Re: Forward and reply messages

[David Hopwood <david.hopwood@zetnet.co.uk>]

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George Danezis wrote:
> Some additional comments would be:
> 1) One has to be careful about the fact the header length is variable and
> that some nodes know it. It would be nice if the total length of the path
> cannot be deduced (so for example we require a minimum size of header
> padded with junk).

Each node obviously knows the header that is addressed to it, and there is
a limit on the sum of the header lengths, but other than that, no information
is leaked about the lengths of headers for other nodes, or the total path length,
or the position within the path. Minimum header sizes are not required for this.

Note that variable-length headers can be used for any of the approaches to
doing reply blocks.

> 2) If we require the message to be encrypted end-to-end we have to specify
> how, and make sure that it looks like a normal encrypted message.

Yes, but that's straightforward, and should be supported anyway whatever
approach to reply blocks we use (it is essential for the "two payloads"
approach).

> In general, while I find the proposal in itself ok, I am still not
> convinced about dropping the requirement about un-distinguish-ability of
> reply and forward messages. If the objective is to avoid the remaining
> tagging attacks that the swap like approach leave I think it is a
> disproportionate penalty.

I'm still undecided. I've reluctantly come to the conclusion that the
optimal approach to reply blocks depends on the path selection and batching
strategies - I'll explain why in another message.

- -- 
David Hopwood <david.hopwood@zetnet.co.uk>

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