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Predictable SPIs (was: Re: A pothole in ISAKMP/Oakley)
> From: Stephen Kent <kent@bbn.com>
>
> If one selects SPIs to be small integers so that they represent
> indices into local tables, then they may be very predictable and thus an
> attacker without passive wiretapping ability may be able to formulate
> credible-loooking SPIs for existing SAs. Remember, the SPI plus
> destination address completely defines the SA, so there is no source
> address check that would be pereformed at this stage of packet processing.
If one believes that inject-only attackers are a threat, then it is
possible to get both unpredictable (to the attacker) SPIs and efficient
SPI lookup at the destination by picking a random offset once at boot
time and adding it to small integer indices to form the SPIs.
Since SPI selection does not affect interoperability, since DOS in
general is a hard problem, and since entropy testing of SPI sequences
for conformance testing purposes is impossible, it seems unwise to write
a MUST requirement on SPI generation. I'd write a note on the rationale
for unpredictable SPIs in the Security Considerations section and leave
it at that.
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