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..
  This file is part of GNU TALER.

  Copyright (C) 2014-2020 Taler Systems SA

  TALER is free software; you can redistribute it and/or modify it under the
  terms of the GNU General Public License as published by the Free Software
  Foundation; either version 2.1, or (at your option) any later version.

  TALER is distributed in the hope that it will be useful, but WITHOUT ANY
  WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
  A PARTICULAR PURPOSE.  See the GNU General Public License for more details.

  You should have received a copy of the GNU General Public License along with
  TALER; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>

  @author Christian Grothoff

GNU Taler Exchange Operator Manual
##################################

Introduction
============

This manual is an early draft that still needs significant editing work
to become readable.

About GNU Taler
---------------

GNU Taler is an open protocol for an electronic payment system with a
free software reference implementation. GNU Taler offers secure, fast
and easy payment processing using well understood cryptographic
techniques. GNU Taler allows customers to remain anonymous, while
ensuring that merchants can be held accountable by governments. Hence,
GNU Taler is compatible with anti-money-laundering (AML) and
know-your-customer (KYC) regulation, as well as data protection
regulation (such as GDPR).

GNU Taler is not yet production-ready, after following this manual you
will have a backend that can process payments in “KUDOS”, but not
regular currencies. This is not so much because of limitations in the
backend, but because we are not aware of a Taler exchange operator
offering regular currencies today.

About this manual
-----------------

This tutorial targets system administrators who want to install and
operate a GNU Taler exchange.

Organizational prerequisites
----------------------------

Operating a GNU Taler exchange means that you are operating a payment
service provider, which means that you will most likely need a bank
license and/or follow applicable financial regulation.

GNU Taler payment service providers generally need to ensure high
availability and have *really* good backups (synchronous replication,
asynchronous remote replication, off-site backup, 24/7 monitoring,
etc.).  This manual will not cover these aspects of operating a
payment service provider.

We will assume that you can operate a (high-availability,
high-assurance) Postgres database. Furthermore, we expect some moderate
familiarity with the compilation and installation of free software
packages. You need to understand the cryptographic concepts of private
and public keys and must be able to protect private keys stored in files
on disk.


Architecture overview
---------------------

Taler is a pure payment system, not a new crypto-currency. As such, it
operates in a traditional banking context. In particular, this means
that in order to receive funds via Taler, the merchant must have a
regular bank account, and payments can be executed in ordinary
currencies such as USD or EUR. Similarly, the Taler exchange must
interact with a bank. The bank of the exchange holds the exchange’s
funds in an escrow account.

Note that, given the technical burden (XML-based communications,
additional cryptography, and a vast variety of standards) due to
interact with banks, the exchange uses a intermediary system to talk
to its bank.  Such intermediary system abstracts the native banking
protocol by exposing the *Taler Wire Gateway API*; this way, the exchange
can conduct its banking operations in a simplified and JSON-based style.

When customers wire money to the escrow account, the bank notifies the
exchange about the incoming wire transfers. The exchange then creates a
*reserve* based on the subject of the wire transfer. The wallet which
knows the secret key matching the wire transfer subject can then
withdraw coins from the reserve, thereby draining it. The liability of
the exchange against the reserve is thereby converted into a liability
against digital coins issued by the exchange. When the customer later
spends the coins at a merchant, and the merchant *deposits* the coins at
the exchange, the exchange first *aggregates* the amount from multiple
deposits from the same merchant and then instructs its bank to make a
wire transfer to the merchant, thereby fulfilling its obligation and
eliminating the liability. The exchange charges *fees* for some or all
of its operations to cover costs and possibly make a profit.

*Auditors* are third parties, for example financial regulators, that
verify that the exchange operates correctly. The same software is also
used to calculate the exchange’s profits, risk and liabilities by the
accountants of the exchange.

The Taler software stack for an exchange consists of the following
components:

-  HTTP frontend
   The HTTP frontend interacts with Taler wallets and merchant backends.
   It is used to withdraw coins, deposit coins, refresh coins, issue
   refunds, map wire transfers to Taler transactions, inquire about the
   exchange’s bank account details, signing keys and fee structure. The
   binary is the ``taler-exchange-httpd``.

-  Crypto-Helpers
   The ``taler-exchange-secmod-rsa`` and ``taler-exchange-secmod-eddsa``
   are two programs that are responsible for managing the exchange's
   online signing keys. They must run on the same machine as the
   ``taler-exchange-httpd`` as the HTTP frontend communicates with the
   crypto helpers using UNIX Domain Sockets.

