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Design Doc 013: Wallet-to-Wallet Payments
#########################################

Summary
=======

This design document proposes an extension of the Taler protocol that allows
payments from wallet-to-wallet without a merchant.


Motivation
==========

To be usable as an electronic payment system with cash-like properties,
customers should be able to transfer money between themselves without
needing to setup anything beyond their wallet(s).

This will be used for payments via e-mail and other messaging apps, as well as
possibly for transfers via NFC/QR code between mobile phones.

Invoice Flow User Experience
----------------------------------

.. graphviz::

   digraph invoice {
     settings [
           label = "Invoice flow";
     ];
     ranksep="0.5"
     { rank = same; "inbox"; "begin"; }
     { rank = same; "sending"; "receiving2"; }
     { rank = same; "receiving"; "paying"; }
     { rank = same; "mid"; "midbox"; }
     { rank = same; "body"; "amount"; }
     begin [label="Seller Inbox",shape=box];
     body [label="compose\nE-mail message"];
     amount [label="specify\ninvoice details"];
     receiving [label="receiving...",shape=diamond];
     sending [label="transmitting...",shape=diamond];
     mid [label="Seller Inbox",shape=box];
     notified [label="Notification:\npayment received"];
     end [label="Seller Inbox",shape=box];
     begin -> body [label="(1) new"];
     body -> amount [label="(2) attach invoice"];
     amount -> body [label="(3) Ok"];
     body -> sending [label="(4) send"];
     sending -> mid [style=dashed];
     mid -> receiving [style=dashed];
     receiving -> notified [style=dashed];
     notified -> end [label="(9) Acknowledge"];
     inbox [label="Buyer Inbox",shape=box];
     receiving2 [label="receiving...",shape=diamond];
     midbox [label="Buyer Inbox",shape=box];
     open [label="message with\nattached invoice"];
     confirm [label="review invoice"];
     paying [label="paying...", shape=diamond];
     paid [label="message with\npaid invoice"];
     finbox [label="Buyer Inbox",shape=box];
     inbox -> receiving2 [style=dashed];
     receiving2 -> sending [label="Internet\n(pEp)",style=dashed,dir=back];
     receiving2 -> midbox [style=dashed];
     midbox -> open [label="(5) select message"];
     open -> confirm [label="(6) view invoice"];
     confirm -> paying [label="(7) pay"];
     paying -> paid [style=dashed];
     paid -> finbox [label="(8) back"];
     paying -> receiving [style=dashed, label="Internet\n(Taler)"];
   }

Donation Flow User Experience
-------------------------------------

.. graphviz::

   digraph donation {
     ranksep="0.5"
     settings [
           label = "Donation flow";
     ];
     { rank = same; "inbox"; "begin"; }
     { rank = same; "sending"; "receiving2"; }
     { rank = same; "body"; "amount"; }
     { rank = same; "mid"; "midbox"; }
     { rank = same; "accepting"; "timeout"; "receiving"; }
     begin [label="Donor Inbox",shape=box];
     body [label="compose\nE-mail message"];
     amount [label="specify\npayment details"];
     receiving [label="receiving...",shape=diamond];
     timeout [label="timeout...",shape=diamond];
     sending [label="transmitting...",shape=diamond];
     mid [label="Donor Inbox",shape=box];
     notified [label="Notification:\npayment confirmed"];
     notified2 [label="Notification:\npayment refunded"];
     end [label="Donor Inbox",shape=box];
     begin -> body [label="(1) new"];
     body -> amount [label="(2) attach payment"];
     amount -> body [label="(3) Ok"];
     body -> sending [label="(4) send"];
     sending -> mid [style=dashed];
     mid -> receiving [style=dashed];
     receiving -> notified [style=dashed];
     mid -> timeout [style=dashed];
     timeout -> notified2 [style=dashed];
     notified -> end [label="(9a) Acknowledge"];
     notified2 -> end [label="(9b) Acknowledge"];
     inbox [label="Recipient Inbox",shape=box];
     receiving2 [label="receiving...",shape=diamond];
     midbox [label="Recipient Inbox",shape=box];
     open [label="message with\nattached payment"];
     confirm [label="accept payment?"];
     accepting [label="accepting...", shape=diamond];
     paid [label="message with\naccepted payment"];
     finbox [label="Recipient Inbox",shape=box];
     inbox -> receiving2 [style=dashed];
     receiving2 -> sending [label="Internet\n(pEp)",style=dashed,dir=back];
     receiving2 -> midbox [style=dashed];
     midbox -> open [label="(5) select message"];
     open -> confirm [label="(6) review payment details"];
     confirm -> accepting [label="(7) yes"];
     accepting -> paid [style=dashed];
     paid -> finbox [label="(8) back"];
     accepting -> receiving [style=dashed, label="Internet\n(Taler)"];
   }

