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Mohamed G. Gouda                                        	CS 386S
Spring 2008                                        		Quiz 7
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A computer, whose IP address is d, receives a packet pk whose ultimate 
destination is d. Packet pk has an Authentication Header AH. How many checks
does packet pk need to pass before computer d can accept it (rather than
discard it)? List these checks in the order in which they need to be performed
by computer d assuming that computer d first uses the Security Association
Database (SAD) then uses the Security Policy Database (SPD). (Recall that
the AH of packet pk has four important fields: next header, SPI of the 
security association of pk, sequence number, and the value of a hash.)

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Solution:

Three checks need to be carried out. They are as follows.

i. Use SAD:

1. Use the sequence number in AH and the window and the right edge in
SAD to check that the packet is fresh.

2. Use the hash value in AH and the session key in SAD to check that 
the hash value is consistent with the rest of the packet.

ii. Use SPD:

3. Use the IP header and the TCP header of the packet and SPD to check
that the packet needs to have an AH


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