-  Aggregator
   The aggregator combines multiple deposits made by the same merchant
   and (eventually) triggers wire transfers for the aggregate amount.
   The merchant can control how quickly wire transfers are made. The
   exchange may charge a fee per wire transfer to discourage
   excessively frequent transfers. The binary is the
   ``taler-exchange-aggregator``.

-  Closer
   The ``taler-exchange-closer`` tool check that reserves are properly
   closed. If a customer wires funds to an exchange and then fails
   to withdraw them, the closer will (eventually) trigger a wire
   transfer that sends the customer's funds back to the originating
   wire account.

-  Transfer
   The ``taler-exchange-transfer`` tool is responsible for actually
   executing the aggregated wire transfers. It is the only process
   that needs to have the credentials to execute outgoing wire
   transfers.  The tool uses the Taler Wire Gateway API to execute
   wire transfers.  This API is provided by the Taler Python Bank
   for stand-alone deployments (like those with ``KUDOS``) and
   by LibEuFin.  LibEuFin is an adapter which maps the Taler Wire
   REST API to traditional banking protocols like EBICS and FinTS.

-  Wirewatch
   The ``taler-exchange-wirewatch`` tool is responsible for observing
   incoming wire transfers to the exchange. It needs to have the
   credentials to obtain a list of incoming wire transfers.
   The tool also uses the Taler Wire Gateway API to observe such
   incoming transfers.  It is possible that observing incoming and
   making outgoing wire transfers is done via different bank accounts
   and/or credentials.

-  Wire adapter
   A wire adapter is a component that enables exchange to talk to a bank.

   (1) The libtalerfakebank implements a bank with a wire adapter API
       inside of a testcase.
   (2) For the demonstration Web site (or local currencies),
       the Python bank provides a bank that directly provides
       the wire adapter API.
   (3) For production, libeufin's Nexus component implements a wire
       adapter towards the traditional SEPA banking system with IBAN
       accounts.

   The client-side wire adapter API is implemented in libtalerbank and
   is used by the transfer to execute wire transfers and for the
   auditor to query bank transaction histories.


-  DBMS
   The exchange requires a DBMS to stores the transaction history for
   the Taler exchange and aggregator, and a (typically separate) DBMS
   for the Taler auditor. For now, the GNU Taler reference implementation
   only supports Postgres, but the code could be easily extended to
   support another DBMS.
   .. index:: Postgres

-  Auditor
   The auditor verifies that the transactions performed by the exchange
   were done properly. It checks the various signatures, totals up the
   amounts and alerts the operator to any inconsistencies. It also
   computes the expected bank balance, revenue and risk exposure of the
   exchange operator. The main binary is the ``taler-auditor``.
   Aside from the key setup procedures, the most critical setup for
   deploying an auditor is providing the auditor with an up-to-date
   copy of the database.


Offline keys
------------

The exchange (and ideally also auditors) uses a long-term offline master
siging key that identifies the operator and is used to authenticate critical
information, such as the exchange's bank account and the actual keys the
exchange uses online.

Interactions with the offline system are performed using the
``taler-exchange-offline`` tool.  To use the offline system will require
exchange operators to copy JSON files from or to the offline system (say using
an USB stick).  The offline system does not need any significant amount of
computing power, a Raspberry-Pi is perfectly sufficient and the form-factor
might be good for safe-keeping! (You should keep a copy of the (encrypted)
private offline key on more than one physical medium though.)

Exchange operators are strongly advised to secure your private master key and
any copies on encrypted, always-offline computers. Again, we assume that you
are familiar with good best practices in operational security, including
securing key material.



Online signing key security
---------------------------

To provide an additional level of protection for the private *online* signing
keys used by the exchange, the actual cryptographic signing operations are
performed by two helper processes, the ``taler-exchange-secmod-rsa`` and the
``taler-exchange-secmod-eddsa``.

The current implementation does not yet support the use of a hardware security
module (HSM).  If you have such a device with adequate functionality and are
interested in Taler supporting it, please contact the developers for HSM
integration support.


Functionality
^^^^^^^^^^^^^

The UNIX domain sockets have mode 0620 (u+rw, g+w).  The exchange process
MUST be in the same group as the crypto helper processes.

The two helper processes will create the required private keys, and allow
anyone with access to the UNIX domain socket to sign arbitrary messages with
the keys or to inform them about a key being revoked.  The helper processes
are also responsible for deleting the private keys if their validity period
expires or if they are informed about a key having been revoked.

