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Re: replay counter size
Derrell Piper wrote sometime back:
> The latest HMAC AH draft (the one following
Montreal) specifies a 64-bit
> replay field. The latest Combined ESP draft uses
only a 32-bit field.
>
> Jim, was it your intention for these specs to diverge
like this? I
> would like to understand why these fields need to be
different for AH
> and ESP. I would rather see them be the same. I
personally believe that
> 2^32 packets is too much data to encrypt under one
key anyway, so I
> think 32-bits is the right number. But I'm more
concerned that AH and
> ESP be equally protected.
>
> I recall some discussion in Montreal about the
performance of replay
> window checks being dependent on the underlying
hardware register size,
> which supports our desire to make this
implementation dependent. I do
> not recall discussing changing the replay counter
from 32 to 64 bits,
> though I confess to being a bit late for the first
working group meeting,
> due to not being able to get into the room due to
overcrowding.
I don't remember that I saw an explanation about the
question on the
mailing list. To me, different replay counter size for
different AH/ESP
transforms would definitely complicate an IPsec
implementation in terms
of storing and using the per session (SA) replay
checking state - the
largest counter seen so far. I'd appreciate that
someone could enlighten
me on this.
CJ Lee (cj_lee@novell.com)