12. Developer’s Manual


This manual contains information for developers working on GNU Taler and related components. It is not intended for a general audience.

12.3. Fundamentals

12.3.1. Bug Tracking

Bug tracking is done with Mantis (https://www.mantisbt.org/). The bug tracker is available at https://bugs.taler.net. A registration on the Web site is needed in order to use the bug tracker, only read access is granted without a login.

12.3.2. Code Repositories

Taler code is versioned with Git. For those users without write access, all the codebases are found at the following URL:


A complete list of all the existing repositories is currently found at https://git.taler.net/.

12.3.3. Committing code

Before you can obtain Git write access, you must sign the copyright agreement. As we collaborate closely with GNUnet, we use their copyright agreement – with the understanding that your contributions to GNU Taler are included in the assignment. You can find the agreement on the GNUnet site. Please sign and mail it to Christian Grothoff as he currently collects all the documents for GNUnet e.V.

To obtain Git access, you need to send us your SSH public key. Most core team members have administrative Git access, so simply contact whoever is your primary point of contact so far. You can find instructions on how to generate an SSH key in the Git book. If you have been granted write access, you first of all must change the URL of the respective repository to:


For an existing checkout, this can be done by editing the .git/config file.

The server is configured to reject all commits that have not been signed with GnuPG. If you do not yet have a GnuPG key, you must create one, as explained in the GNU Privacy Handbook. You do not need to share the respective public key with us to make commits. However, we recommend that you upload it to key servers, put it on your business card and personally meet with other GNU hackers to have it signed such that others can verify your commits later.

To sign all commits, you should run

$ git config --global commit.gpgsign true

You can also sign individual commits only by adding the -S option to the git commit command. If you accidentally already made commits but forgot to sign them, you can retroactively add signatures using:

$ git rebase -S

Whether you commit to a personal branch (recommended: dev/$USER/...), a feature branch or to master should depend on your level of comfort and the nature of the change. As a general rule, the code in master must always build and tests should always pass, at least on your own system. However, we all make mistakes and you should expect to receive friendly reminders if your change did not live up to this simple standard. We plan to move to a system where the CI guarantees this invariant in the future.

In order to keep a linear and clean commits history, we advise to avoid merge commits and instead always rebase your changes before pushing to the master branch. If you commit and later find out that new commits were pushed, the following command will pull the new commits and rebase yours on top of them.

# -S instructs Git to (re)sign your commits
$ git pull --rebase -S

12.3.4. Observing changes

Every commit to the master branch of any of our public repositories (and almost all are public) is automatically sent to the gnunet-svn@gnu.org mailinglist. That list is for Git commits only, and must not be used for discussions. It also carries commits from our main dependencies, namely GNUnet and GNU libmicrohttpd. While it can be high volume, the lists is a good way to follow overall development.

12.3.5. Communication

We use the #taler channel on the Freenode IRC network and the taler@gnu.org public mailinglist for discussions. Not all developers are active on IRC, but all developers should probably subscribe to the low-volume Taler mailinglist. There are separate low-volume mailinglists for gnunet-developers (@gnu.org) and for libmicrohttpd (@gnu.org).

12.4. Debian and Ubuntu Repositories

We package our software for Debian and Ubuntu.

12.4.1. Nightly Repositories

To try the latest, unstable and untested versions of packages, you can add the nightly package sources.

# For Ubuntu (focal-fossa)
$ echo "deb https://deb.taler.net/apt-nightly focal-taler-nightly main" > /etc/apt/sources.list.d/taler.list

# For Debian (bullseye)
$ echo "deb https://deb.taler.net/apt-nightly bullseye-taler-nightly main" > /etc/apt/sources.list.d/taler.list

# Both: Install signing key for nightly packages
$ wget -O - https://taler.net/taler-systems-nightly.gpg.key | apt-key add -

12.6. Taler Deployment on gv.taler.net

This section describes the GNU Taler deployment on gv.taler.net. gv is our server at BFH. It hosts the Git repositories, Web sites, CI and other services. Developers can receive an SSH account and e-mail alias for the system. As with Git, ask your primary team contact for shell access if you think you need it.

Our old server, tripwire, is currently offline and will likely go back online to host production systems for operating real Taler payments at BFH in the future.

12.6.1. DNS

DNS records for taler.net are controlled by the GNU Taler maintainers, specifically Christian and Florian. If you need a sub-domain to be added, please contact one of them.

12.6.2. User Acccounts

On gv.taler.net, there are four system users that are set up to serve Taler on the Internet:

  • taler-test: serves *.test.taler.net and gets automatically built by Buildbot.
  • taler-internal: serves *.int.taler.net, and does NOT get automatically built.

The following two users are never automatically built, and they both serve *.demo.taler.net. At any given time, only one is active and serves the HTTP requests from the outside; the other one can so be compiled without any downtime. If the compilation succeeds, the inactive user can be switched to become active (see next section), and vice versa.

