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Re: Bellovin's and Ashar's attacks
Russ makes the good point that:
>If the attacker has access to a machine behind the firewall, then
>that attacker can simply listen to the plaintext traffic as it is sent from
>that host to the firewall. There is not reason to mount a complex
>replay
>attach -- just listen. I do not want to add a huge amount of complexity >to
>protect against an attacker who can read the traffic before it even gets
>protected. If we want to protect data from other users of the same
>host,
>then the encryption better be applied before it is tranmitted at all. In
>other words, not firewall crypto.
I'd like to extend this argument by an analogy: just as IPSEC between
firewalls cannot protect against reading the clear-text traffic on the networks
inside the firewalls, IPSEC between hosts cannot protect against reading the
clear-text traffic within the hosts. Encryption applied at the network layer
cannot protect against user1 spying on the traffic on user2's socket within the
same host.
Similarly, just as IPSEC between firewalls cannot completely protect against
replay attacks done by hosts, IPSEC between hosts cannot completely protect
against replay attacks mounted by users of those hosts.
My argument here is against user-oriented keying. I argue that if it's
important to authenticate or provide privacy of user1's data versus user2, the
only way to really ensure it is at some layer higher than the network layer.
Exactly because the network layer is a common resource shared by multiple users.