Security goals
^^^^^^^^^^^^^^

From a security point of view, the helpers are designed to *only* make it
harder for an attacker who took control of the HTTP daemon's account to
extract the private keys, limiting the attackers ability to creating
signatures to the duration of their control of that account.

.. note::
   In the future, the helper processes should additionally provide a mechanism
   to track the total number of signatures they have made for the various keys.

Setup
^^^^^

The helper processes should be run under a user ID that is separate from that
of the user running the main ``taler-exchange-httpd`` service.  For security,
it is important that helpers run under a different user ID than the main HTTP
frontend, in fact ideally each helper should run under its own user ID.  The
``taler-exchange-httpd`` service's will securely communicate with the helpers
using UNIX domain sockets.  To enable access to the keys, the service's user
must be in the group of the helper processes (and no other users should be in
that group).

Configuration
^^^^^^^^^^^^^

The helpers and the HTTP service need both access to the same configuration
information.  Having divergent configurations may result in run-time failures.
It is recommended that the configuration file (``-c`` option) is simply shared
between all of the different processes, even though they run as different
system users. The configuration does not contain any sensitive information.




Installation
============

Before installing a Taler exchange, please make sure that your
system does not have swap space enabled.  Swap space is a security
risk that Taler does not try to mitigate against.

Please install the following packages before proceeding with the
exchange compilation.

.. include:: frags/list-of-dependencies.rst

-  GNU Taler exchange (from `download directory <http://ftpmirror.gnu.org/taler/>`__,
   see `release announcement <https://mail.gnu.org/archive/cgi-bin/namazu.cgi?query=taler&idxname=info-gnu&max=20&result=normal&sort=date:late>`__)

Except for the last two, these are available in most GNU/Linux
distributions and should just be installed using the respective package
manager.


Installing from source
----------------------

The following instructions will show how to install libgnunetutil and
the GNU Taler exchange from source.

.. include:: frags/installing-gnunet.rst

.. include:: frags/installing-taler-exchange.rst


Installing the GNU Taler binary packages on Debian
--------------------------------------------------

.. include:: frags/installing-debian.rst

To install the Taler exchange, you can now simply run:

.. code-block:: console

   # apt install taler-exchange

Note that the package does not perform any configuration work except for
setting up the various users and the systemd service scripts. You still must
configure at least the database, HTTP reverse proxy (typically with TLS
certificates), denomination and fee structure, bank account, auditor(s),
offline signing and the terms of service.

Sample configuration files for the HTTP reverse proxy can be found in
``/etc/taler-exchange/``.


Installing the GNU Taler binary packages on Ubuntu
--------------------------------------------------

.. include:: frags/installing-ubuntu.rst

To install the Taler exchange, you can now simply run:

.. code-block:: console

   # apt install taler-exchange

Note that the package does not perform any configuration work except for
setting up the various users and the systemd service scripts. You still must
configure at least the database, HTTP reverse proxy (typically with TLS
certificates), denomination and fee structure, bank account, auditor(s),
offline signing and the terms of service.

Sample configuration files for the HTTP reverse proxy can be found in
``/etc/taler-exchange/``.


Configuration
=============

This chapter provides an overview of the exchange configuration. Or at
least eventually will do so, for now it is a somewhat wild description
of some of the options.


.. include:: frags/configuration-format.rst


.. _Using-taler_002dconfig-exchange:

.. include:: frags/using-taler-config.rst


.. _Keying:

Keying
------

The exchange works with four types of keys:

-  master key (kept offline)

   To create a master key, use:

   .. code-block:: console

      $ taler-exchange-offline setup

-  sign keys (signs normal messages from the exchange)

-  denomination keys (signs electronic coins, see section Coins)

-  security module keys (signs sign keys and denomination keys)

Additionally, the exchange is sometimes concerned with the auditor's public
key (to verify messages signed by auditors approved by the exchange operator)
and the merchant's public key (to verify refunds are authorized by the
merchant).


Key options include:

-  ``[exchange-offline/MASTER_PRIV_FILE]``: Path to the exchange’s master private file.  Only needs to be provided on the offline system where the ``taler-exchange-offline`` command is used.

-  ``[exchange/MASTER_PUBLIC_KEY]``: Must specify the exchange’s master public key.  Needed for the exchange to verify information signed by the offline system.


.. _Serving:

Serving
-------

The exchange can serve HTTP over both TCP and UNIX domain socket.

The following options are to be configured in the section ``[exchange]``:

-  ``SERVE``: Must be set to ``tcp`` to serve HTTP over TCP, or ``unix`` to serve
   HTTP over a UNIX domain socket.