  • demo-blue
  • demo-green

12.7. Demo Upgrade Procedure

Upgrading the demo environment should be done with care, and ideally be coordinated on the mailing list before. It is our goal for demo to always run a “working version” that is compatible with various published wallets.

Before deploying on demo, the same version of all components must be deployed and tested on int.

Please use the demo upgrade checklist to make sure everything is working.

12.7.1. Tagging components

All Taler components must be tagged with git before they are deployed on the demo environment, using a tag of the following form:

YYYY = year
MM = month
DD = day
SS = serial

12.7.2. Environment Layout

Environments have the following layout:

  deployment (deployment.git checkout)
  envcfg.py  (configuration of the Taler environment)
  activate   (bash file, sourced to set environment variables)
  logs/      (log files)
  local/     (locally installed software)
  sources/   (sources repos of locally build components)
  sockets/   (unix domain sockets of running components)
  taler-data (on-disk state, public and private keys)
  .config/taler.conf (main Taler configuration file)

On demo-blue and demo-green, taler-data is a symlink pointing to $HOME/demo/shared-data instead of a directory.

12.7.3. Using envcfg.py

The $HOME/envcfg.py file contains (1) the name of the environment and (2) the version of all components we build (in the form of a git rev).

The envcfg.py for demo looks like this:

env = "demo"
tag = "demo-2019-10-05-01:
tag_gnunet = tag
tag_libmicrohttpd = tag
tag_exchange = tag
tag_merchant = tag
tag_bank = tag
tag_twister = tag
tag_landing = tag
tag_donations = tag
tag_blog = tag
tag_survey = tag
tag_backoffice = tag
tag_sync = tag

Currently only the variables env and tag_${component} are used.

When deploying to demo, the envcfg.py should be committed to deployment.git/envcfg/envcfg-demo-YYYY-MM-DD-SS.py.

12.7.4. Bootstrapping an Environment

$ git clone https://git.taler.net/deployment.git ~/deployment
$ cp ~/deployment/envcfg/$ENVCFGFILE ~/envcfg.py
$ ./deployment/bin/taler-deployment bootstrap
$ source ~/activate
$ taler-deployment build
$ taler-deployment-prepare
$ taler-deployment-start
$ taler-deployment-arm -I # check everything works
# The following command sets up the 'blog' and 'donations' instances.
$ taler-config-instances

12.7.5. Upgrading an Existing Environment

$ rm -rf ~/sources ~/local
$ git -C ~/deployment pull
$ cp ~/deployment/envcfg/$ENVCFGFILE ~/envcfg.py
$ taler-deployment bootstrap
$ taler-deployment build
$ taler-deployment-prepare
$ taler-deployment-restart
$ taler-deployment-arm -I # check everything works

12.7.6. Switching Demo Colors

As the demo user, to switch to color ${COLOR}, run the following script from deployment/bin:

$ taler-deployment switch-demo

12.8. Environments and Builders on taler.net

12.8.1. Buildbot implementation

GNU Taler uses a buildbot implementation (front end at https://buildbot.taler.net) to manage continuous integration. Buildbot documentation is at https://docs.buildbot.net/.

Here are some highlights:

  • The WORKER is the config that that lives on a shell account on a localhost (taler.net), where this host has buildbot-worker installed. The WORKER executes the commands that perform all end-functions of buildbot.
  • The WORKER running buildbot-worker receives these commands by authenticating and communicating with the buildbot server using parameters that were specified when the worker was created in that shell account with the buildbot-worker command.
  • The buildbot server’s master.cfg file contains FACTORY declarations which specify the commands that the WORKER will run on localhost.
  • The FACTORY is tied to the WORKER in master.cfg by a BUILDER.
  • The master.cfg also allows for SCHEDULER that defines how and when the BUILDER is executed.
  • Our master.cfg file is checked into git, and then periodically updated on a particular account on taler.net (ask Christian for access if needed). Do not edit this file directly/locally on taler.net, but check changes into Git.

Best Practices:

  • When creating a new WORKER in the master.cfg file, leave a comment specifying the server and user account that this WORKER is called from. (At this time, taler.net is the only server used by this implementation, but it’s still good practice.)
  • Create a worker from a shell account with this command: buildbot-worker create-worker <workername> localhost <username> <password>

Then make sure there is a WORKER defined in master.cfg like: worker.Worker("<username>", "<password>")

12.8.2. Documentation Builder

All the Taler documentation is built by the user docbuilder that runs a Buildbot worker. The following commands set the docbuilder up, starting with an empty home directory.

