7. GNU Taler Exchange Operator Manual

7.1. Introduction

7.1.1. 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).

7.1.2. About this manual

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

7.1.3. 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. Exceptions may apply, especially if you are operating a regional currency or a payment system for an event with a closed user group.

GNU Taler payment service providers generally need to ensure high availability and should 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 (sufficiently high-availability, high-assurance) PostgreSQL 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.

Note

Taler may store sensitive business and customer data in the database. Any operator SHOULD thus ensure that backup operations are encrypted and secured from unauthorized access.

7.1.4. Architecture overview

GNU Taler is a pure payment system, not a crypto-currency. As such, it operates in a traditional banking context. In particular, this means that payments can be executed in ordinary currencies such as USD or EUR. Furthermore, a typical merchant in Taler has a regular bank account, and would use it to receive funds via Taler.

Consequently, a typical Taler exchange must interact with a bank. The bank of the exchange holds funds in an account where the balance is basically equivalent to the value of all coins in circulation. (Small mismatches arise whenever customers are about to withdraw coins and have already send the funds into the bank account, or if merchants just deposited coins and are about to receive wire transfers for deposited coins, or due to fees charged by the exchange and the operator not yet having drained the fees from the account.)

The exchange uses an intermediary system to talk to its bank. This shifts the technical burden (XML-based communications, additional cryptography, and a vast variety of standards) for this interaction into another bank-specific subsystem. 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 exchange’s bank account, the Taler Wire Gateway API must notify 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, taler-exchange-secmod-cs and taler-exchange-secmod-eddsa are three 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. Each wire adapter must implement the Taler Wire Gateway API. Three wire adapters are currently provided:

    1. The libtalerfakebank implements a bank with a wire adapter API inside of a testcase. taler-fakebank-run is a stand-alone process using libtalerfakebank. Note that this adapter is only useful for tests, as all transaction data is kept in memory.
    2. For production, libeufin’s libeufin-nexus component implements a wire adapter towards the traditional SEPA banking system with IBAN accounts using the EBICS protocol.
    3. To use GNU Taler with blockchains, the Depolymerization component provides a wire gateway API that runs on top of blockchains like Bitcoin and Ethereum.

    The client-side wire adapter API is implemented in libtalerbank and is used by taler-exchange-transfer to execute wire transfers and by taler-exchange-wirewatch and the Taler auditor 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 PostgreSQL, but the code could be easily extended to support another DBMS. .. index:: PostgreSQL

  • 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 exchange’s database.

7.1.5. Key Types

The exchange works with four types of keys:

  • master key (kept offline, configured manually at merchants and wallets)
  • online message signing keys (signs normal messages from the exchange)
  • denomination keys (signs digital coins)
  • security module keys (signs online message signing 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).

Most of the keys are managed fully automatically or configured as part of the denomination configuration. Some configuration settings must be manually set with regards to the exchange’s master key.

7.1.6. Offline keys

The exchange (and ideally also its auditor(s)) 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 their private master key and any copies on encrypted, always-offline computers. Again, this manual assumes that you are familiar with good best practices in operational security, including securing key material.

7.1.7. 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 three helper processes, taler-exchange-secmod-rsa, taler-exchange-secmod-cs and 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 of the secmod helpers have mode 0620 (u+rw, g+w). The exchange process MUST thus be in the same group as the crypto helper processes to enable access to the keys. No other users should be in that group!

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. To get any security benefit from this, 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.

Configuration

The helpers and the exchange 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.

7.2. 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.

We recommend the setup of offline signing keys to be done on a second machine that does not have Internet access.

In this guide’s shell-session fragments, the command prompt shows two pieces of information:

  • Who is performing the command ($user vs root, and ending character $ vs #).
  • Host where the command is supposed to be executed (exchange-offline vs exchange-online). It is possible to do the entire setup on one machine, but we do not recommend this for security reasons.

7.2.1. Before you start

To deploy this with a real bank, you need:

  • IBAN of the bank account to use
  • BIC of the bank
  • EBICS host, user and partner IDs

Information to write down during the installation:

  • LibEuFin Nexus superuser password
  • Taler facade base URL
  • exchange Nexus username and password

7.2.2. Installing from source

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

The package sources can be find in our download directory.

GNU Taler components version numbers follow the MAJOR.MINOR.MICRO format. The general rule for compatibility is that MAJOR and MINOR must match. Exceptions to this general rule are documented in the release notes. For example, Taler merchant 1.3.0 should be compatible with Taler exchange 1.4.x as the MAJOR version matches. A MAJOR version of 0 indicates experimental development, and you are expected to always run all of the latest releases together (no compatibility guarantees).

First, the following packages need to be installed before we can compile the backend:

  • “Sphinx RTD Theme” Python package aka python3-sphinx-rtd-theme on Debian-based systems (for GNUnet documentation support, can be omitted if GNUnet is configured with --disable-documentation)
  • libsqlite3 >= 3.16.2
  • GNU libunistring >= 0.9.3
  • libcurl >= 7.26 (or libgnurl >= 7.26)
  • libqrencode >= 4.0.0 (Taler merchant only)
  • GNU libgcrypt >= 1.6 (1.10 or later highly recommended)
  • libsodium >= 1.0
  • libargon2 >= 20171227
  • libjansson >= 2.7
  • PostgreSQL >= 13, including libpq
  • GNU libmicrohttpd >= 0.9.71
  • GNUnet >= 0.19 (from source tarball)
  • Python3 with jinja2

If you are on Debian stable or later, the following command may help you install these dependencies:

# apt-get install \
  libqrencode-dev \
  libsqlite3-dev \
  libltdl-dev \
  libunistring-dev \
  libsodium-dev \
  libargon2-dev \
  libcurl4-gnutls-dev \
  libgcrypt20-dev \
  libjansson-dev \
  libpq-dev \
  libmicrohttpd-dev \
  python3-jinja2 \
  postgresql-13

Before you install GNUnet, you must download and install the dependencies mentioned in the previous section, otherwise the build may succeed, but could fail to export some of the tooling required by GNU Taler.