-  ``PORT``: Set to the TCP port to listen on if ``SERVE`` is ``tcp``.

-  ``UNIXPATH``: Set to the UNIX domain socket path to listen on if ``SERVE`` is
   ``unix``.

-  ``UNIXPATH_MODE``: Number giving the mode with the access permission mask
   for the ``UNIXPATH`` (i.e. 660 = ``rw-rw---``).

.. _Currency:

Currency
--------

The exchange supports only one currency. This data is set under the
respective option ``CURRENCY`` in section ``[taler]``.

.. _Database:

Database
--------

The option ``DB`` in section ``[exchange]`` gets the database backend’s name the
exchange is going to use. So far, only ``db = postgres`` is supported. After
choosing the backend, it is mandatory to supply the connection string
(namely, the database name). This is possible in two ways:

-  via an environment variable: ``TALER_EXCHANGEDB_POSTGRES_CONFIG``.

-  via configuration option ``CONFIG``, under section ``[exchangedb-BACKEND]``.
   For example, the demo exchange is configured as follows:

.. code-block:: ini

   [exchange]
   ...
   DB = postgres
   ...

   [exchangedb-postgres]
   CONFIG = postgres:///talerdemo


Given this database configuration, the database can be initialized using:

.. code-block:: console

   $ taler-exchange-dbinit

Note that to run this command, the user must have ``CREATE TABLE``, ``CREATE
INDEX``, ``ALTER TABLE`` and (in the future possibly even) ``DROP TABLE``
permissions.  Those permissions are only required for this step (which may
have to be repeated when upgrading a deployment).  Afterwards, during normal
operation, permissions to ``CREATE`` or ``ALTER`` tables are not required by
any of the Taler exchange processes and thus should not be granted.
For more information, see :doc:`manpages/taler-exchange-dbinit.1`.

Commands, like ``taler-exchange-dbinit``, that support the ``-l LOGFILE``
command-line option, send logging output to standard error by default.

.. _Coins-denomination-keys:

Coins (denomination keys)
-------------------------

Sections specifying denomination (coin) information start with ``coin_``.  By
convention, the name continues with ``$CURRENCY_[$SUBUNIT]_$VALUE_$REVISION``,
i.e. ``[coin_eur_ct_10_0]`` for a 10 cent piece. However, only the ``coin_``
prefix is mandatory.  Once configured, these configuration values must not
change.  The ``$REVISION`` part of the section name should be incremented if
any of the coin attributes in the section changes.  Each ``coin_``-section
must then have the following options:

-  ``VALUE``: How much is the coin worth, the format is
   CURRENCY:VALUE.FRACTION. For example, a 10 cent piece is "EUR:0.10".

-  ``DURATION_WITHDRAW``: How long can a coin of this type be withdrawn?
   This limits the losses incurred by the exchange when a denomination
   key is compromised.

-  ``DURATION_SPEND``: How long is a coin of the given type valid? Smaller
   values result in lower storage costs for the exchange.

-  ``DURATION_LEGAL``: How long is the coin of the given type legal?

-  ``FEE_WITHDRAW``: What does it cost to withdraw this coin? Specified
   using the same format as value.

-  ``FEE_DEPOSIT``: What does it cost to deposit this coin? Specified using
   the same format as value.

-  ``FEE_REFRESH``: What does it cost to refresh this coin? Specified using
   the same format as value.

-  ``FEE_REFUND``: What does it cost to refund this coin?
   Specified using the same format as value.

-  ``RSA_KEYSIZE``: How many bits should the RSA modulus (product of the two
   primes) have for this type of coin.

See :doc:`manpages/taler.conf.5` for information on *duration* values
(i.e. ``DURATION_WITHDRAW`` and ``DURATION_SPEND`` above,
and ``OVERLAP_DURATION`` and ``DURATION`` below).
Additionally, there are two global configuration options of note:

-  ``[taler-exchange-secmod-rsa/OVERLAP_DURATION]``: What is the overlap of the
   withdrawal timespan for denomination keys?  The value given here must
   be smaller than any of the ``DURATION_WITHDRAW`` values for any of the coins.

-  ``[taler-exchange-secmod-rsa/LOOKAHEAD_SIGN]``: For how far into the future
   should denomination keys be pre-generated?  This allows the exchange and
   auditor operators to download, offline-sign, and upload denomination key
   signatures for denomination keys that will be used in the future by the
   exchange.