# Log-in as the 'docbuilder' user.

$ cd $HOME
$ git clone git://git.taler.net/deployment
$ ./deployment/bootstrap-docbuilder

# If the previous step worked, the setup is
# complete and the Buildbot worker can be started.

$ buildbot-worker start worker/

12.8.3. Website Builder

Taler Websites, www.taler.net and stage.taler.net, are built by the user taler-websites by the means of a Buildbot worker. The following commands set the taler-websites up, starting with an empty home directory.

# Log-in as the 'taler-websites' user.

$ cd $HOME
$ git clone git://git.taler.net/deployment
$ ./deployment/bootstrap-sitesbuilder

# If the previous step worked, the setup is
# complete and the Buildbot worker can be started.

$ buildbot-worker start worker/

12.8.4. Code coverage

Code coverage tests are run by the lcovworker user, and are also driven by Buildbot.

# Log-in as the 'lcovworker' user.

$ cd $HOME
$ git clone git://git.taler.net/deployment
$ ./deployment/bootstrap-taler lcov

# If the previous step worked, the setup is
# complete and the Buildbot worker can be started.

$ buildbot-worker start worker/

The results are then published at https://lcov.taler.net/.

12.8.5. Service Checker

The user demo-checker runs periodic checks to see if all the *.demo.taler.net services are up and running. It is driven by Buildbot, and can be bootstrapped as follows.

# Log-in as the 'demo-checker' user

$ cd $HOME
$ git clone git://git.taler.net/deployment
$ ./deployment/bootstrap-demochecker

# If the previous step worked, the setup is
# complete and the Buildbot worker can be started.

$ buildbot-worker start worker/

12.8.6. Tipping reserve top-up

Both ‘test’ and ‘demo’ setups get their tip reserve topped up by a Buildbot worker. The following steps get the reserve topper prepared.

# Log-in as <env>-topper, with <env> being either 'test' or 'demo'

$ git clone git://git.taler.net/deployment
$ ./deployment/prepare-reservetopper <env>

# If the previous steps worked, then it should suffice to start
# the worker, with:

$ buildbot-worker start worker/

12.8.7. Producing auditor reports

Both ‘test’ and ‘demo’ setups get their auditor reports compiled by a Buildbot worker. The following steps get the reports compiler prepared.

# Log-in as <env>-auditor, with <env> being either 'test' or 'demo'

$ git clone git://git.taler.net/deployment
$ ./deployment/prepare-auditorreporter <env>

# If the previous steps worked, then it should suffice to start
# the worker, with:

$ buildbot-worker start worker/

12.8.8. Database schema versioning

The Postgres databases of the exchange and the auditor are versioned. See the 0000.sql file in the respective directory for documentation.

Every set of changes to the database schema must be stored in a new versioned SQL script. The scripts must have contiguous numbers. After any release (or version being deployed to a production or staging environment), existing scripts MUST be immutable.

Developers and operators MUST NOT make changes to database schema outside of this versioning.

12.9. Releases

12.9.1. Release Process and Checklists

Please use the release checklist

This document describes the process for releasing a new version of the various Taler components to the official GNU mirrors.

The following components are published on the GNU mirrors

  • taler-exchange (exchange.git)
  • taler-merchant (merchant.git)
  • talerdonations (donations.git)
  • talerblog (blog.git)
  • taler-bank (bank.git)
  • taler-wallet-webex (wallet-webex.git)

12.9.2. Tagging

Tag releases with an annotated commit, like

$ git tag -a v0.1.0 -m "Official release v0.1.0"
$ git push origin v0.1.0

12.9.3. Database for tests

For tests in the exchange and merchant to run, make sure that a database talercheck is accessible by $USER. Otherwise tests involving the database logic are skipped.


The Taler merchant backend stores private keys and other sensitive business and customer data in the database. The backend operator SHOULD ensure that backup operations are encrypted and secured from unauthorized access.

12.9.4. Exchange, merchant

Set the version in configure.ac. The commit being tagged should be the change of the version.

Tag the current GANA version that works with the exchange and merchant and checkout that tag of gana.git (instead of master). Otherwise, if there are incompatible changes in GANA (like removed symbols), old builds could break.

Update the Texinfo documentation using the files from docs.git:

# Get the latest documentation repository
$ cd $GIT/docs
$ git pull
$ make texinfo
# The *.texi files are now in _build/texinfo
# This checks out the prebuilt branch in the prebuilt directory
$ git worktree add prebuilt prebuilt
$ cd prebuilt
# Copy the pre-built documentation into the prebuilt directory
$ cp -r ../_build/texinfo .
# Push and commit to branch
$ git commit -a -S -m "updating texinfo"
$ git status
# Verify that all files that should be tracked are tracked,
# new files will have to be added to the Makefile.am in
# exchange.git as well!
$ git push
# Remember $REVISION of commit
# Go to exchange
$ cd $GIT/exchange/doc/prebuilt
# Update submodule to point to latest commit
$ git checkout $REVISION

Finally, the Automake Makefile.am files may have to be adjusted to include new *.texi files or images.