To install GNUnet, unpack the tarball and change into the resulting directory, then proceed as follows:

$ ./configure [--prefix=GNUNETPFX]
$ # Each dependency can be fetched from non standard locations via
$ # the '--with-<LIBNAME>' option. See './configure --help'.
$ make
# make install
# ldconfig

If you did not specify a prefix, GNUnet will install to /usr/local, which requires you to run the last step as root. The ldconfig command (also run as root) makes the shared object libraries (.so files) visible to the various installed programs.

Please note that unlike most packages, if you want to run the make check command, you should run it only after having done make install. The latter ensures that necessary binaries are copied to the right place.

In any case, if make check fails, please consider filing a bug report with the Taler bug tracker.

There is no need to actually run a GNUnet peer to use the Taler merchant backend – all the merchant needs from GNUnet is a number of headers and libraries!

After installing GNUnet, unpack the GNU Taler exchange tarball, change into the resulting directory, and proceed as follows:

$ ./configure [--prefix=EXCHANGEPFX] \
              [--with-gnunet=GNUNETPFX]
$ # Each dependency can be fetched from non standard locations via
$ # the '--with-<LIBNAME>' option. See './configure --help'.
$ make
# make install

If you did not specify a prefix, the exchange will install to /usr/local, which requires you to run the last step as root. You have to specify --with-gnunet=/usr/local if you installed GNUnet to /usr/local in the previous step.

Please note that unlike most packages, if you want to run the make check command, you should run it only after having done make install. The latter ensures that necessary binaries are copied to the right place.

In any case, if make check fails, please consider filing a bug report with the Taler bug tracker.

7.2.3. Installing the GNU Taler binary packages on Debian

To install the GNU Taler Debian packages, first ensure that you have the right Debian distribution. At this time, the packages are built for Debian bookworm.

You need to add a file to import the GNU Taler packages. Typically, this is done by adding a file /etc/apt/sources.list.d/taler.list that looks like this:

deb [signed-by=/etc/apt/keyrings/taler-systems.gpg] https://deb.taler.net/apt/debian stable main

Next, you must import the Taler Systems SA public package signing key into your keyring and update the package lists:

# wget -P /etc/apt/keyrings/ \
    https://taler.net/taler-systems.gpg
# apt update

Note

You may want to verify the correctness of the Taler Systems SA key out-of-band.

Now your system is ready to install the official GNU Taler binary packages using apt.

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

[root@exchange-online]# 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.

On the offline system, you should run at least:

[root@exchange-offline]# apt install taler-exchange-offline

7.2.4. Installing the GNU Taler binary packages on Trisquel

To install the GNU Taler Trisquel packages, first ensure that you have the right Trisquel distribution. Packages are currently available for Trisquel GNU/Linux 10.0. Simply follow the same instructions provided for Ubuntu.

7.2.5. Installing the GNU Taler binary packages on Ubuntu

To install the GNU Taler Ubuntu packages, first ensure that you have the right Ubuntu distribution. At this time, the packages are built for Ubuntu Kinetic.

A typical /etc/apt/sources.list.d/taler.list file for this setup would look like this:

deb [signed-by=/etc/apt/keysrings/taler-systems.gpg] https://deb.taler.net/apt/ubuntu/ stable main

The last line is crucial, as it adds the GNU Taler packages.

Next, you must import the Taler Systems SA public package signing key into your keyring and update the package lists:

# wget -P /etc/apt/keyrings/ \
    https://taler.net/taler-systems.gpg
# apt update

Note

You may want to verify the correctness of the Taler Systems key out-of-band.

Now your system is ready to install the official GNU Taler binary packages using apt.

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

[root@exchange-online]# 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.

On the offline system, you should run at least:

[root@exchange-offline]# apt install taler-exchange-offline

7.2.6. Services, users, groups and file system hierarchy

The taler-exchange package will create several system users to compartmentalize different parts of the system:

  • taler-exchange-httpd: runs the HTTP daemon with the core business logic.
  • taler-exchange-secmod-rsa: manages the RSA private online signing keys.
  • taler-exchange-secmod-cs: manages the CS private online signing keys.
  • taler-exchange-secmod-eddsa: manages the EdDSA private online signing keys.
  • taler-exchange-closer: closes idle reserves by triggering wire transfers that refund the originator.
  • taler-exchange-aggregator: aggregates deposits into larger wire transfer requests.
  • taler-exchange-transfer: performs wire transfers with the bank (via LibEuFin/Nexus).
  • taler-exchange-wirewatch: checks for incoming wire transfers with the bank (via LibEuFin/Nexus).
  • postgres: runs the PostgreSQL database (from postgresql package).
  • www-data: runs the frontend HTTPS service with the TLS keys (from nginx package).

Note

The taler-merchant package additionally creates a taler-merchant-httpd user to run the HTTP daemon with the merchant business logic.

The exchange setup uses the following system groups:

  • taler-exchange-db: group for all Taler users with direct database access, specifically taler-exchange-httpd, taler-exchange-wire, taler-exchange-closer and taler-exchange-aggregator.
  • taler-exchange-secmod: group for processes with access to online signing keys; this group must have four users: taler-exchange-secmod-rsa, taler-exchange-secmod-cs, taler-exchange-secmod-eddsa and taler-exchange-httpd.
  • taler-exchange-offline: group for the access to the offline private key (only used on the offline host and not used on the online system).

