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Re: [Taler] transaction state/history; fulfillment

From: Florian Dold
Subject: Re: [Taler] transaction state/history; fulfillment
Date: Thu, 7 Jan 2016 19:58:51 +0100
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On 01/07/2016 04:05 PM, Christian Grothoff wrote:
> Well, but the protocol says that the merchant replies with a "200
> OK" if (and only if) the payment was successful.  Oh, and there is
> a reason why there is no signed message here: we don't have a way
> to reasonably get a public key from the merchant (beyond TLS /
> X.509), and sigs by just any random key are somewhat useless.  We
> could introduce it, as the contract contains the merchant's public
> key (but that's for refunds and thus different). But legally it
> buys us nothing as the merchant can always say: "that was not my
> public key, you made this up".

No, I strongly disagree with you on that point.  Your argument might
work for the "immediate fulfillment" case that we have with NFC tokens
and online news articles.  But it does not work in other scenarios,
where we would also like to use Taler.

Let's say Alice needs a new $GADGET quickly, so she buys it online
using Taler; the contract mentions that the merchant will deliver the
$GADGET within 24 hours to Alice.

24h later, Alice's $GADGET still hasn't arrived.  She calls the
merchant, who tells her "Oh, your payment didn't go through.
Unfortunately $GADGET was on a limited offer, but we can offer you
$EXPENSIVE_ALTERNATIVE, which will arrive in 24 hours if you order now!"

Alice doesn't have a way to prove that the merchant received her coins
and pledged to fulfill the contract.

If the protocol would have included a fulfillment signature, she could
use it against the merchant, who might have to pay her damages for the
late/missed delivery of $GADGET.  When she pays but the merchant
doesn't send her the fulfillment signature, Alice can always escalate
this and/or go shopping somewhere else, so that she receives her
$GADGET in time.

There might be an alternative to handle this case (without fulfillment
signatures), but we definitely need to consider it!

IANAL, but AFAIK with e.g. German law, the "contract" would just count
as a "invitatio ad offerendum" [1, 2nd. bullet point "Einzelfaelle"].
 Now you could say that Taler merchant contracts are legally binding
offers (with an expiration date), but do (a) merchants really want the
risk associated with this and (b) wouldn't the customer still need
assurance that the merchant received the contract in time?

Having fulfillment signatures is IMHO beneficial to the customer (who
as more assurances / can react more timely) and to the merchant (since
contracts are less binding).  This model also seems to map better to
existing contract law, though I might be wrong on this one.

I agree with the other points you made, we've discussed them offline

- - Florian

[1] https://de.wikipedia.org/wiki/Aufforderung_zur_Abgabe_eines_Angebots
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