.. index:: maintenance
.. note::
   We recommend setting the ``LOOKAHEAD_SIGN`` value to at least one year and
   then to perform the offline-signing procedure at least once every 6 months
   to ensure that there is sufficient time for wallets to learn the new keys
   and to avoid unavailability in case this critical maintenance procedure is
   delayed.

.. note::
   It is crucial that the configuration provided in these sections is identical (!)
   for the exchange and the crypto helpers.  We recommend pointing both users
   to the same configuration file!



.. _Sign-keys:

Sign keys
---------

There are three global configuration options of note for sign keys:

-  ``[taler-exchange-secmod-eddsa/DURATION]``: How long are sign keys
   used to sign messages? After this time interval expires, a fresh
   sign key will be used (key rotation).  We recommend using
   a ``DURATION`` of a few weeks to a few months for sign keys.

-  ``[taler-exchange-secmod-eddsa/OVERLAP_DURATION]``: What is the overlap of the
   timespan for sign keys?  We recommend a few minutes or hours.  Must
   be smaller than ``DURATION``.

-  ``[taler-exchange-secmod-eddsa/LOOKAHEAD_SIGN]``: For how far into the future
   should sign keys be pre-generated?  This allows the exchange and
   auditor operators to download, offline-sign, and upload sign key
   signatures for sign keys that will be used in the future by the exchange.

.. note::
   We recommend setting the ``LOOKAHEAD_SIGN`` value to at least one year and
   then to perform the offline-signing procedure at least once every 6 months
   to ensure that there is sufficient time for wallets to learn the new keys
   and to avoid unavailability in case this critical maintenance procedure is
   delayed.


Terms of Service
----------------

The exchange has an endpoint "/terms" to return the terms of service
(in legal language) of the exchange operator.  The wallet will show
those terms of service to the user when the user is first withdrawing
coins.  Terms of service are optional for experimental deployments,
if none are configured, the exchange will return a simple statement
saying that there are no terms of service available.

To configure the terms of service response, there are two options
in the ``[exchange]`` section:

-   ``TERMS_ETAG``: The current "Etag" to return for the terms of service.
    This value must be changed whenever the terms of service are
    updated. A common value to use would be a version number.
    Note that if you change the ``TERMS_ETAG``, you MUST also provide
    the respective files in ``TERMS_DIR`` (see below).
-   ``TERMS_DIR``: The directory that contains the terms of service.
    The files in the directory must be readable to the exchange
    process.

The ``TERMS_DIR`` directory structure must follow a particular layout.
First, inside of ``TERMS_DIR``, there should be sub-directories using
two-letter language codes like "en", "de", or "jp".  Each of these
directories would then hold translations of the current terms of
service into the respective language.  Empty directories are
permitted in case translations are not available.

Then, inside each language directory, files with the name of the
value set as the ``TERMS_ETAG`` must be provided. The extension of
each of the files should be typical for the respective mime type.
The set of supported mime types is currently hard-coded in the
exchange, and includes HTML, PDF and TXT files. If other files are
present, the exchange may show a warning on startup.

Example
^^^^^^^

A sample file structure for a ``TERMS_ETAG`` of "v1" would be:

-   TERMS_DIR/en/v1.txt
-   TERMS_DIR/en/v1.html
-   TERMS_DIR/en/v1.pdf
-   TERMS_DIR/de/v1.txt
-   TERMS_DIR/de/v1.html
-   TERMS_DIR/de/v1.pdf
-   TERMS_DIR/fr/v1.pdf

If the user requests an HTML format with language preferences "fr" followed by "en",
the exchange would return ``TERMS_DIR/en/v1.html`` lacking an HTML version in
French.


.. _Bank-account:

Bank account
------------

To configure a bank account in Taler, we need to furnish two pieces of
information:

-  The ``payto://`` URI of the bank account, which uniquely idenfies the
   account. Examples for such URIs include
   ``payto://iban/CH9300762011623852957`` for a bank account with
   an IBAN or
   ``payto://x-taler-bank/localhost:8080/2`` for the 2nd bank account a
   the Taler bank demonstrator running at ``localhost`` on port 8080.
   The first part of the URI following ``payto://`` (“iban” or
   “x-taler-bank”) is called the wire method.

-  The ``taler-exchange-wirewatch`` and ``taler-exchange-transfer``
   tools needs to be provided resources for authentication
   to the respective banking service. The format in which the
   authentication information is currently a username and password
   for HTTP basic authentication.