Requirements
============

* The protocol must permit transacting arbitrary amounts in any currency,
  as long as both parties trust the exchange involved.
* The control data for wallet-to-wallet payments should be small
  enough to fit into a QR code or short message (so ideally less than 64 bytes).
* No other direct communication channel between payer and payee should
  be required.
* The wallet-to-wallet 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, we need transactional semantics
  resulting in exclusive control over the funds by the recipient.
* The wallet-to-wallet payment protocol must not allow users to circumvent income
  transparency.  That is, each wallet-to-wallet transaction must be visible
  on a KYCed transaction ledger (such as a bank account).
* The money received via a wallet-to-wallet payment must be usable for
  further Taler payments with minimal delay (after KYC).
* It must still be possible to associate payments with a contract that is
  effectively (alas not necessarily directly) signed by both parties:
  the payer with the coin private keys, and the payee with their KYC'ed
  account private key.
* The contract must be able to satisfy laws like the German TSE law,
  which implies that the payer must be able to obtain a payment receipt.
* Two payment scenarios must be possible: (1) one where the payee first
  transmits a proposal to the payer (request-to-pay) that the payer
  accepts by making the payment, and (2) completely uni-directional
  payments where the payer includes a proposal with the payment and the
  payee accepts the proposal by taking the offered payment.
* If the payment fails (i.e. the receiver refuses to accept the money
  or the message is lost), the payer must automatically recover the
  funds (minus applicable fees) without the need for further communication.
* If funds flow back to the payer due to an aborted payment, it must be
  provable for the payer that these funds were not income but merely an aborted
  transaction.  Furthermore, in this case, no KYC should be required from the
  payer.
* If a payment would partially succeed, i.e. because the payer inadvertedly
  used some double-spent coins and some valid coins, this must fail before the
  uni-directional communication and be correctable payer-side.  In other words,
  the actual payment must be atomic.
* The usual properties of Taler (everything auditable, unlinkability,
  high-performance in terms of CPU, bandwidth, latency, storage
  requirements, and the ability to levy fees on every operation that
  is costly for the exchange) need to be preserved.
* The system must handle the case where a customer no longer intends to
  use the KYCed account (due to disuse, death, or key compromise).



New Terminology
===============

* An ``account`` is a non-expiring reserve for which entity knowing the
  reserve private key has completed a KYC procedure sufficient to enable
  receiving income under that address.
* A ``purse`` is a public-private key pair where the public key is
  an exchange address from which any owners of an account
  can ``merge`` the amount left at a ``purse`` into their
  account balance assuming they know the purse private key.
* A ``wad`` is an exchange-to-exchange wire transfer that wires money
  into a group of accounts at the target exchange.


