?
Answer: The nonce is used to ensure that the sender is
“live”.
5. What does it mean for a signed document to be verifiable,
nonforgeable, and nonrepudiable?
Answer: That is, it must be possible to “prove” that a document
signed by an individual was indeed signed by that individual (the
signature must be verifiable) and that only that individual could
have signed the document (the signature cannot be forged, and a
signer cannot later repudiate or deny having signed the document.)