You can configure multiple accounts for an exchange by creating sections
starting with “exchange-account-” for the section name. You can ENABLE for
each account whether it should be used, and for what (incoming or outgoing
wire transfers):

.. code-block:: ini

   [exchange-account-1]
   # With x-taler-bank (say for PyBank)
   PAYTO_URI = "payto://x-taler-bank/bank.demo.taler.net/Exchange"

   # Example using IBAN (for use with LibEuFin)
   # PAYTO_URI = "payto://iban/CH9300762011623852957"

   # URL for talking to the bank wire the wire API.
   WIRE_GATEWAY_URL = https://bank.demo.taler.net/taler-wire-gateway/Exchange

   # Use for exchange-aggregator (outgoing transfers)
   ENABLE_DEBIT = YES
   # Use for exchange-wirewatch (and listed in /wire)
   ENABLE_CREDIT = YES

   # Authentication options for exchange bank account go here.
   # (Next sections have examples of authentication mechanisms)
   WIRE_GATEWAY_AUTH_METHOD = basic
   USERNAME = exchange
   PASSWORD = super-secure


The command line tool ``taler-exchange-offline`` must be used to
sign the ``payto://`` URI in a way suitable to convince wallets that
this is the correct address to wire funds to.
For example, the utility may be invoked as
follows to enable a wire account:

.. code-block:: console

   $ taler-exchange-offline enable-account payto://iban/CH9300762011623852957

The resulting JSON output must be uploaded to the exchange using
``taler-exchange-offline upload``.
For details, see :doc:`manpages/taler-exchange-offline.1`.


.. _Wire-fee-structure:

Wire fee structure
^^^^^^^^^^^^^^^^^^

.. index:: wire fee
.. index:: fee

For each wire method (“sepa” or “x-taler-wire”) the
exchange must know about applicable wire fees. This is also done
using the ``taler-exchange-offline`` tool:

.. code-block:: console

   $ taler-exchange-offline wire-fee iban 2040 EUR:0.05 EUR:0.10

The above sets the wire fees for wire transfers involving ``iban`` accounts
(in Euros) in the year 2040 to 5 cents (wire fee) and 10 cents (closing fee).
The tool only supports setting fees that applies for the entire calendar year.

We recommend provisioning an exchange with wire fees at least for the next two
years.  Note that once the fees have been set for a year, they cannot be
changed (basically, by signing the fees the exchange makes a legally binding
offer to the customers).

.. index:: maintenance
.. note::
   Provisioning future wire fees, like provisioning future denomination
   and signing keys, are key regular maintenance procedures for every
   exchange operator.  We recommend setting automated reminders for
   this maintenance activity!


.. _Auditor-configuration:

Auditor configuration
---------------------

The exchange must be informed about any auditor that is expected to provision
it with auditor signatures.  This is also done using the
``taler-exchange-offline`` tool on the offline system.  First, the auditor
must be configured and provide the exchange operator with its public key and
the URL of it's REST API.  The exchange operator also needs a human-readable
name that may be shown to users to identify the auditor.  Given this
information, the exchange operator can enable the auditor:

.. code-block:: console

   $ taler-exchange-offline enable-auditor $PUB_KEY $REST_URL "$AUDITOR_NAME" > auditor.json

As before, the *auditor.json* file must then be copied from the offline system
to a system connected to the exchange and there ``uploaded`` to the exchange.



.. _Deployment:

Deployment
==========

This chapter describes how to deploy the exchange once it has been
configured.


.. _Launch:

Launching an exchange
---------------------

A running exchange requires starting the following processes:

-   ``taler-exchange-secmod-rsa`` (as special user, sharing group with the HTTPD)
-   ``taler-exchange-secmod-eddsa`` (as special user, sharing group with the HTTPD)
-   ``taler-exchange-httpd`` (needs database access)
-   ``taler-exchange-aggregator`` (only needs database access)
-   ``taler-exchange-closer`` (only needs database access)
-   ``taler-exchange-wirewatch`` (needs bank account read credentials and database access)
-   ``taler-exchange-transfer`` (needs credentials to initiate outgoing wire transfers and database access)

The crypto helpers must be started before the ``taler-exchange-httpd`` and
they should use the same configuration file.

For the most secure deployment, we recommend using separate users for each of
these processes to minimize information disclosures should any of them be
compromised.  The helpers do not need access to the Postgres database (and
thus also should not have it).

The processes that require access to the bank account need to have a
configuration file with the respective credentials in it. We recommend using a
separate configuration at least for ``taler-exchange-transfer`` which is the
*only* process that needs to know the credentials to execute outgoing wire
transfers.