The package will deploy systemd service files in /usr/lib/systemd/system/ for the various components:

  • taler-exchange-aggregator.service: service that schedules wire transfers which combine multiple deposits to the same merchant.
  • taler-exchange-closer.service: service that watches for reserves that have been abandoned and schedules wire transfers to send the money back to the originator.
  • taler-exchange-httpd.service: main Taler exchange logic with the public REST API.
  • taler-exchange-httpd.socket: systemd socket activation for the Taler exchange HTTP daemon.
  • taler-exchange-secmod-eddsa.service: software security module for making EdDSA signatures.
  • taler-exchange-secmod-rsa.service: software security module for making RSA signatures.
  • taler-exchange-secmod-cs.service: software security module for making CS signatures.
  • taler-exchange-transfer.service: service that triggers outgoing wire transfers (pays merchants).
  • taler-exchange-wirewatch.service: service that watches for incoming wire transfers (first step of withdraw).
  • taler-exchange.target: Main target for the Taler exchange to be operational.

The deployment creates the following key locations in the system:

  • /etc/taler/: configuration files.
  • /run/taler/: contains the UNIX domain sockets for inter-process communication (IPC).
  • /var/lib/taler/: serves as the $HOME for all Taler users and contains sub-directories with the private keys; which keys are stored here depends on the host:
    • online system: exchange-secmod-eddsa, exchange-secmod-cs and exchange-secmod-rsa keys.
    • offline system: exchange-offline keys.

7.3. Configuration Fundamentals

This chapter provides fundamental details about the exchange configuration.

The configuration for all Taler components uses a single configuration file as entry point: /etc/taler/taler.conf.

System defaults are automatically loaded from files in /usr/share/taler/config.d. These default files should never be modified.

The default configuration taler.conf configuration file also includes all configuration files in /etc/taler/conf.d. The settings from files in conf.d are only relevant to particular components of Taler, while taler.conf contains settings that affect all components.

The directory /etc/taler/secrets contains configuration file snippets with values that should only be readable to certain users. They are included with the @inline-secret@ directive and should end with .secret.conf.

To view the entire configuration annotated with the source of each configuration option, you can use the taler-config helper:

[root@exchange-online]# taler-config --diagnostics
< ... annotated, full configuration ... >

Warning

While taler-config also supports rewriting configuration files, we strongly recommend to edit configuration files manually, as taler-config does not preserve comments and, by default, rewrites /etc/taler/taler.conf.

7.3.1. Configuration format

All GNU Taler components are designed to possibly share the same configuration files. When installing a GNU Taler component, the installation deploys default values in configuration files located at ${prefix}/share/taler/config.d/ where ${prefix} is the installation prefix. Different components must be installed to the same prefix.

In order to override these defaults, the user can write a custom configuration file and either pass it to the component at execution time using the -c option, or name it taler.conf and place it under $HOME/.config/ which is where components will look by default. Note that the systemd service files pass -c /etc/taler.conf, thus making /etc/taler.conf the primary location for the configuration.

A config file is a text file containing sections, and each section contains maps options to their values. Configuration files follow basically the INI syntax:

[section1]
value1 = string
value2 = 23

[section2]
value21 = string
value22 = /path22

Comments start with a hash (#). Throughout the configuration, it is possible to use $-substitution for options relating to names of files or directories. It is also possible to provide defaults values for those variables that are unset, by using the following syntax: ${VAR:-default}. There are two ways a user can set the value of $-prefixable variables:

  1. by defining them under a [paths] section:
[paths]
TALER_DEPLOYMENT_SHARED = ${HOME}/shared-data
..
[section-x]
path-x = ${TALER_DEPLOYMENT_SHARED}/x
  1. or by setting them in the environment:
$ export VAR=/x

The configuration loader will give precedence to variables set under [path] over environment variables.

The utility taler-config, which gets installed along with the exchange, can be used get and set configuration values without directly editing the configuration file. The option -f is particularly useful to resolve pathnames, when they use several levels of $-expanded variables. See taler-config --help.

The repository git://git.taler.net/deployment contains example code for generating configuration files under deployment/netzbon/.

7.3.2. Using taler-config

The tool taler-config can be used to extract or manipulate configuration values; however, the configuration use the well-known INI file format and is generally better edited by hand to preserve comments and structure.

Run

$ taler-config -s $SECTION

to list all of the configuration values in section $SECTION.

Run

$ taler-config -s $SECTION -o $OPTION

to extract the respective configuration value for option $OPTION in section $SECTION.

Finally, to change a setting, run

$ taler-config -s $SECTION -o $OPTION -V $VALUE

to set the respective configuration value to $VALUE. Note that you have to manually restart affected Taler components after you change the configuration to make the new configuration go into effect.

Some default options will use $-variables, such as $DATADIR within their value. To expand the $DATADIR or other $-variables in the configuration, pass the -f option to taler-config. For example, compare:

$ taler-config --section exchange-offline --option MASTER_PRIV_FILE
$ taler-config -f --section exchange-offline --option MASTER_PRIV_FILE

While the configuration file is typically located at $HOME/.config/taler.conf, an alternative location can be specified to any GNU Taler component using the -c option.

7.4. Exchange Database Setup

The access credentials for the exchange’s database are configured in /etc/taler/secrets/exchange-db.secret.conf. Currently, only PostgreSQL is supported as a database backend.

The following users must have access to the exchange database:

  • taler-exchange-httpd
  • taler-exchange-wire
  • taler-exchange-aggregator
  • taler-exchange-closer

These users are all in the taler-exchange-db group, and the exchange-db.secret.conf should already be only readable by users in this group.

To create a database for the Taler exchange on the local system, run:

[root@exchange-online]# su - postgres
[postgres@exchange-online]# createuser taler-exchange-httpd
[postgres@exchange-online]# createuser taler-exchange-wire
[postgres@exchange-online]# createuser taler-exchange-aggregator
[postgres@exchange-online]# createuser taler-exchange-closer
[postgres@exchange-online]# createdb -O taler-exchange-httpd taler-exchange
[postgres@exchange-online]# exit

This will create a taler-exchange database owned by the taler-exchange-httpd user. We will use that user later to perform database maintenance operations.

