Re: I-D Action: draft-ietf-bfd-secure-sequence-numbers-08.txt

[Jeffrey Haas <[e-mail]>]

Alan,

On Mon, Jul 26, 2021 at 10:35:01AM -0400, Alan DeKok wrote:
> 
>   That should be possible.
[...]
>   Yes.
[...]
>   Yes.
> 
> > This means that the benefit for the feature would require a function that
> > can be run on a window of packets for predicted inputs and generate the pool
> > of next expected sequence numbers.
> 
>   Yes.
> 
>   I think a cryptographic random number generator here is likely OK.  Those are usually simple, and fast.  The system can be seeded with a strong secret, or maybe hash of a secret and other information.
> 
>   My suggestion to calculate a hash over the packet is that it prevents certain kinds of attacks.  i.e. an attacker could take packet X, and sequence number Y, and put the two together, to spoof / forge state.
> 
>   Fixing that requires that the sequence number is somehow tied to a particular packet.

I don't dispute that these things are possible.

What's being requested is that our specifications have some specificity and
a proposal be made for a suitable mechanism and how it integrates into BFD.
:-)

In any case, you've minimally confirmed that the current text in the
secure-sequence numbers draft is misleading in suggesting a reversible
operation.  The procedural text there thus needs work to incorporate the
points discussed.

-- Jeff