Proposed Solution
=================

Principles
----------

* Purses are ephemeral and only serve for one transaction.
* The purse's transaction amount is fixed when the purse is created, and
  specified together with the maximum deposit fee acceptable to the payee.
  Deposit fees exceeding this limit must be paid by the payer.
* Each purse is associated with a contract terms hash and an expiration date.
* The contract is optionally stored encrypted at the exchange.
  The contract must be encrypted to the purse private key.
  An additional ephemeral public key (for DH encryption) should be
  part of the POSTed payload.
* The exchange deletes the encrypted contract at this expiration date.
* The exchange may limit the encrypted contract size and storage duration.
* Either payer or payee can create the purse and associate it with the contract.
* By merging the purse into the account, the payee accepts the contract.
* By paying the purse to the designated amount, the payer accepts the contract.
* Until the purse is fully paid, the payer can abort the payment.
* The exchange may charge a **purse fee** for managing the purse, but we
  want most scenarios to not require it to be effectively charged.
* By associating a purse with an account upon creation, the purse fee can be
  made optional for account holders as long as the number of purses created
  per account is below a configurable threshold.
* By charging the purse fee only in case the payee did **not** merge the
  purse into their account, the purse fee can be limited for payers to
  the case where they are receiving a refund --- and here it could be
  then entirely avoided if the refund fee is non-zero.


W2W Payment Metadata
--------------------

The standard Taler Customer-to-Merchant payments always use a contract terms
JSON object to record the modalities of the payment and the resulting
obligations of a successful payment.

The contract terms concept does not directly carry over to W2W payments,
because:

* Either party may initiate the payment.
* The payee does not have a merchant public key, and
  at the time of payment initiation, the payee account might not yet
  be known.
* There is no nonce that the customer generates and uses to prove that they
  uniquely "own" the contract terms.
* There is no negotiation of trusted auditors / exchanges possible.

As a result, some of the existing fields of the contract terms no longer apply
to wallet-to-wallet payments.

Contract metadata for W2W payments can be exchanged in three ways:

1. Inline, as part of the payment request / payment offer.  In this case,
   both parties are already aware of the contract's contents and
   the exchange's contract exchange facility is simply not used.
2. The payee can create a **purse** and immediately associate it with
   an **account** by sending a signed **merge** request together with
   the (encrypted) contract.

   a. The purse creation request created by the payee must include a
      signature with the account private key of the payee signing the purse
      public key and the hash of the contract, thereby affirming that
      the contract was pre-approved by the account owner.
   b. The exchange may wave the purse fee for a certain number of
      active purses per account. Additional purses can be purchased
      by paying the **purse fee**.
3. The payer can store a contract with the exchange by POSTing
   an encrypted contract to the exchange as part of creating a **purse**.

   a. The exchange charges the **purse fee** to payers for purses that
      are refunded after not being **merged**.
   b. When paying into a purse, the coin signature includes the purse
      public key, the contract hash and the desired expiration date
      (how long a merge is allowed).
   c. Payment offers are not allowed if the amount transacted is below
      the purse fee.
   d. The exchange auto-refunds coins in **purses** with deposits
      matching expired contracts.

      .. note::

         While the **refund fee** amount 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.


Account creation
----------------

1. The payee generates an account key, which also yields a
   ``payto://taler/$EXCHANGE_BASE_URL/$ACCOUNT_PUB``
   target address (for which the payee knows the corresponding
   account private key).
2. When withdrawing from an account, the exchange first checks if the
   customer has satisfied the KYC requirements. If not, the customer
   is redirected to a Web page where they can perform the necessary
   KYC operation.
3. For this, the exchange wire gateway is extended with a request to
   check the KYC status of a customer based on an ``ACCOUNT_PUB``.
   Possible replies are ``in-progress`` and ``succeeded``. An ``in-progress``
   status should be accompanied with
   information how the customer may complete the KYC check.
4. A new exchange endpoint ``/account/$ACCOUNT_PUB/kyc``
   allows wallets to request a KYC for an
   ``$ACCOUNT_PUB``.  Such a request may include the requirement to pay
   a **KYC fee**.  The KYC fee may be charged to that account (if it exists),
   or could be waved if the account was established via a wire transfer
   from a partner bank.
5. If the account owner fails to perform the KYC check, the funds
   in an account remain inaccessible.  After a configurable duration,
   the funds may be considered forfeit and become the property of
   the exchange where the account is located.