Assuming the above database setup, the database credentials to configure in the configuration file would simply be:

/etc/taler/secrets/exchange-db.secret.conf
[exchange]
DB = postgres

[exchangedb-postgres]
CONFIG=postgres:///taler-exchange

If the database is run on a different host, please follow the instructions from the PostgreSQL manual for configuring remote access.

After configuring the database credentials, the exchange database needs to be initialized with the following command:

[root@exchange-online]# sudo -u taler-exchange-httpd taler-exchange-dbinit

..note::

  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`.

Finally we need to grant the other accounts limited access:

[root@exchange-online]# sudo -u taler-exchange-httpd bash
[taler-exchange-httpd@exchange-online]# echo 'GRANT SELECT,INSERT,UPDATE ON ALL TABLES IN SCHEMA public TO "taler-exchange-aggregator";' \
  | psql taler-exchange
[taler-exchange-httpd@exchange-online]# echo 'GRANT SELECT,INSERT,UPDATE ON ALL TABLES IN SCHEMA public TO "taler-exchange-closer";' \
  | psql taler-exchange
[taler-exchange-httpd@exchange-online]# echo 'GRANT SELECT,INSERT,UPDATE ON ALL TABLES IN SCHEMA public TO "taler-exchange-wire";' \
  | psql taler-exchange
[taler-exchange-httpd@exchange-online]# echo 'GRANT USAGE ON ALL SEQUENCES IN SCHEMA public TO "taler-exchange-aggregator";' \
  | psql taler-exchange
[taler-exchange-httpd@exchange-online]# echo 'GRANT USAGE ON ALL SEQUENCES IN SCHEMA public TO "taler-exchange-closer";' \
  | psql taler-exchange
[taler-exchange-httpd@exchange-online]# echo 'GRANT USAGE ON ALL SEQUENCES IN SCHEMA public TO "taler-exchange-wire";' \
  | psql taler-exchange
[taler-exchange-httpd@exchange-online]# exit

Note

The above instructions for changing database permissions only work after having initialized the database with taler-exchange-dbinit, as the tables to exist before permissions can be granted on them. The taler-exchange-dbinit tool cannot setup these permissions, as it does not know which users will be used for which processes.

7.5. Basic Setup: Currency, Denominations and Keys

A Taler exchange only supports a single currency. The currency and the smallest currency unit supported by the bank system must be specified in /etc/taler/taler.conf.

/etc/taler/taler.conf
 [taler]
 CURRENCY = EUR
 CURRENCY_ROUND_UNIT = EUR:0.01

 # ... rest of file ...

Warning

When editing /etc/taler/taler.conf, take care to not accidentally remove the @inline-matching@ directive to include the configuration files in conf.d.

7.5.1. Coins (denomination keys)

Next, the electronic cash denominations that the exchange offers must be specified.

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.
  • CIPHER: Which cipher to use for this coin? Must be either RSA or CS.
  • RSA_KEYSIZE: How many bits should the RSA modulus (product of the two primes) have for this type of coin.
  • AGE_RESTRICTED: Set to YES to make this a denomination with support
    for age restrictions. See age restriction extension below for details. This option is optional and defaults to NO.

See 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.

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!

The taler-wallet-cli has a helper command that generates a reasonable denomination structure.

[root@exchange-online]# taler-wallet-cli deployment gen-coin-config \
                          --min-amount EUR:0.01 \
                          --max-amount EUR:100 \
                          > /etc/taler/conf.d/exchange-coins.conf

You can manually review and edit the generated configuration file. The main change that is possibly required is updating the various fees. Note that you MUST NOT edit a coin configuration section after the initial setup. If you must change the values, you must instead create a new section with a different unique name (still with the coin- prefix) and comment out or remove the existing section. Do take care to not introduce the name of the disabled section again in the future.

7.5.2. 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.

7.5.3. Setting up the offline signing key

Before launching an exchange, the offline signing (master) key must be generated and set in the configuration. The offline signing keys of the exchange should be stored on a different machine. The responsibilities of this offline signing machine are:

  • Generation of the exchange’s offline master signing key.
  • Secure storage of the exchange’s offline master signing key.
  • Generation of certificates (signed with the offline master signing key) that will be imported by the exchange.
  • Revocation of keys when the online system was compromised or is being terminated

Configuration file options related to the master key are:

  • [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. The default value is usually fine and does not require adjustment.
  • [exchange/MASTER_PUBLIC_KEY]: Must specify the exchange’s master public
    key. Needed for the exchange to verify information signed by the offline system. This value must almost always be set explicitly by hand.
[root@exchange-offline]# taler-exchange-offline setup
< ... prints the exchange master public key >

The public key printed as the output of this command must be put into the configuration of the online machine:

/etc/taler/conf.d/exchange-business.conf
 [exchange]
 MASTER_PUBLIC_KEY = YE6Q6TR1ED...

 # ... rest of file ...

7.6. Wire Gateway Setup

The Taler Wire Gateway is an API that connects the Taler exchange to the underlying core banking system.

LibEuFin is an implementation of the Wire Gateway API for the EBICS protocol. This section will walk through (1) installing and configuring LibEuFin and (2) connecting the Taler Exchange to LibEuFin.

Note

If you do not have a bank account with EBICS but want to test these instructions, you can use the EBICS sandbox as described in the LibEuFin Tutorial.

7.6.1. Installation and Basic Configuration

First, install the libeufin package. This can be done on the exchange-online machine or a different one.

[root@exchange-online]# apt-get install -y libeufin

The main component of LibEuFin is called the Nexus. It implements a Web service that provides a JSON abstraction layer to access bank accounts.