All of these processes should also be started via a hypervisor like
``systemd`` or ``gnunet-arm`` that automatically re-starts them should they
have terminated unexpectedly.  If the bank is down (say for maintenance), it is
*possible* to halt the ``taler-exchange-wirewatch`` and/or
``taler-exchange-transfer`` processes (to avoid them making requests to the
bank API that can only fail) without impacting other operations of the
exchange. Naturally, incoming wire transfers will only be observed once
``taler-exchange-wirewatch`` is resumed, and merchants may complain if the
disabled ``taler-exchange-transfer`` process causes payment deadlines to be
missed.

.. note::
   The ``taler-exchange-httpd`` does not ship with HTTPS enabled by default.
   In production, it should be run behind an HTTPS reverse proxy that performs
   TLS termination on the same system.  Thus, it would typically be configured
   to listen on a UNIX domain socket.  The ``/management`` and ``/auditors``
   APIs do technically not have to be exposed on the Internet (only to the
   administrators running ``taler-exchange-offline``) and should be blocked
   by the reverse proxy for requests originating from outside of the bank.
   (However, this is not a strong security assumption: in principle having
   these endpoints available should do no harm. However, it increases the
   attack surface.)


.. _Keys-generation:

Keys generation
---------------

Once the configuration is properly set up, all the keys can be signed using
the offline key on the offline system by the tool ``taler-exchange-offline``.
To do this, one must first start the crypto helpers and the ``taler-exchange-httpd``
process (the tools for wire transfers may also be started, but do not have to
run yet).

Next, the *future* key material should be downloaded using:

.. code-block:: console

   $ taler-exchange-offline download > future-keys.json

Afterwards, *future-keys.json* contains data about denomination and
online signing keys that the exchange operator needs to sign with
the offline tool.  The file should be copied to the offline system.
There, the operator should run:

.. code-block:: console

   $ taler-exchange-offline show < future-keys.json

and verify that the output contains the fee structure and key lifetimes
they expect to see. They should also note the public keys being shown
and communicate those to the *auditors* over a secure channel.  Once
they are convinced the file is acceptable, they should run:

.. code-block:: console

   $ taler-exchange-offline sign < future-keys.json > offline-sigs.json

The *offline-sigs.json* file must then be copied to an online system
that is able to again communicate with the exchange. On that system, run:

.. code-block:: console

   $ taler-exchange-offline upload < offline-sigs.json

to provision the signatures to the exchange.  At this point, the
exchange will be able to use those keys, but wallets and merchants
may not yet trust them!  Thus, the next step is for the auditor
to affirm that they are auditing this exchange.  Details about
this are described in :doc:`taler-auditor-manual`.

The simplistic (without using offline keys for the auditor) way
to do this would be:

.. code-block:: console

   $ taler-auditor-offline download sign upload

For more information, see :doc:`manpages/taler-auditor-offline.1`.


Private key storage
-------------------

Keeping the private keys the helpers create secret is paramount. If the
private keys are lost, it is easy to provision fresh keys (with the help of
the auditor).  Thus, we recommend that the private keys of the crypto helpers
are *not* backed up: in the rare event of a disk failure, they can be
regenerated.  However, we do recommend using RAID (1+1 or 1+1+1) for all
disks of the system.


.. _Database-upgrades:

Database upgrades
-----------------

Currently, there is no way to upgrade the database between Taler
versions.

The exchange database can be re-initialized using:

.. code-block:: console

   $ taler-exchange-dbinit -r

However, running this command will result in all data in the database
being lost, which may result in significant financial liabilities as the
exchange can then not detect double-spending. Hence this operation must
not be performed in a production system.


.. _Revocations:

Revocations
^^^^^^^^^^^

When an exchange goes out of business or detects that the private key of
a denomination key pair has been compromised, it may revoke some or all
of its denomination keys. At this point, the hashes of the revoked keys
must be returned as part of the ``/keys`` response under “recoup”.
Wallets detect this, and then return unspent coins of the respective
denomination key using the ``/recoup`` API.

To revoke a denomination key, you need to know the hash of the denomination
public key, ``$HDP``.  The ``$HDP`` value is usually included in the security
report that is generated when a compromise is detected).  Given this
value, the key revocation can be approved on the offline system:

.. code-block:: console

   $ taler-exchange-offline revoke-denominatin $HDP > revocation.json

The resulting *revocation.json* must be copied to a system connected to the
exchange and uploaded to the exchange using the ``upload`` subcommand
of ``taler-exchange-offline``.

.. note::
   Denomination key revocations should only happen
   under highly unusual (“emergency”) conditions and not in normal
   operation.