Withdrawing from accounts
-------------------------

1. When requesting an account's history (which can get quite long),
   the exchange only returns the last 3 months of data.  Requesting
   the full history requires paying an **account history fee**
   (which is not done via a 402, but simply charged to the account
   when requested; full account histories for accounts with an
   insufficient balance cannot be requested -- except of course
   the wallet could simply top up the account balance first, see below).
2. If the exchange has **merged** a **purse** into an account, or
   received an inbound wire transfer from a **wad** matching the
   account (see below), it adds the respective amount(s) to the
   account's balance, allowing the KYC'ed customer to withdraw the funds,
   similar to withdrawals from a reserve.
3. The account history 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.


Account deletion
----------------

1. The account owner can delete an account by signing a deletion message
   with the account private key.
2. This basically resets the KYC data at the exchange, preventing further use of
   the account. This is helpful in case a user is concerned about having
   accidentally disclosed the account private key to a third party.
3. If funds remain in the account, an error message is generated instead.  The
   user can pass an extra override parameter to delete accounts even if
   they still contain funds.
4. A related endpoint should exist for the exchange operator, possibly
   using messages signed with the exchange offline key. This could be
   useful in case customers die or are otherwise in need for manual
   intervention that requires an account to be deleted. In this case,
   remaining funds in the account should be wired to a bank account
   designated in the message with the offline signature. The audit
   report should contain a special note for all of these account deletions.



Payment offers
--------------

In this protocol variant, the payer is initiating the process.

1. The payer creates a **purse** by computing a public-private key pair.
2. The payer POSTs to the ``/purse/$PURSE_PUB/depost`` endpoint to
   deposit coins into the purse and optionally upload the encrypted contract terms.
   The deposit signatures should use ``payto://taler/$PURSE_PUB``
   as the target address and signing over the ``$CONTRACT_HASH`` as
   usual in deposit operations. Note that the lack of a hostname
   indicates that the target address is a local purse.
3. The payer shares the purse's private key and the base URL
   of the exchange where the purse was created with the payee.
   This can be done using a ``taler://purse/$BASE_URL/$PURSE_PRIV`` URL.
4. The payee uses the new ``/purse/$PURSE_PUB`` endpoint to retrieve
   the encrypted contract (if available) and purse balance, which includes all
   (coin) deposits and **merges** involving the purse.
5. The payee's wallet must ensure that either:

   a. The purse has an attached encrypted contract terms, the contract
      terms can be decrypted and are valid, and their hash matches the contract
      terms hash of the purse.
   b. The wallet received detached contract terms, and their hash matches
      contract terms of the purse.

   If neither case applies, the payee's wallet must reject the payment.
6. The payee can then POST to ``/purse/$PURSE_PUB/merge`` a
   request signed by the purse's private key to **merge** the
   funds into an account.  A second signature must be provided
   by the account private key, signing the ``$CONTRACT_HASH`` thereby
   affirming that the payee accepted the contract.
   The account is of the form ``payto://taler/$EXCHANGE_BASE_URL/$ACCOUNT_PUB``.
7. Processing continues depending on the location of the account:

   a. If the ``$EXCHANGE_BASE_URL`` matches the local exchange, then
      the exchange processes the **merge** request akin to the logic
      for payments into known accounts, as detailed above.
   b. If the ``$EXCHANGE_BASE_URL`` does not match the local exchange,
      a **wad fee** is charged, and the remaining amount are placed into a **wad**
      to inform the target exchange, as detailed below.  Wad fees may be covered
      by the merchant, just like deposit fees, depending on the contract.
8. The exchange confirms the merge (per response to the **merge** request).
   This allows the payee software to instantly
   affirm to the users that the transaction is final (even if it may not
   be instantly available to the payee if the payee did not complete the
   KYC process for the account).
9. The payer uses the GET ``/purse/$PURSE_PUB`` endpoint
   to obtain the receipt from the payee (in the form of the
   **merge** signature).  Query parameters are used to avoid
   downloading the (already known) encrypted contract and the
   deposit operations.  Long-polling must also be possible for
   this request.