The HTTP port and database connection string can be edited in the configuration:

/etc/libeufin/nexus.env
LIBEUFIN_NEXUS_PORT=5017
LIBEUFIN_NEXUS_DB_CONNECTION=jdbc:sqlite:/var/lib/libeufin/nexus/nexus-db.sqlite3

After configuring the database, you can start the service. The database is initialized automatically.

[root@exchange-online]# systemctl enable libeufin-nexus
[root@exchange-online]# systemctl start libeufin-nexus

You can now create a superuser account. The command to create the superuser needs direct database access, thus the configuration file is sourced first, and the relevant environment variable is exported.

[root@exchange-online]# source /etc/libeufin/nexus.env
[root@exchange-online]# export LIBEUFIN_NEXUS_DB_CONNECTION
[root@exchange-online]# NEXUS_ADMIN_PW=$(tr -dc A-Za-z0-9 </dev/urandom | head -c 13)
[root@exchange-online]# libeufin-nexus superuser admin --password $NEXUS_ADMIN_PW

If you omit --password $NEXUS_ADMIN_PW, you will interactively be asked for a password. For simplicity, a superuser can as well act as a normal user, but an API to create less privileged users is offered.

Note

User and permissions management in LibEuFin is still under development. In particular, permissions for non-superusers are very limited at the moment.

7.6.2. Connecting Nexus with an EBICS account

The command line interface of the LibEuFin Nexus needs the following three values to be defined in the environment: LIBEUFIN_NEXUS_URL, LIBEUFIN_NEXUS_USERNAME, and LIBEUFIN_NEXUS_PASSWORD. In this example, LIBEUFIN_NEXUS_USERNAME should be set to admin, and LIBEUFIN_NEXUS_PASSWORD to the value hold in NEXUS_ADMIN_PW from the previous step (the libeufin-nexus superuser command). The LIBEUFIN_NEXUS_URL could be given as http://localhost:5017/.

Next, we create a EBICS bank connection that Nexus can use to communicate with the bank.

[root@exchange-online]# libeufin-cli \
    connections \
      new-ebics-connection \
        --ebics-url $EBICS_BASE_URL \
        --host-id $EBICS_HOST_ID \
        --partner-id $EBICS_PARTNER_ID \
        --ebics-user-id $EBICS_USER_ID \
        $CONNECTION_NAME

If this step executes correctly, Nexus will have created all the cryptographic material that is needed on the client side; in this EBICS example, it created the signature and identification keys. It is therefore advisable to make a backup copy of such keys.

[root@exchange-online]# libeufin-cli \
    connections \
      export-backup \
        --passphrase $SECRET \
        --output-file $BACKUP_FILE \
        $CONNECTION_NAME

At this point, Nexus needs to both communicate its keys to the bank, and download the bank’s keys. This synchronization happens through the INI, HIA, and finally, HPB message types.

After the electronic synchronization, the subscriber must confirm their keys by sending a physical mail to the bank. The following command helps generating such letter:

[root@exchange-online]# libeufin-cli connections get-key-letter $CONNECTION_NAME out.pdf

[root@exchange-online]# libeufin-cli \
    connections \
      connect \
        $CONNECTION_NAME

Once the connection is synchronized, Nexus needs to import locally the data corresponding to the bank accounts offered by the bank connection just made. The command below downloads the list of the bank accounts offered by $CONNECTION_NAME.

[root@exchange-online]# libeufin-cli \
    connections \
      download-bank-accounts \
        $CONNECTION_NAME

It is now possible to list the accounts offered by the connection.

[root@exchange-online]# libeufin-cli \
    connections \
      list-offered-bank-accounts \
        $CONNECTION_NAME

Note

The nexusBankAccountId field should at this step be null, as we have not yet imported the bank account and thus the account does not yet have a local name.

Nexus now needs an explicit import of the accounts it should manage. This step is needed to let the user pick a custom name for such accounts.

[root@exchange-online]# libeufin-cli \
    connections \
      import-bank-account \
        --offered-account-id testacct01 \
        --nexus-bank-account-id $LOCAL_ACCOUNT_NAME \
        $CONNECTION_NAME

Once a Nexus user imported a bank account ($LOCAL_ACCOUNT_NAME) under a certain connection ($CONNECTION_NAME), it is possible to accomplish the usual operations for any bank account: asking for the list of transactions, and making a payment.

Testing: Requesting the transaction history

The LibEuFin Nexus keeps a local copy of the bank account’s transaction history. Before querying transactions locally, it is necessary to request transactions for the bank account via the bank connection.

This command asks Nexus to download the latest transaction reports/statements through the bank connection:

[root@exchange-online]# libeufin-cli accounts fetch-transactions $LOCAL_ACCOUNT_NAME

Note

By default, the latest available transactions are fetched. It is also possible to specify a custom date range (or even all available transactions) and the type of transactions to fetch (inter-day statements or intra-day reports).

Once Nexus has stored all the information in the database, the client can ask to actually see the transactions:

[root@exchange-online]# libeufin-cli accounts transactions $LOCAL_ACCOUNT_NAME

Testing: Making payments

Payments pass through two phases: preparation and submission. The preparation phase assigns the payment initiation a unique ID, which prevents accidental double submissions of payments in case of network failures or other disruptions.

The following command prepares a payment:

[root@exchange-online]# libeufin-cli accounts prepare-payment \
        --creditor-iban=$IBAN_TO_SEND_MONEY_TO \
        --creditor-bic=$BIC_TO_SEND_MONEY_TO \
        --creditor-name=$CREDITOR_NAME \
        --payment-amount=$AMOUNT \
        --payment-subject=$SUBJECT \
        $LOCAL_ACCOUNT_NAME

Note: the $AMOUNT value needs the format X.Y:CURRENCY; for example EUR:10, or EUR:1.01.

The previous command should return a value ($UUID) that uniquely identifies the prepared payment in the Nexus system. That is needed in the next step, to send the payment instructions to the bank:

[root@exchange-online]# libeufin-cli accounts submit-payments \
      --payment-uuid $UUID \
      $LOCAL_ACCOUNT_NAME

Automatic scheduling

With an EBICS bank connection, the LibEuFin Nexus needs to regularly query for new transactions and (re-)submit prepared payments.

