[taler-docs] branch master updated: working on DD13

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grothoff pushed a commit to branch master
in repository docs.

The following commit(s) were added to refs/heads/master by this push:
     new 2746d09  working on DD13
     new 771e46d  Merge branch 'master' of git+ssh://git.taler.net/docs
2746d09 is described below

commit 2746d09ef06cc06581f5448727ed8bd8d47706ed
Author: Christian Grothoff <christian@grothoff.org>
AuthorDate: Wed Jan 20 22:19:11 2021 +0100

    working on DD13
---
 design-documents/013-peer-to-peer-payments.rst | 154 +++++++++++++++++++++++--
 1 file changed, 145 insertions(+), 9 deletions(-)

diff --git a/design-documents/013-peer-to-peer-payments.rst 
b/design-documents/013-peer-to-peer-payments.rst
index c5e529d..8921644 100644
--- a/design-documents/013-peer-to-peer-payments.rst
+++ b/design-documents/013-peer-to-peer-payments.rst
@@ -11,8 +11,8 @@ Motivation
 ==========
 
 To be usable as an electronic payment system with cash-like properties,
-customers should be able to transfer money between themselves without any
-external provider.
+customers should be able to transfer money between themselves without
+needing to setup anything beyond their wallet(s).
 
 
 Requirements
@@ -23,15 +23,109 @@ Requirements
   direct communication channel between payer and payee should be required.
 * This customer-to-customer payment must be possible without trusting the other
   party beyond the point where the money has been received by the payee.  Thus,
-  sharing of coin private keys is not sufficient.
+  sharing of coin private keys is not sufficient, we need transactional 
semantics
+  resulting in exclusive control over the funds by the recipient.
 * The P2P payment protocol must not allow users to circumvent income
   transparency.  That is, each P2P transaction must be visible
   on a KYCed transaction ledger (such as a bank account).
 * The money received via a P2P payment must be usable for
   further Taler payments with minimal delay.
 
-Proposed Solution
-=================
+Proposed Solution (CG)
+======================
+
+.. note::
+
+   The notion of ``pouch`` used in this proposal differs completely from
+   the notion of ``pouch`` in the Dold approach.  In this proposal,
+   pouches are used to represent aggregated wire transfers combining
+   multiple p2p payments between pairs of exchanges.  WE NEED A BETTER
+   WORD.
+
+A complete solution for customer-to-customer payments consists of three
+separate parts:
+
+1. The payee generates a ``payto://taler/$EXCHANGE_BASE_URL/$RESERVE_PUB``
+   target address for which the payee knows the corresponding reserve
+   private key.  The parties must also create a fixed-form contract.
+   The the contract must specify the amount, a nonce, the reserve public key,
+   and may include further a **forgettable** details like a subject.
+   The required information should still be sufficiently compact for a
+   small QR code or NFC transmission.
+2. The payer then performs an ordinary ``/deposit`` operation.
+3. The exchange detects the use of the ``taler`` wire method, and if the base
+   URL of the target exchange differs, initiates a wire transfer to the
+   ``/wire`` endpoint of the target exchange.  For this wire transfer,
+   the wire transfer subject is set to provide the exchange base URL and
+   a WTID as with other outgoing wire transfers. Aggregation is possible.
+   An inbound wire transfer of this type creates a **pouch**.
+4. If the (aggregated) wire transfer fails (say the
+   ``/wire`` endpoint does not resolve to a valid bank account, or
+   the target exchange closed the pouch), the originating exchange
+   automatically creates a full refund for the deposit (refund fees still
+   apply).
+
+   .. note::
+
+      While the refund **fee** can be reused, these types of refunds
+      are not approved by a merchant's signature. Thus, we will need
+      a new message type in the coin history to represent these events.
+
+5. When the exchange is requested to provide information about
+   aggregated transfers under the WTID, it provides the contract's
+   512-bit hash and the amount.
+6. The exchange wire gateway is extended with a request to check the KYC
+   status of a customer based on a RESERVE_PUB.  Possible replies are
+   in-progress, failed and succeeded.  Failed KYCs ultimately result in
+   pouches being closed.  An in-progress status should be accompanied with
+   information how the customer may complete the KYC check.
+
+   .. note::
+
+      If a target exchange closes a pouch, the closing fee
+      applies for every non-merged contract hash associated with the pouch.
+      Closing a pouch requires the target exchange to perform
+      a wire transfer for each (non-merged) contract hash, using
+      a truncated 256-bit contract hash for the wire transfer subject.
+      That way, the originating exchange can refund the respective
+      coins.
+
+7. A new exchange endpoint allows wallets to request a KYC for a
+   RESERVE_PUB.  Such a request may include the requirement to pay
+   a KYC fee.  The KYC fee may be charged to that reserve (if it exists),
+   or could be waved if the reserve was established via a wire transfer
+   from a partner bank.
+8. When an exchange receives a wire transfer with an exchange base URL
+   and a WTID (instead of a RESERVE_PUB), it performs a request to the
+   ``/transfers/$WTID`` API of the originating exchange to obtain a
+   list of contract hashes.
+9. The payee can try to merge incoming payments into a KYC'ed reserve by
+   POSTing to a new ``/reserve/$RESERVE_PUB/merge`` endpoint.
+   The POSTing must include a signature by the reserve private key
+   over the 512-bit contract hash and the (maximally ``forgotten``)
+   contract (so only nonce, amount and RESERVE_PUB are in the clear).
+   By doing so, the receiver demonstrates knowledge of the full contract
+   (the ``forgotten`` fields provide privacy for details irrelevant to
+   the exchange).
+
+   .. note::
+
+      A possible optimization is to forgo including the reserve public key
+      field in the transmitted exchange contract, as it is already in the URL;
+      however, the RESERVE_PUB MUST be in a well-defined
+      field of the contract when hashing the contract.
+
+10. If the exchange has received a matching inbound wire transfer, it adds the
+   respective balance to the reserve's balance, allowing the KYC'ed customer
+   to withdraw the funds.  The new ``merge`` endpoint should also allow
+   long-polling. Note that long-polling should be limited to short durations,
+   as inbound transfers via ``taler-exchange-wirewatch`` cannot cause the long
+   polling to be resumed, only transfers within the same exchange can benefit
+   from long polling acceleration.
+
+
+Proposed Solution (Dold)
+========================
 