Payment requests
----------------

1. The payee creates a **purse** by computing a public-private key pair.
2. The payee POSTs to the ``/purse/$PURSE_PUB/merge`` endpoint to
   both upload the encrypted contract, associate it with the payer's
   account and signal its agreement to the contract.  The
   **merge** request must be signed by the purse's private key.
   A second signature must be provided by the account private key,
   signing the ``$CONTRACT_HASH`` thereby affirming that the payee
   accepted the contract.
3. The payee provides the ``$PURSE_PRIV`` to the payer.
4. The payer computes the corresponding public key and uses the
   new ``/purse/$PURSE_PUB`` endpoint to retrieve
   the encrypted contract and the merge request, which signifies that
   the payee would agree to the contract.
5. The payer software decrypts the encrypted contract using the purse private
   key and the payer accepts the contract in the user interface.
6. Processing continues depending on the source of the coins:

   a. If the payer's coins originate from the same exchange, the
      payer software POSTs to the ``/purse/$PURSE_PUB/depost`` endpoint to
      deposit coins into the purse. The deposit signatures should use
      ``payto://taler/$PURSE_PUB``
      as the target address and signing over the ``$CONTRACT_HASH`` as
      usual in deposit operations. Note that the lack of a hostname
      indicates that the target address is a local purse.
   b. If the payer's coins originate from another exchange, the
      payer software deposits the coins at the originating exchange
      using the traditional ``/deposit`` endpoint and a target account of the form
      ``payto://taler/$EXCHANGE_BASE_URL/$ACCOUNT_PUB``.  In this case,
      the remote exchange charges a **wad fee** and places the remaining
      amount into a **wad** to inform the target exchange, as detailed below.
7. The exchange confirms the deposit. This allows the payer software to instantly
   affirm to the users that the transaction is final, or to abort or try again
   in case of errors.
8. The payee uses the GET ``/purse/$PURSE_PUB`` endpoint (possibly with long-polling)
   to be notified about the successful deposit and subsequent completion of the
   **merge** request.


Payment into accounts at remote exchanges
-----------------------------------------

In case the coins and the accounts in the transaction flows above are at
different exchanges, an aggregated exchange-to-exchange payment (short
**wad**) is used.

1. Exchanges specify a new **wad fee** that they charge for exchange-to-exchange
   payments.  They also specify their wad policy, that is how often they
   perform exchange-to-exchange transfers.

   .. note::

      We may want to consider allowing for different wad-speed levels, where
      express payments (without aggregation) are allowed in return for higher
      wad fees.

2. The payer's exchange creates a **wad** by grouping all wad requests
   to the same target exchange. It executes
   the transaction when either the **wad threshold** (maximum number
   of transactons aggregated per wad) or the **wad delay** (maximum
   delay for transfers) has been reached.
3. If the (aggregated) wire transfer fails (say the
   ``/wire`` endpoint of the payee exchange does not
   resolve to a valid bank account), the
   originating exchange automatically creates a full refund for
   all involved coins (**refund fees** apply).

   .. note::

      While the **refund fee** amount 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.

4. The payee's exchange observes the wire transfer with a wire transfer
   subject with the originating exchange base URL and a ``$WATID``,
   and uses a GET ``/wad/$WATID`` request to obtain
   details about the target accounts.
5. When the payer's exchange is requested to provide information about
   aggregated transfers under the ``$WATID``, it provides a signed list of
   account public keys and associated amounts that must add up to an
   amount below the total amount transferred. If they do not, the
   payee's exchange does not credit any of the accounts and instead
   preserves the bogus reply (to justify its inaction with its own
   auditor) and reports the issue to the auditor of the payer's exchange
   (keeping the received funds for future manual resolution).
6. ``taler-exchange-wirewatch`` and the Taler wire gateway API will
   need to be extended to allow passing inbound wire transfers with ``$WATID``
   and exchange base URL to the exchange. Furthermore, another tool
   is needed to lookup the **wad** data at remote exchanges.
7. If the payee trusts the originating exchange, it may consider the
   transaction ``final`` once the originating exchange has affirmed the
   deposit (assuming the payer has a way to submit the evidence of that
   payment, which may not apply in uni-directional scenarios).
   Otherwise, the payee may simply only trust its own exchange,
   resulting in the transfer only being considered final after the
   receiving exchange has confirmed that the **wad** has arrived.


