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RE: Q: Why IPSEC to be used only in CBC mode & not other like CFB or OFB ?
>>>>> "Helger" == Helger Lipmaa <helger@cyber.ee> writes:
Helger> The problem with sequence number concatenated with packet
Helger> number used as counters is that some counter space would be
Helger> lost: e.g. if sequence numbers are 32-bit numbers and packets
Helger> are not longer than 2^16 blocks (where a block could be 8, 16
Helger> or 32 bytes) in length, there would be no more than 2^48
Helger> different counters. Of course, that is still better than the
Helger> security of 2^32 offered by the CBC mode. And in this case
Helger> more than 2^48 encrypted blocks should not be sent anyways
Helger> (otherwise ESP counter would zero again).
Any deterministic way of maintaining the counter would have that
property for any SA with replay protection, since you're not allowed
to send more than 2^32 packets before rekeying and no IP packet is
more than 2^16 bytes long. But while that's a *property* of the
system, it doesn't strike me as a *problem*. If you had a counter
construction rule that limited you to fewer distinct counter values
than you might want to use naturally, that would be different -- but
that isn't the case here.
paul
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