 A complete solution for customer-to-customer payments consists of three
 separate parts:
@@ -39,8 +133,8 @@ separate parts:
 1. The mechanism whereby coins are deposited into some address
    associated with the receiver.
 2. A mechanism to ensure that the receiver of a customer-to-customer
-   payment must have undergone a KYC check.
-3. If more than once exchange should be supported, there must be a real-time
+   payment has undergone a KYC check.
+3. If more than once exchange is involved, there must be a real-time
    settlement layer between involved exchanges.
 
 
@@ -122,9 +216,51 @@ Alternatives
      to transfer via a QR code or short text message.
 
 
-Drawbacks
-=========
+Drawbacks (Dold)
+================
 
+* pouches introduce new 'pouch' loophole, which could allow a customer
+  to collect 'income' into a pouch and then 'pay' a merchant by allowing
+  the merchant to empty the pouches.
 * The exchange needs to be extended with customer-to-customer pouches (probably
   the easy part) and bound reserves (possibly falls into the responsibility of
   LibEuFin).
+* error handling (failure in exchange-to-exchange wire transfer,
+  failure to drain reserve, etc) is unclear from the proposal. Can money get
+  lost in the case of failures (bad keys, failed KYC)?
+
+
+
+Drawbacks (Grothoff)
+====================
+
+The overall changes required are not small:
+
+* New KYC fee required in ``/keys`` endpoint (similar to closing and wire 
fees),
+  requires extensive work across toolchain (offline signature, etc.)
+* New ``taler`` wire method needs special case to possibly bypass
+  (same exchange scenario, with long-poll trigger) the usual aggregation logic.
+* New exchange table(s) required to store inbound amounts by contract hash.
+  Maybe two tables, one for local exchange p2p and one for remote exchange p2p
+  payments.
+* New exchange table for pouches required (for remote p2p payments).
+* New exchange logic required to make ``transfers`` requests for pouches
+  (another separate process).
+* New ``/reserve/$RESERVE_PUB/kyc`` endpoint required.
+* New ``/reserve/$RESERVE_PUB/merge`` endpoint required.
+* New type of reserve history entries required (``merge``).
+* Additional tables to be verified by the auditor.
+* ``taler-exchange-closer`` needs additional logic to ``close`` pouches.
+* ``taler-exchange-wirewatch`` needs to support receiving pouch closures
+  and exchange-to-exchange wire transfers with WTIDs.
+* A new type of ``refund`` (better name needed) is required to handle
+  coins re-gaining value from closed pouches.
+
+Aside from implementation complexity, the solution has the following drawbacks:
+
+* We have no good way to determine that a p2p payment failed and that the
+  wallets should try to refresh the spent coin to get the ``refund``.
+  Wallets will have to basically 'probe' for a plausible time period to
+  recover funds from failed p2p payments.  Alternatively, we may create a
+  special URI that the payee wallet will generate to inform the payer
+  about failed p2p payments. Alas, that seems to not be very user-friendly.

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