Some cryptography questions

From: AIREGG (DONT@SEND.ME)
Date: 03/30/03

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    From: "AIREGG" <DONT@SEND.ME>
Date: Sun, 30 Mar 2003 20:35:37 +0800

    I've just started to study cryptography and network security. Anyone could
    help me to answer these questions:

    1. Given that there is a revocation mechanism, why do certificates need an
    expiration date?

    2. The IPSec architecture document states that when two transport mode SAs
    are bundled to allow both AH and ESP protocols on the same end-to-end flow,
    only one ordering of security protocols seems appropriate: performing the
    ESP protocol before performing the AH protocol. Why is this approach
    recommended rather than authentication before encryption?

    3. In PGP, key IDs are used to distinguish different public keys of a user.
    A key ID is obtained by taking the least significant 64 bits of a public key
    of the user. Using this key ID generation scheme, what is the probability
    that a user with public keys will have at least one duplicate key ID?

    Thanks very much.

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