It is possible to schedule these tasks via an external task scheduler such as cron(8). However, the nexus also has an internal task scheduling mechanism for accounts.

The following three commands create a schedule for submitting payments hourly, fetching transactions (intra-day reports) every 5 minutes, and (inter-day statements) once at 11pm every day:

[root@exchange-online]# libeufin-cli accounts task-schedule $LOCAL_ACCOUNT_NAME \
    --task-type="submit" \
    --task-name='submit-payments-hourly' \
    --task-cronspec='0 0 *'

[root@exchange-online]# libeufin-cli accounts task-schedule $LOCAL_ACCOUNT_NAME \
    --task-type="fetch" \
    --task-name='fetch-5min' \
    --task-cronspec='0 */5 *' \
    --task-param-level=report \
    --task-param-range-type=latest

[root@exchange-online]# libeufin-cli accounts task-schedule $LOCAL_ACCOUNT_NAME \
    --task-type="fetch" \
    --task-name='fetch-daily' \
    --task-cronspec='0 0 23' \
    --task-param-level=statement \
    --task-param-range-type=latest

The cronspec has the following format, which is slightly non-standard due to the SECONDS field

SECONDS MINUTES HOURS DAY-OF-MONTH[optional] MONTH[optional] DAY-OF-WEEK[optional]

Creating a Taler facade

Facades are additional abstraction layers that can serve specific purposes. For example, one application might need a filtered version of the transaction history, or it might want to refuse payments that do not conform to certain rules.

At this moment, only the Taler facade type is implemented in the Nexus, and the command below instantiates one under a existing bank account / connection pair. You can freely assign an identifier for the $FACADE_NAME below:

[root@exchange-online]# libeufin-cli facades new-taler-wire-gateway-facade \
    --currency EUR \
    --facade-name $FACADE_NAME \
    $CONNECTION_NAME \
    $LOCAL_ACCOUNT_NAME

At this point, the additional taler-wire-gateway API becomes offered by the Nexus. The purpose is to let a Taler exchange rely on Nexus to manage its bank account.

The base URL of the facade that can be used by the Taler exchange as the Taler Wire Gateway base URL can be seen by listing the facades:

[root@exchange-online]# libeufin-cli facades list

Managing Permissions and Users

This guide has so far assumed that a superuser is accessing the LibEuFin Nexus. However, it is advisable that the Nexus is accessed with users that only have a minimal set of permissions.

The Nexus currently only has support for giving non-superusers access to Taler wire gateway facades.

To create a new user, use the users subcommand of the CLI:

[root@exchange-online]# libeufin-cli users list
# [ ... shows available users ... ]

[root@exchange-online]# libeufin-cli users create $USERNAME
# [ ... will prompt for password ... ]

Permissions are managed with the permissions subcommand. The following commands grant permissions to view the transaction history and create payment initiations with a Taler wire gateway facade:

[root@exchange-online]# libeufin-cli permissions grant \
   user $USERNAME \
   facade $FACADENAME \
   facade.talerwiregateway.history

[root@exchange-online]# libeufin-cli permissions grant \
   user $USERNAME \
   facade $FACADENAME \
   facade.talerwiregateway.transfer

The list of all granted permissions can be reviewed:

[root@exchange-online]# libeufin-cli permissions list

7.6.3. Exchange Bank Account Configuration

An exchange must be configured with the right settings to access its bank account via a Taler Wire Gateway. An exchange can be configured to use multiple bank accounts by using multiple Wire Gateways. Typically only one Wire Gateway is used.

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.

A Taler Wire Gateway is configured in a configuration section that follows the pattern exchange-account-$id, where $id is an internal identifier for the bank account accessed by the exchange. The basic information for an account should be put in /etc/taler/conf.d/exchange-business.conf. The secret credentials to access the Taler Wire Gateway API should be put into a corresponding exchange-accountcredentials-$id section in /etc/taler/secrets/exchange-accountcredentials.conf. The latter file should already be only readable for the taler-exchange-wire user. Other exchange processes should not have access to this information.

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):

/etc/taler/conf.d/exchange-business.conf
[exchange-account-1]
# Account identifier in the form of an RFC-8905 payto:// URI.
# For SEPA, looks like payto://iban/$IBAN?receiver-name=$NAME
# Make sure to URL-encode spaces in $NAME!
#
# With x-taler-bank (for Fakebank)
# PAYTO_URI = "payto://x-taler-bank/bank.demo.taler.net/Exchange?receiver-name=exop"
#
# Example using IBAN (for use with LibEuFin)
PAYTO_URI = "payto://iban/CH9300762011623852957?receiver=name=exop"

# 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

@inline-secret@ exchange-accountcredentials-1 ../secrets/exchange-accountcredentials.secret.conf
/etc/taler/secrets/exchange-accountcredentials.secret.conf
[exchange-accountcredentials-1]

# LibEuFin expects basic auth.
WIRE_GATEWAY_AUTH_METHOD = basic

# Username and password set in LibEuFin.
USERNAME = ...
PASSWORD = ...

# Base URL of the wire gateway set up with LibEuFin.
WIRE_GATEWAY_URL = ...

Such a Wire Gateway configuration can be tested with the following commands:

[root@exchange-online]# taler-exchange-wire-gateway-client \
  --section exchange-accountcredentials-1 --debit-history
[root@exchange-online]# taler-exchange-wire-gateway-client \
  --section exchange-accountcredentials-1 --credit-history

7.8. Deployment

This chapter describes how to deploy the exchange once the basic installation and configuration are completed.

7.8.1. 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---). Make sure to set it in such a way that your reverse proxy has permissions to access the UNIX domain socket. The default (660) assumes that the reverse proxy is a member of the group under which the exchange HTTP server is running.