Examples
---------

Cross-exchange W2W payment request:

* Bob borrowed 15 EUR from Alice to buy a train ticket.  A few days later,
  Alice wants her money back.  She creates a request for payment in her wallet.
  The wallet creates a purse for 15 EUR at the only exchange that Alice is currently
  using.  The wallet shows her a
  ``taler://purse/{EXCHANGE_URL}/{PURSE_PRIV}``
  link that she can share with Bob.  Bob receives the link and opens it with
  his Taler wallet.  Bob is using a different EUR exchange than Alice.  Bob's
  wallet makes a ``/deposit`` request to his own exchange.  Shortly after, Alice's
  exchange receives the wad from Bob's exchange, and credits the money into
  Alice's purse.

  * Q: How does Bob find out if Alice's exchange supports a wad transfer from
    Bob's exchange? A: This needs to be part of the wad policy.
  * Q: How does Bob get a "receipt" to prove that he paid Alice?
    A: He has Alice's account public key and the associated signature
    chain leading to her payment request. If he paid someone else by accident,
    the KYC of Alice's exchange could be used to find out who received the funds.

Cross-exchange W2W payment offer:

* Carol wants to send some money to Dave as a birthday gift.  Carol knows that
  Dave is using Taler, but she does not know which exchange he is using.  She
  opens her Taler wallet and initiates a P2P payment.  She sends the resulting
  ``taler://purse/{EXCHANGE_URL}/{PURSE_PRIV}`` in an e-mail to Dave.
  Dave opens they link in the e-mail with his Taler wallet.
  Since Dave is using a different exchange than Alice, Dave's wallet
  issues a **merge** request to Alice's exchange pointing Alice's exchange
  to Dave's account at his exchange.  Shortly after,
  Dave's exchange receives a **wad** from Alice's exchange,
  and credits Dave's account with the money.


State machine for Purses
------------------------

.. code-block:: none

   // The "OPEN-ACCOUNT" start state implies that the purse is associated
   // with an account and a merge request for that account.
   -> OPEN-ACCOUNT

   // "Partial" means that it is filled with a fraction of the coins
   // indicated in the creation request.
   OPEN-ACCOUNT -> PARTIAL

   // The purse was filled with as many coins
   // as indicated in the creation request, resulting in the transaction to complete.
   OPEN-ACCOUNT -> ACCEPTED

   // The offer expired before any payment was received.
   OPEN-ACCOUNT -> CLOSED

   // The purse was filled with as many coins
   // as indicated in the creation request, resulting in the transaction to complete.
   PARTIAL -> ACCEPTED

   // During an abort, already deposited coins are being taken out of the purse.
   PARTIAL -> OPEN-ACCOUNT

   // All coins put into the purse are refunded because the
   // payer never completed the purchase before the timeout.
   PARTIAL -> CLOSED

   // The "OPEN-DEPOSIT" start state implies that the purse is filled with
   // deposited coins.
   -> OPEN-DEPOSIT

   // Paid and merged with an account (locally or via a wad)
   OPEN-DEPOSIT -> ACCEPTED

   // The offer expired without a merge request.
   OPEN-DEPOSIT -> CLOSED


Additional considerations
-------------------------

* Creation of additional accounts per customer can
  be discouraged by asking for higher fees.
* The global transaction volume of one customer can be easily
  determined by authorities, which can then trigger further audits
  of the customer
* As a technically expensive but more water-tight measure, normal
  withdrawals from reserves could be disallowed.  Instead,
  a modified refresh protocol could ensure that whoever has knowledge
  of the account private key can also learn the private keys
  of coins withdrawn from that account, thereby removing
  Taler's "one-hop withdrawal loohole".


