CS6223 Distributed Systems: Tutorial 10

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CS6223 Distributed Systems Tutorial 10

• Q1. Can we safely adapt the authentication
  protocol shown in the figure in the next slide,
  such that message 3 consists only of RB?
• Q2. Can secret key be used for digit signature?
  List the main features if secret key is used for
  digit signature.
• Q3. Initial exchanges of public keys are
  vulnerable to the man-in-the-middle attack.
  Describe as many ways against it as you can.
• Q4. Does it make sense to restrict the lifetime
  of a session key? If so, give an example how that
  could be established.

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CS6223 Distributed Systems Tutorial 10

• Q1 Mutual authentication in a public-key
  cryptosystem.

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CS6223 Distributed Systems Tutorial 10

• Q5. How are ACLs implemented in a UNIX file
  system?
• Q6. In message 2 of the Needham-Schroeder
  authentication protocol, the ticket is encrypted
  with the secret key shared between Alice and the
  KDC. Is this encryption necessary?
• Q7. Complete the figure in the next slide by
  adding the communication for authentication
  between Alice and Bob.

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0CS6223 Distributed Systems Tutorial 10

• Q7 Authentication in Kerberos.

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Tutorial 10 Q1 Ans.

• Q1 Ans.
• In principle, if RB is never used again, then
  returning it unencrypted should be enough.
  However, such randomness is seldom found.
  Therefore, by encrypting RB, it becomes much more
  difficult for Chuck to break in and forge message
  3.

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Tutorial 10 Q2 Ans.

• Q2 Ans.
• Digit signature can also be implemented using
  secret keys.
• Encryption of a document in a secret key as a
  signature has the following features
• impossible for others to perform without
  knowledge of the secret key
• strong authentication of document
• strong protection against forgery
• weak against repudiation (signer could claim key
  was compromised).

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Tutorial 10 Q3 Q4 Ans.

• Q3. Ans.
• 2.1. Use a private channel for the delivery of
  initial keys, such as a CDROM delivered by hand
  or by some other reliable method.
• 2.2. Include the Domain Name in the certificate
  and deal only with the correct corresponding IP
  address.
• Q4 Ans.
• Session keys should always have a restricted
  lifetime as they are easier to break than other
  types of cryptographic keys. The way to restrict
  their lifetime is to send along the expiration
  time when the key is generated and distributed.

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Tutorial 10 Q5 Q6 Ans.

• Q5 Ans.
• Each file has three associated entries one
  for the owner, one for a group that is associated
  with the file, and one for everyone else. For
  each entry, the access rights can essentially be
  specified as read, write, execute.
• Q6 Ans.
• No. Because Bob is the only one who can
  decrypt the ticket, it might as well have been
  sent as plaintext.

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Tutorial 10 Q7 Ans.

• Q7 Ans.
• Alice sends to Bob the message
• MKB,AS(A,KA,B),KA,B(t ), where KB,AS is
  the secret key shared between Bob and the AS. At
  that point, Bob knows he is talking to Alice. By
  responding with KA,B(t 1), Bob proves to Alice
  that he is indeed Bob.