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Authors:
James F. Kurose, Keith W. Ross
Chapter:
Security In Computer Networks
Exercise:
Problems
Question:23 | ISBN:9780132856201 | Edition: 6

Question

Consider the example in Figure 8.28. Suppose Trudy is a woman-in-the- middle, who can insert datagrams into the stream of datagrams going from R1 and R2. As part of a replay attack, Trudy sends a duplicate copy of one of the datagrams sent from R1 to R2. Will R2 decrypt the duplicate datagram and forward it into the branch-office network? If not, describe in detail how R2 detects the duplicate datagram.

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Answer

Consider the following scenario:

  • Trudy is a woman in the middle.
  • In the stream of datagrams going from R1 and R2, she can insert a datagram.
  • Trudy sends one of the datagrams duplicate copy to perform a reply attack.

 

In the above scenario, R2 will not decrypt the datagram. It will not forward the datagram to the branch office network.

 

Reasons:

  • First verification: R2 verifies the sequence number of the datagram packet received. Decryption is done only if the ESP header and sequence number counter value are equal. If they won’t match, the packet will be discarded. This is the.
  • Second verification: R2 verifies the sub field of sequence number in ESP field header. It compares with sequence number sub field of the already received datagram packets. The datagram packet will be discarded if the number matches with the existing packets.

 

Therefore, R2 discards the duplicate packet sent by Trudy.

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