For bootstrap, you will need to install GNU Recutils.

For the exchange test cases to pass, make install must be run first. Without it, test cases will fail because plugins can’t be located.

$ ./bootstrap
$ ./configure # add required options for your system
$ make dist
$ tar -xf taler-$COMPONENT-$VERSION.tar.gz
$ make install check

12.9.5. Wallet WebExtension

The version of the wallet is in manifest.json. The version_name should be adjusted, and version should be increased independently on every upload to the WebStore.

$ ./configure
$ make dist

12.9.6. Upload to GNU mirrors

See https://www.gnu.org/prep/maintain/maintain.html#Automated-FTP-Uploads

Directive file:

version: 1.2
directory: taler
filename: taler-exchange-0.1.0.tar.gz

Upload the files in binary mode to the ftp servers.

12.9.7. Creating Debian packages

Our general setup is based on https://wiki.debian.org/DebianRepository/SetupWithReprepro

First, update at least the version of the Debian package in debian/changelog, and then run:

$ dpkg-buildpackage -rfakeroot -b -uc -us

in the respective source directory (GNUnet, exchange, merchant) to create the .deb files. Note that they will be created in the parent directory. This can be done on gv.taler.net, or on another (secure) machine.

On gv, we use the aptbuilder user to manage the reprepro repository.

Next, the *.deb files should be copied to gv.taler.net, say to /home/aptbuilder/incoming. Then, run

# cd /home/aptbuilder/apt
# reprepro includedeb bullseye ~/incoming/*.deb

to import all Debian files from ~/incoming/ into the bullseye distribution. If Debian packages were build against other distributions, reprepro may need to be first configured for those and the import command updated accordingly.

Finally, make sure to clean up ~/incoming/ (by deleting the now imported *.deb files).

12.10. Continuous integration

CI is done with Buildbot (https://buildbot.net/), and builds are triggered by the means of Git hooks. The results are published at https://buildbot.taler.net/ .

In order to avoid downtimes, CI uses a “blue/green” deployment technique. In detail, there are two users building code on the system, the “green” and the “blue” user; and at any given time, one is running Taler services and the other one is either building the code or waiting for that.

There is also the possibility to trigger builds manually, but this is only reserved to “admin” users.

12.11. Internationalization

Internationalization (a.k.a “Translation”) is handled with Weblate (https://weblate.org) via our instance at https://weblate.taler.net/ .

At this time, this system is still very new for Taler.net and this documentation may be incorrect and is certainly incomplete.

12.11.1. Who can Register

At this time, anyone can register an account at https://weblate.taler.net/ to create translations. Registered users default to the Users and Viewers privilege level.

12.11.2. About Privilege Levels

This is the breakdown of privilege levels in Weblate:

  • Users/Viewers = Can log in, view Translations (applies to new users)
  • Reviewers = Can contribute Translations to existing Components
  • Managers = Can create new Components of existing Projects
  • Superusers = Can create new Projects

12.11.3. Upgrading Privileges

To upgrade from Users/Viewers, a superuser must manually augment your privileges. At this time, superusers are Christian, Florian, and Buck.

12.11.4. How to Create a Project

The GNU Taler project is probably the correct project for most Components and Translations falling under this guide. Please contact a superuser if you need another Project created.

12.11.5. How to Create a Component

Reference: https://docs.weblate.org/en/weblate-4.0.3/admin/projects.html#component-configuration

In Weblate, a Component is a subset of a Project and each Component contains N translations. A Component is generally associated with a Git repo.

To create a Component, log into https://weblate.taler.net/ with your Manager or higher credentials and choose + Add from the upper-right corner.

What follows is a sort of Wizard. You can find detailed docs at https://docs.weblate.org/. Here are some important notes about connecting your Component to the Taler Git repository:

Under https://weblate.taler.net/create/component/vcs/:

  • Source code repository - Generally git+ssh://git@git.taler.net/<reponame>`. Check with git remote -v.
  • Repository branch - Choose the correct branch to draw from and commit to.
  • Repository push URL - This is generally git+ssh://git@git.taler.net/<reponame>` Check with git remote -v.
  • Repository browser - This is the www URL of the Git repo’s file browser. Example https://git.taler.net/<repositoryname>.git/tree/{{filename}}?h={{branch}}#n{{line}} where <repositoryname> gets replaced but {{filename}} and other items in braces are actual variables in the string.
  • Merge style - Rebase, in line with GNU Taler development procedures
  • Translation license - GNU Affero General Public License v3.0 or Later
  • Adding new translation - Decide how to handle adding new translations

12.11.6. How to Create a Translation

1 - Log into https://weblate.taler.net

2 - Navigate to Projects > Browse all projects

3 - Choose the Project you wish to contribute to.