Alternatives
============

* The payer could directly give deposit permissions to the payee.
  This has two problems:

  1. The payer does not know the wire details of the payee.
     Thus we would need to introduce some "wildcard deposit permission",
     where the exchange allows any wire details on ``/deposit``.
  2. The payment information would be rather large, making it difficult
     to transfer via a QR code or short text message.

* Account history exceeding a configurable time limit (like 6 years)
  could be subject to garbage collection. However, doing so may be difficult to
  square with onboarding new auditors in the presence of existing
  accounts, as the auditors could then not reconstruct the account
  balances from cryptographic proofs.

* Accounts without KYC check could be eventually closed. However,
  even if the coins used to fill the account are refunded, it
  would be difficult to inform the originating wallet that the
  coins have received a refund. This applies even more strongly
  in case of accounts filled via wads, where in theory the
  originating exchange may not even be in business anymore.
  Thus, it is cleaner and simpler to declare such funds forfeit.


Drawbacks
=========

The overall changes required are not small:

* New **KYC fee**, **wad fee** and **account history fee**
  required in ``/keys`` endpoint (similar to closing and wire fees),
  requires some 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 account.
  Likely two tables, one for local exchange p2p and one for remote exchange p2p
  payments.
* New exchange table for purses required (for remote p2p payments).
* New exchange logic required to make ``transfers`` requests for purses
  (another separate process).
* New ``/account/$ACCOUNT_PUB/kyc`` endpoint required.
* New ``/purse/$PURSE_PUB/merge`` endpoint required.
* Additional tables to be verified by the auditor.
* ``taler-exchange-wirewatch`` needs to support receiving purses closures
  and exchange-to-exchange wire transfers with WTIDs.

Aside from implementation complexity, the solution has the following drawbacks:

* If a W2W payment failed (say the receiver lost the account private key),
  the customer's money can be forfeit.  Alas, this should be very, very rare
  as the wallet software can trivially ensure that a backup was made of the
  account private key before initiating the KYC process.


Q / A
=====

* Q: Why are direct payments into accounts allowed?

  * A: Direct payments into accounts may be used by the customer
    to fund the expenses for using the account.  They should not
    be used for payments between customers, as contract terms for
    the ``/deposit`` of coins cannot be negotiated.  Furthermore,
    the sender of the payment cannot be sure that the account of
    the sender is still valid.

* Q: Who "owns" a purse?  The payer of payee?

  * Both.  Ownership is shared.  Either the payer issues
    a refund on the purse, or the payee claims it by merging
    it with one of their accounts.

* Q: Are purses created with a pre-determined "capacity"?

  * A: Yes.  Otherwise there would be weird failure modes when the payee
    merges the purse before the payer fully deposited into it.

* Q: Are account public keys considered private or public data?

  * A: Public.  The payer needs a signature from the payee affirming
    that they accepted the contract, and this requires a key that
    is linked to the KYC process to be meaningful.  However, the
    software should NOT permit direct payments into foreign accounts
    because it would be too easy to accidentally send payments that
    nobody can receive, because the account public key is wrong/lost.

* Q: Why do traditional merchant payments not use purses?

  * Refunds are not possible with purses after they are closed.
  * The customer cannot prove that they own the contract terms
    (Contract terms claiming requires interactivity that is not
    possible in all W2W scenarios.) Thus, while payers can prove
    that they paid, the payee may claim someone else also
    bought the same product. A secure channel must thus be used to
    exchange the purchase offer.

* Q: What determines when a wad transfer can happen between two exchanges?

  * Exchanges should explicitly state which other exchanges they are willing
    to do wad transfers with (and how often, at what cost). This may involve
    abstract policies like sharing an auditor, using the same currency and the
    same (banking) protocol, or other constraints (like a specific list of
    exchanges).