Testing a deployment
====================

We recommend testing whether an exchange deployment is functional by using the
Taler wallet command line interface.  The tool can be used to withdraw and
deposit electronic cash via the exchange without having to deploy and operate a
separate merchant backend and storefront.  For more information, see
:doc:`taler-wallet-cli-manual`.


.. _Diagnostics:

Diagnostics
===========

This chapter includes various sections on specific topics that might be
helpful to understand how the exchange operates. The information may also be
helpful for diagnostics.

.. _Internal-audit:

Internal audits
---------------

While an exchange should use an external auditor to attest to regulators that
it is operating correctly, an exchange operator can also use the auditor's
logic to perform internal checks.  For this, an exchange opeator can generally
follow the auditor guide.  However, instead of using ``taler-auditor-sync``,
an internal audit can and likely should be performed either directly against
the production exchange database or against a synchronous copy created using
standard database replication techniques. After all, the exchange operator
runs this for diagnostics and can generally trust its own database to maintain
the database invariants.

Running the auditor against a the original the production database (without
using ``taler-auditor-sync``) enables the auditing logic to perform a few
additional checks that can detect inconsistencies.  These checks are enabled
by passing the **-i** option to the ``taler-auditor`` command.  As always,
the resulting report should be read carefully to see if there are any problems
with the setup.

Reports are generally created incrementally, with ``taler-auditor`` reporting
only incidents and balance changes that were not covered in previous reports.
While it is possible to reset the auditor database and to restart the audit
from the very beginning, this is generally not recommended as this may be too
expensive.


.. _Database-Scheme:

Database Scheme
---------------

The exchange database must be initialized using ``taler-exchange-dbinit``.
This tool creates the tables required by the Taler exchange to operate.
The tool also allows you to reset the Taler exchange database, which is
useful for test cases but should never be used in production. Finally,
``taler-exchange-dbinit`` has a function to garbage collect a database,
allowing administrators to purge records that are no longer required.

The database scheme used by the exchange looks as follows:

.. image:: exchange-db.png


.. _ExchangeBenchmarking:

Benchmarking
============

This chapter describes how to run the Taler exchange benchmark.  The benchmark
can be used to measure the performance of the exchange by running a (possibly
large) number of simulated clients against one Taler deployment with a bank,
exchange and auditor.  For the bank, both a "fakebank" (``-f``) and a
"Pythonbank" deployment are currently supported.  The
``taler-exchange-benchmark`` program can launch all required services and
clients, or only launch the parallel clients (``-m``), for example for
distributed testing over a network.

For each *parallel* (``-p``) client, a number of *reserves* (``-r``) is first established by
**transferring** money from a "user" account (42) to the Exchange's account
with the respective reserve public key as wire subject.  Next, the
client will **withdraw** a *number of coins* (``-n``) from the reserve and
**deposit** them. Additionally, a *fraction* (``-R``) of the dirty coins will then be
subject to **refreshing**.  For some deposits, the auditor will receive
**deposit confirmations**.

Operations that are not covered today include closing reserves, refunds and
recoups.

The existing ``benchmark.conf`` file in ``src/benchmark/`` can be used as a
starting point for a configuration to run the benchmark. The existing
configuration file only requires that the ``talercheck`` database already
exists and will launch all required services locally as needed.

You can run a first simple benchmark using:

.. note::
   FIXME-TTN/CG: these instructions are incomplete and untested for the
   current iteration of the code...

.. code-block:: console

   $ createdb talercheck # if it does not yet exist
   $ taler-exchange-dbinit -c benchmark.conf
   $ taler-exchange-httpd -c benchmark.conf &
   $ HTTPD_PID=$!
   $ taler-exchange-offline -c benchmark.conf \
     download sign \
     enable-account FIXME-DETAILS-MISING-HERE \
     wire-fee FIXME-DETAILS-MISING-HERE \
     upload
   $ kill -TERM $HTTPD_PID
   $ taler-exchange-benchmark -c benchmark.conf -p 4 -r 1 -n 10

This will run 4 parallel clients withdrawing 10 coins from 1 reserve and then
depositing those coins. The default refresh probability is 10 percent.  Note
that the tiny run should only take a few seconds, most of it will be spent in
the setup of the original key material. For meaningful runs, all three values
should likely be increased.

The output of ``taler-exchange-benchmark`` will include for each parallel
client the total time spent in each of the major operations, possible
repetitions (i.e. if the operation failed the first time), total execution
time (operating system and user space) and other details.

Naturally, additional instrumentation (including using features of the
Postgres database itself) may help discover performance issues.