4 - Choose the Component you wish to contribute to.

5 - Find the language you want to translate into. Click “Translate” on that line.

6 - Find a phrase and translate it.

You may also wish to refer to https://docs.weblate.org/ .

12.11.7. Translation Standards and Practices

By default, our Weblate instance is set to accept translations in English, French, German, Italian, Russian, Spanish, and Portuguese. If you want to contribute a translation in a different language, navigate to the Component you want to translate for, and click “Start new translation” to begin. If you require a privilege upgrade, please contact a superuser with your request.

When asked, set the license to GPLv3 or later.

Set commit/push to manual only.

12.11.8. GPG Signing of Translations

weblate.taler.net signs GPG commits with the GPG key CD33CE35801462FA5EB0B695F2664BF474BFE502, and the corresponding public key can be found at https://weblate.taler.net/keys/.

This means that contributions made through weblate will not be signed with the individual contributor’s key when they are checked into the Git repository, but with the weblate key.

12.12. Android Apps

12.12.1. Android App Nightly Builds

There are currently three Android apps in the official Git repository:

  • Wallet [CI]
  • Merchant PoS Terminal [CI]
  • Cashier [CI]

Their git repositories are mirrored at Gitlab to utilize their CI and F-Droid’s Gitlab integration to publish automatic nightly builds for each change on the master branch.

All three apps publish their builds to the same F-Droid nightly repository (which is stored as a git repository): https://gitlab.com/gnu-taler/fdroid-repo-nightly

You can download the APK files directly from that repository or add it to the F-Droid app for automatic updates by clicking the following link (on the phone that has F-Droid installed).


Nightly apps can be installed alongside official releases and thus are meant only for testing purposes. Use at your own risk!

12.12.2. Building apps from source

Note that this guide is different from other guides for building Android apps, because it does not require you to run non-free software. It uses the Merchant PoS Terminal as an example, but works as well for the other apps if you replace merchant-terminal with wallet or cashier.

First, ensure that you have the required dependencies installed:

  • Java Development Kit 8 or higher (default-jdk-headless)
  • git
  • unzip

Then you can get the app’s source code using git:

# Start by cloning the Android git repository
$ git clone https://git.taler.net/taler-android.git

# Change into the directory of the cloned repository
$ cd taler-android

# Find out which Android SDK version you will need
$ grep -i compileSdkVersion merchant-terminal/build.gradle

The last command will return something like compileSdkVersion 29. So visit the Android Rebuilds project and look for that version of the Android SDK there. If the SDK version is not yet available as a free rebuild, you can try to lower the compileSdkVersion in the app’s merchant-terminal/build.gradle file. Note that this might break things or require you to also lower other versions such as targetSdkVersion.

In our example, the version is 29 which is available, so download the “SDK Platform” package of “Android 10.0.0 (API 29)” and unpack it:

# Change into the directory that contains your downloaded SDK
$ cd $HOME

# Unpack/extract the Android SDK
$ unzip android-sdk_eng.10.0.0_r14_linux-x86.zip

# Tell the build system where to find the SDK
$ export ANDROID_SDK_ROOT="$HOME/android-sdk_eng.10.0.0_r14_linux-x86"

# Change into the directory of the cloned repository
$ cd taler-android

# Build the merchant-terminal app
$ ./gradlew :merchant-terminal:assembleRelease

If you get an error message complaining about build-tools

> Failed to install the following Android SDK packages as some licences have not been accepted.
build-tools;29.0.3 Android SDK Build-Tools 29.0.3

you can try changing the buildToolsVersion in the app’s merchant-terminal/build.gradle file to the latest “Android SDK build tools” version supported by the Android Rebuilds project.

After the build finished successfully, you will find your APK in merchant-terminal/build/outputs/apk/release/.

12.12.3. Update translations

Translations are managed with Taler’s weblate instance: https://weblate.taler.net/projects/gnu-taler/

To update translations, enter the taler-android git repository and ensure that the weblate remote exists:

$ git config -l | grep weblate

If it does not yet exist (empty output), you can add it like this:

$ git remote add weblate https://weblate.taler.net/git/gnu-taler/wallet-android/

Then you can merge in translations commit from the weblate remote:

# ensure you have latest version
$ git fetch weblate

# merge in translation commits
$ git merge weblate/master

Afterwards, build the entire project from source and test the UI to ensure that no erroneous translations (missing placeholders) are breaking things.

12.12.4. Release process

After extensive testing, the code making up a new release should get a signed git tag. The current tag format is:

  • cashier-$VERSION
  • pos-$VERSION
  • wallet-$VERSION (where $VERSION has a v prefix)
$ git tag -s $APP-$VERSION


Nightly builds get published automatically (see above) after pushing code to the official repo. Actual releases get picked up by F-Droid’s official repository via git tags. So ensure that all releases get tagged properly.