7.8.2. Reverse Proxy Setup

By default, the taler-exchange-httpd service listens for HTTP connections on a UNIX domain socket. To make the service publicly available, a reverse proxy such as nginx should be used. We strongly recommend to configure nginx to use TLS.

The public URL that the exchange will be served under should be put in /etc/taler/conf.d/exchange-business.conf configuration file.

/etc/taler/conf.d/exchange-business.conf
 [exchange]
 BASE_URL = https://example.com/

 # ... rest of file ...

The taler-exchange package ships with a sample configuration that can be enabled in nginx:

[root@exchange-online]# vim /etc/nginx/sites-available/taler-exchange
< ... customize configuration ... >
[root@exchange-online]# ln -s /etc/nginx/sites-available/taler-exchange \
                              /etc/nginx/sites-enabled/taler-exchange
[root@exchange-online]# systemctl reload nginx

Note that the reverse proxy must set a HTTP X-Forwarded-Host header to refer to the hostname used by nginx and a HTTP X-Forwarded-Proto header to inform the exchange whether the external protocol was http or https. Thus, depending on your setup, you will likely have to edit those parts of the provided taler-exchange configuration file.

With this last step, we are finally ready to launch the main exchange process.

7.8.3. 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-cs (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 (secmod) 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 PostgreSQL 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.)

Given proper packaging, all of the above are realized via a simple systemd target. This enables the various processes of an exchange service to be started using a simple command:

[root@exchange-online]# systemctl start taler-exchange.target

Note

At this point, the exchange service is not yet fully operational.

To check whether the exchange is running correctly under the advertized base URL, run:

[root@exchange-online]# export BASE_URL=$(taler-config -s exchange -o base_url)
[root@exchange-online]# wget ${BASE_URL}management/keys

The request might take some time to complete on slow machines, because a lot of key material will be generated.

7.9. Offline Signing Setup, Key Maintenance and Tear-Down

The exchange HTTP service must be running before you can complete the following offline signing procedure. Note that when an exchange is running without offline keys its not fully operational. To make the exchange HTTP service fully operational, the following steps involving the offline signing machine must be completed:

  1. The public keys of various online keys used by the exchange service are exported via a management HTTP API.
  2. The offline signing system validates this request and signs it. Additionally, the offline signing system signs policy messages to configure the exchange’s bank accounts and associated fees.
  3. The messages generated by the offline signing system are uploaded via the management API of the exchange HTTP service.

A typical minimal setup would look something like this:

[anybody@exchange-online]# taler-exchange-offline \
  download > sig-request.json

[root@exchange-offline]# taler-exchange-offline \
  sign < sig-request.json > sig-response.json
[root@exchange-offline]# taler-exchange-offline \
  enable-account payto://iban/$IBAN?receiver-name=$NAME > acct-response.json
[root@exchange-offline]# taler-exchange-offline \
  wire-fee now iban EUR:0 EUR:0 > fee-response.json
[root@exchange-offline]# taler-exchange-offline \
  global-fee now EUR:0 EUR:0 EUR:0 4w 6y 4 > global-response.json

[anybody@exchange-online]# taler-exchange-offline upload < sig-response.json
[anybody@exchange-online]# taler-exchange-offline upload < acct-response.json
[anybody@exchange-online]# taler-exchange-offline upload < fee-response.json
[anybody@exchange-online]# taler-exchange-offline upload < global-response.json

The following sections will discuss these steps in more depth.

7.9.1. Signing the online signing keys

To sign the online signing keys, first the future key material should be downloaded using:

$ 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:

$ 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:

$ 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:

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

to provision the signatures to the exchange.

The download sign upload sequence in the commands above has to be done periodically, as it signs the various online signing keys of the exchange which periodically expire.

7.9.2. Account signing

The enable-account step is important to 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. Note that for each bank account, additional options must be set in the configuration file to tell the exchange how to access the bank account. The offline tool only configures the externally visible portions of the setup. The chapter on Bank account configuration has further details.

taler-exchange-offline accepts additional options to configure the use of the account. For example, additional options can be used to add currency conversion or to restrict interactions to bank accounts from certain countries:

$ taler-exchange-offline \
    enable-account payto://iban/CH9300762011623852957
      conversion-url https://conversion.example.com/

For details on optional enable-account arguments, see taler-exchange-offline(1).

7.9.3. Wire fee structure

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

$ taler-exchange-offline wire-fee 2040 iban 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).

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!

7.9.4. Auditor configuration

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 an auditor to affirm that they are auditing this exchange. Before an auditor can do this, the exchange service 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 (using taler-auditor-offline setup) 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. For more information on how to setup and operate an auditor, see taler-auditor-offline(1) and GNU Taler Auditor Operator Manual.

Given this information, the exchange operator can enable the auditor:

$ 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 using taler-exchange-offline upload.

7.9.5. 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:

$ 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.

7.9.6. AML Configuration

The AML configuration steps are used to add or remove keys of exchange operator staff that are responsible for anti-money laundering (AML) compliance. These AML officers are shown suspicious transactions and are granted access to the KYC data of an exchange. They can then investigate the transaction and decide on freezing or permitting the transfer. They may also request additional KYC data from the consumer and can change the threshold amount above which a further AML review is triggered.

AML Officer Setup

To begin the AML setup, AML staff should launch the GNU Taler exchange AML SPA Web interface. (FIXME-Sebastian: how?). The SPA will generate a public-private key pair and store it in the local storage of the browser. The public key will be displayed and must be securely transmitted to the offline system for approval. Using the offline system, one can then configure which staff has access to the AML operations:

[root@exchange-offline]# taler-exchange-offline \
   aml-enable $PUBLIC_KEY "Legal Name" rw > aml.json
[root@exchange-online]# taler-exchange-offline \
   upload < aml.json

The above commands would add an AML officer with the given “Legal Name” with read-write (rw) access to the AML officer database. Using “ro” instead of “rw” would grant read-only access to the data, leaving out the ability to actually make AML decisions. Once AML access has been granted, the AML officer can use the SPA to review cases and (with “rw” access) take AML decisions.

