There should not be a requirement at this level to ensure validity. That would too constrain use cases of implementations of your protocol. It is not difficult to imagine use cases where parties generate chained transactions on top of unconfimed transactions. Although malleability currently makes this difficult to do safely in general, it is not inconceivable that there are circumstances where it would nevertheless be safe or otherwise desireable.

It is a good security recommendation that clients validate the inputs to a shuffle they are participating in. What this means depends on the client -- checking the UTXO set for a full node, making some getutxos queries for a SPV client. But this should remain a recommendation, not a requirement.

On Mon, Aug 11, 2014 at 4:38 AM, Tim Ruffing <tim.ruffing@mmci.uni-saarland.de> wrote:

Hmm, you are right. Lightweight clients are an interesting point, we have to
think about a policy for them.

As you said, the worst case is that the tx will not confirm. So the only
possible attack is DoS. For clients that rely on servers it's reasonable to
trust their servers not to perform DoS. (Anyway, the servers could do worse
attacks.)

For SPV-clients (without servers), I'm not sure at the moment. Something like
getUTXO seems to be a possibility. I think even SPV-clients can verify the
validity of the tx that created the input that is designated for mixing. Then
the only remaining reason why it could be invalid is that the input could have
been spent already otherwise. But in this case, only one honest client with
full information would suffice: a signed transaction that spends the money
would convince even SPV-clients that the participant with this inputs tries to
cheat. This transaction could even be provided by lightweight client that got
if from a server; the transaction is signed by the cheating participant
anyway.

Tim

On Monday 11 August 2014 02:30:16 Chris Pacia wrote:
> Actually getUTXO would probably work here as well. It isn't authenticated
> but it should be good enough for this purpose. The worst that would happen
> is the tx doesn't confirm.
>
> On Aug 11, 2014 2:25 AM, "Chris Pacia" <ctpacia@gmail.com> wrote:
> > One issue I do see is the protocol requires participants to check the
> > inputs submitted by others are valid. Lite clients (at least of the p2p
> > variety) cannot perform this check.
> >
> > You could skip the verification part and if the inputs turn out to be
> > invalid then you'll find out when it doesn't confirm. This would problem
> > open the protocol up to dos attacks and prevent part of the "blame" phase
> > from working properly.
> >
> > Alternatively you can have the participants submit the merkle proof for
> > the input. This would require inputs to have at least one confirmation,
> > however.

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