Some information for F-Droid official repository debugging:

Google Play

Google Play uploads are managed via Fastlane. Before proceeding, ensure that this is properly set up and that you have access to the Google Play API.

To release an app, enter into its respective folder and run fastlane:

$ bundle exec fastlane

Then select the deploy option. Note this requires access to the Google Play upload signing key set via the various environment variables in $app/fastlane/Fastfile.

All uploads are going to the beta track by default. These can be promoted to production later or immediately after upload if you feel daring.

12.13. Code Coverage

Code coverage is done with the Gcov / Lcov (http://ltp.sourceforge.net/coverage/lcov.php) combo, and it is run nightly (once a day) by a Buildbot worker. The coverage results are then published at https://lcov.taler.net/ .

12.14. Coding Conventions

GNU Taler is developed primarily in C, Kotlin, Python and TypeScript.

12.14.1. Components written in C

These are the general coding style rules for Taler.

Naming conventions

  • include files (very similar to GNUnet):
    • if installed, must start with “taler_” (exception: platform.h), and MUST live in src/include/
    • if NOT installed, must NOT start with “taler_” and MUST NOT live in src/include/ and SHOULD NOT be included from outside of their own directory
    • end in “_lib” for “simple” libraries
    • end in “_plugin” for plugins
    • end in “_service” for libraries accessing a service, i.e. the exchange
  • binaries:
    • taler-exchange-xxx: exchange programs
    • taler-merchant-xxx: merchant programs (demos)
    • taler-wallet-xxx: wallet programs
    • plugins should be libtaler_plugin_xxx_yyy.so: plugin yyy for API xxx
    • libtalerxxx: library for API xxx
  • logging
    • tools use their full name in GNUNET_log_setup (i.e. ‘taler-exchange-offline’) and log using plain ‘GNUNET_log’.
    • pure libraries (without associated service) use ‘GNUNET_log_from’ with the component set to their library name (without lib or ‘.so’), which should also be their directory name (i.e. ‘util’)
    • plugin libraries (without associated service) use ‘GNUNET_log_from’ with the component set to their type and plugin name (without lib or ‘.so’), which should also be their directory name (i.e. ‘exchangedb-postgres’)
    • libraries with associated service) use ‘GNUNET_log_from’ with the name of the service, which should also be their directory name (i.e. ‘exchange’)
    • for tools with -l LOGFILE, its absence means write logs to stderr
  • configuration
    • same rules as for GNUnet
  • exported symbols
    • must start with TALER_[SUBSYSTEMNAME]_ where SUBSYSTEMNAME MUST match the subdirectory of src/ in which the symbol is defined
    • from libtalerutil start just with TALER_, without subsystemname
    • if scope is ONE binary and symbols are not in a shared library, use binary-specific prefix (such as TMH = taler-exchange-httpd) for globals, possibly followed by the subsystem (TMH_DB_xxx).
  • structs:
    • structs that are ‘packed’ and do not contain pointers and are thus suitable for hashing or similar operations are distinguished by adding a “P” at the end of the name. (NEW) Note that this convention does not hold for the GNUnet-structs (yet).
    • structs that are used with a purpose for signatures, additionally get an “S” at the end of the name.
  • private (library-internal) symbols (including structs and macros)
    • must not start with TALER_ or any other prefix
  • testcases
    • must be called “test_module-under-test_case-description.c”
  • performance tests
    • must be called “perf_module-under-test_case-description.c”

12.14.2. Shell Scripts

Shell scripts should be avoided if at all possible. The only permissible uses of shell scripts in GNU Taler are:

  • Trivial invocation of other commands.
  • Scripts for compatibility (e.g. ./configure) that must run on as many systems as possible.

When shell scripts are used, they MUST begin with the following set command:

# Make the shell fail on undefined variables and
# commands with non-zero exit status.
$ set -eu

12.14.3. Kotlin

We so far have no specific guidelines, please follow best practices for the language.

12.14.4. Python

Supported Python Versions

Python code should be written and build against version 3.7 of Python.


We use yapf to reformat the code to conform to our style instructions. A reusable yapf style file can be found in build-common, which is intended to be used as a git submodule.

Python for Scripting

When using Python for writing small utilities, the following libraries are useful:

  • click for argument parsing (should be preferred over argparse)
  • pathlib for path manipulation (part of the standard library)
  • subprocess for “shelling out” to other programs. Prefer subprocess.run over the older APIs.

12.15. Testing library

This chapter is a VERY ABSTRACT description of how testing is implemented in Taler, and in NO WAY wants to substitute the reading of the actual source code by the user.

In Taler, a test case is an array of struct TALER_TESTING_Command, informally referred to as CMD, that is iteratively executed by the testing interpreter. This latter is transparently initiated by the testing library.