Access rights can be revoked at any time using:

[root@exchange-offline]# taler-exchange-offline \
   aml-disable $PUBLIC_KEY "Legal Name" > aml-off.json
[root@exchange-online]# taler-exchange-offline \
   upload < aml-off.json

AML Triggers

AML decision processes are automatically triggered under certain configurable conditions. The primary condition that must be configured is the AML_THRESHOLD:

/etc/taler/conf.d/exchange-business.conf
[exchange]
# Accounts or wallets with monthly transaction volumes above this threshold
# are considered suspicious and are automatically flagged for AML review
# and put on hold until an AML officer has reached a decision.
AML_THRESHOLD = "EUR:1000000"

Additionally, certain KYC attributes (such as the user being a politically exposed person) may lead to an account being flagged for AML review. The specific logic is configured by providing the exchange with an external helper program that makes the decision given the KYC attributes:

/etc/taler/conf.d/exchange-business.conf
[exchange]
# Specifies a program to run on KYC attribute data to decide
# whether we should immediately flag an account for AML review.
KYC_AML_TRIGGER = taler-exchange-kyc-aml-pep-trigger.sh

The given program will be given the KYC attributes in JSON format on standard input, and must return 0 to continue without AML and non-zero to flag the account for manual review. To disable this triger, simply leave the option to its default value of ‘[/usr/bin/]true’. To flag all new users for manual review, simply set the program to ‘[/usr/bin/]false’.

7.10. Setup Linting

The taler-wallet-cli package comes with an experimental tool that runs various checks on the current GNU Taler exchange deployment:

[root@exchange-online]# apt install taler-wallet-cli
[root@exchange-online]# taler-wallet-cli deployment lint-exchange

You can optionally pass the --debug option to get more verbose output, and --continue to continue with further checks even though a previous one has failed.

7.11. Testing and Troubleshooting

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.

The following shell session illustrates how the wallet can be used to withdraw electronic cash from the exchange and subsequently spend it. For these steps, a merchant backend is not required, as the wallet acts as a merchant.

# This will now output a payto URI that money needs to be sent to in order to allow withdrawal
# of taler coins.
$ taler-wallet-cli advanced withdraw-manually --exchange $EXCHANGE_URL --amount EUR:10.50

Show the status of the manual withdrawal operation.

$ taler-wallet-cli transactions

At this point, a bank transfer to the exchange’s bank account needs to be made with the correct subject / remittance information as instructed by the wallet after the first step. With the above configuration, it should take about 5 minutes after the wire transfer for the incoming transfer to be observed by the Nexus.

Run the following command to check whether the exchange received an incoming bank transfer:

[root@exchange-online]# taler-exchange-wire-gateway-client \
   --section exchange-accountcredentials-1 --credit-history

Once the transfer has been made, try completing the withdrawal using:

$ taler-wallet-cli run-pending

Afterwards, check the status of transactions and show the current wallet balance:

$ taler-wallet-cli transactions
$ taler-wallet-cli balance

Now, we can directly deposit coins via the exchange into a target account. (Usually, a payment is made via a merchant. The wallet provides this functionality for testing.)

$ taler-wallet-cli deposit create EUR:5 \
  payto://iban/$IBAN?receiver-name=Name
$ taler-wallet-cli run-pending

Check if this transaction was successful (from the perspective of the wallet):

$ taler-wallet-cli transactions

If the transaction failed, fix any open issue(s) with the exchange and run the “run-pending” command.

The wallet can also track if the exchange wired the money to the merchant account. The “deposit group id” can be found in the output of the transactions list.

$ taler-wallet-cli deposit track $DEPOSIT_GROUP_ID

You can also check using the exchange-tools whether the exchange sent the an outgoing transfer:

[root@exchange-online]# taler-exchange-wire-gateway-client \
  --section exchange-accountcredentials-1 --debit-history

After enough time has passed, the money should arrive at the specified IBAN.

For more information on the taler-wallet-cli tool, see GNU Taler Wallet Manual.

7.11.1. 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.

7.11.2. 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 operator 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.

7.11.3. 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:

_images/exchange-db.png

7.11.4. Database upgrades

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

The exchange database can be re-initialized using:

$ taler-exchange-dbinit --reset

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.

7.12. 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-CG: these instructions are incomplete and untested for the current iteration of the code…

$ createdb talercheck # if it does not yet exist
$ export CIPHER="rsa" # or cs
$ export CONF="benchmark-${CIPHER}.conf"
$ taler-exchange-dbinit -c "$CONF"
$ taler-exchange-secmod-rsa -c "$CONF" &
$ taler-exchange-secmod-cs -c "$CONF" &
$ taler-exchange-secmod-eddsa -c "$CONF" &
$ taler-exchange-httpd -c "$CONF" &
$ HTTPD_PID=$!
$ taler-exchange-offline -c "$CONF" \
  download sign \
  enable-account \
    payto://iban/CH9300762011623852957?receiver-name=exchange \
  wire-fee now iban EUR:0 EUR:0 \
  global-fee now EUR:0 EUR:0 EUR:0 4w 6a 42 \
  upload
$ kill -TERM $(jobs -p)
$ taler-exchange-benchmark -c "$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 PostgreSQL database itself) may help discover performance issues.

7.13. FIXMEs

  • We should have some summary with the inventory of services that should be running. Systemd by default doesn’t show this nicely. Maybe suggest running “systemd list-dependencies taler-exchange.target”?
  • What happens when the TWG doesn’t like one particular outgoing transaction? How to recover from that as a sysadmin when it happens in practice?