However, the developer does not have to defined CMDs manually, but rather call the proper constructor provided by the library. For example, if a CMD is supposed to test feature x, then the library would provide the TALER_TESTING_cmd_x () constructor for it. Obviously, each constructor has its own particular arguments that make sense to test x, and all constructor are thoroughly commented within the source code.

Internally, each CMD has two methods: run () and cleanup (). The former contains the main logic to test feature x, whereas the latter cleans the memory up after execution.

In a test life, each CMD needs some internal state, made by values it keeps in memory. Often, the test has to share those values with other CMDs: for example, CMD1 may create some key material and CMD2 needs this key material to encrypt data.

The offering of internal values from CMD1 to CMD2 is made by traits. A trait is a struct TALER_TESTING_Trait, and each CMD contains an array of traits, that it offers via the public trait interface to other commands. The definition and filling of such array happens transparently to the test developer.

For example, the following example shows how CMD2 takes an amount object offered by CMD1 via the trait interface.

Note: the main interpreter and the most part of CMDs and traits are hosted inside the exchange codebase, but nothing prevents the developer from implementing new CMDs and traits within other codebases.

/* Without loss of generality, let's consider the
 * following logic to exist inside the run() method of CMD1 */

struct TALER_Amount *a;
 * the second argument (0) points to the first amount object offered,
 * in case multiple are available.
if (GNUNET_OK != TALER_TESTING_get_trait_amount_obj (cmd2, 0, &a))

use(a); /* 'a' points straight into the internal state of CMD2 */

In the Taler realm, there is also the possibility to alter the behaviour of supposedly well-behaved components. This is needed when, for example, we want the exchange to return some corrupted signature in order to check if the merchant backend detects it.

This alteration is accomplished by another service called twister. The twister acts as a proxy between service A and B, and can be programmed to tamper with the data exchanged by A and B.

Please refer to the Twister codebase (under the test directory) in order to see how to configure it.

12.16. User-Facing Terminology

This section contains terminology that should be used and that should not be used in the user interface and help materials.

12.16.1. Terms to Avoid


Refreshing is the internal technical terminology for the protocol to give change for partially spent coins

Use instead: “Obtaining change”


Coins are an internal construct, the user should never be aware that their balance is represented by coins if different denominations.

Use instead: “(Digital) Cash” or “(Wallet) Balance”


Has bad connotation of consumption.

Use instead: Customer or user.


The term used to describe the process of the merchant facilitating the download of the signed contract terms for an order.

Avoid. Generally events that relate to proposal downloads should not be shown to normal users, only developers. Instead, use “communication with mechant failed” if a proposed order can’t be downloaded.

Anonymous E-Cash

Should be generally avoided, since Taler is only anonymous for the customer. Also some people are scared of anonymity (which as a term is also way too absolute, as anonymity is hardly ever perfect).

Use instead: “Privacy-preserving”, “Privacy-friendly”

Payment Replay

The process of proving to the merchant that the customer is entitled to view a digital product again, as they already paid for it.

Use instead: In the event history, “re-activated digital content purchase” could be used. (FIXME: this is still not nice.)

Session ID
See Payment Replay.

Too ambiguous in the wallet.

Use instead: Purchase

Fulfillment URL
URL that the serves the digital content that the user purchased with their payment. Can also be something like a donation receipt.

12.16.2. Terms to Use

Regulatory entity that certifies exchanges and oversees their operation.
Exchange Operator

The entity/service that gives out digital cash in exchange for some other means of payment.

In some contexts, using “Issuer” could also be appropriate. When showing a balance breakdown, we can say “100 Eur (issued by exchange.euro.taler.net)”. Sometimes we may also use the more generic term “Payment Service Provider” when the concept of an “Exchange” is still unclear to the reader.

A refund is given by a merchant to the customer (rather the customer’s wallet) and “undoes” a previous payment operation.
The act of sending digital cash to a merchant to pay for an order.
Used to refer to the “result” of a payment, as in “view purchase”. Use sparsingly, as the word doesn’t fit for all payments, such as donations.
Contract Terms
Partially machine-readable representation of the merchant’s obligation after the customer makes a payment.
Party that receives a payment.
Also “Taler Wallet”. Software component that manages the user’s digital cash and payments.

12.17. Developer Glossary

This glossary is meant for developers. It contains some terms that we usually do not use when talking to end users or even system administrators.

absolute time
method of keeping time in GNUnet where the time is represented as the number of microseconds since 1.1.1970 (UNIX epoch). Called absolute time in contrast to relative time.
the exchange combines multiple payments received by the same merchant into one larger wire transfer to the respective merchant’s bank account
trusted third party that verifies that the exchange is operating correctly
traditional financial service provider who offers wire transfers between accounts
individual in control of a Taler wallet, usually using it to spend the coins on contracts (see also customer).
operation an exchange performs on a reserve that has not been drained by withdraw operations. When closing a reserve, the exchange wires the remaining funds back to the customer, minus a fee for closing
coins are individual token representing a certain amount of value, also known as the denomination of the coin
refresh commitment
data that the wallet commits to during the melt stage of the refresh protocol where it has to prove to the exchange that it is deriving the fresh coins as specified by the Taler protocol. The commitment is verified probabilistically (see: kappa) during the reveal stage.
formal agreement between merchant and customer specifying the contract terms and signed by the merchant and the coins of the customer
contract terms
the individual clauses specifying what the buyer is purchasing from the merchant
individual that directs the buyer (perhaps the same individual) to make a purchase
unit of currency, specifies both the currency and the face value of a coin, as well as associated fees and validity periods
denomination key
(RSA) key used by the exchange to certify that a given coin is valid and of a particular denomination
operation by which a merchant passes coins to an exchange, expecting the exchange to credit his bank account in the future using an aggregate wire transfer
dirty coin
a coin is dirty if its public key may be known to an entity other than the customer, thereby creating the danger of some entity being able to link multiple transactions of coin’s owner if the coin is not refreshed
a reserve is being drained when a wallet is using the reserve’s private key to withdraw coins from it. This reduces the balance of the reserve. Once the balance reaches zero, we say that the reserve has been (fully) drained. Reserves that are not drained (which is the normal process) are closed by the exchange.
Taler’s payment service operator. Issues electronic coins during withdrawal and redeems them when they are deposited by merchants
Various operations come with time limits. In particular, denomination keys come with strict time limits for the various operations involving the coin issued under the denomination. The most important limit is the deposit expiration, which specifies until when wallets are allowed to use the coin in deposit or refreshing operations. There is also a “legal” expiration, which specifies how long the exchange keeps records beyond the deposit expiration time. This latter expiration matters for legal disputes in courts and also creates an upper limit for refreshing operations on special zombie coin
implementation of the bank API in memory to be used only for test cases.
an exchange charges various fees for its service. The different fees are specified in the protocol. There are fees per coin for withdrawing, depositing, melting, and refunding. Furthermore, there are fees per wire transfer for closing a reserve: and for aggregate wire transfers to the merchant.
fresh coin
a coin is fresh if its public key is only known to the customer
Codebase of various libraries for a better Internet, some of which GNU Taler depends upon.
JavaScript Object Notation
serialization format derived from the JavaScript language which is commonly used in the Taler protocol as the payload of HTTP requests and responses.
security parameter used in the refresh protocol. Defined to be 3. The probability of successfully evading the income transparency with the refresh protocol is 1:kappa.
FIXME: explain
specific step in the refresh protocol that an exchange must offer to prevent abuse of the refresh mechanism. The link step is not needed in normal operation, it just must be offered.
master key
offline key used by the exchange to certify denomination keys and message signing keys
step of the refresh protocol where a dirty coin is invalidated to be reborn fresh in a subsequent reveal step.
party receiving payments (usually in return for goods or services)
message signing key
key used by the exchange to sign online messages, other than coins
FIXME: to be written!
a coin is owned by the entity that knows the private key of the coin
precursor data for a coin. A planchet includes the coin’s internal secrets (coin private key, blinding factor), but lacks the RSA signature of the exchange. When withdrawing, a wallet creates and persists a planchet before asking the exchange to sign it to get the coin.
privacy policy
Statement of an operator how they will protect the privacy of users.
Message that cryptographically demonstrates that a particular claim is correct.
a list of contract terms that has been completed and signed by the merchant backend.
Refers to the overall process of negotiating a contract and then making a payment with coins to a merchant.
Operation by which an exchange returns the value of coins affected by a revocation to their owner, either by allowing the owner to withdraw new coins or wiring funds back to the bank account of the owner.
operation by which a dirty coin is converted into one or more fresh coins. Involves melting the dirty coin and then revealing so-called transfer keys.
operation by which a merchant steps back from the right to funds that he obtained from a deposit operation, giving the right to the funds back to the customer
refund transaction id
unique number by which a merchant identifies a refund. Needed as refunds can be partial and thus there could be multiple refunds for the same purchase.
relative time
method of keeping time in GNUnet where the time is represented as a relative number of microseconds. Thus, a relative time specifies an offset or a duration, but not a date. Called relative time in contrast to absolute time.

accounting mechanism used by the exchange to track customer funds from incoming wire transfers. A reserve is created whenever a customer wires money to the exchange using a well-formed public key in the subject. The exchange then allows the customer’s wallet to withdraw up to the amount received in fresh coins from the reserve, thereby draining the reserve. If a reserve is not drained, the exchange eventually closes it.

Other definition: Funds set aside for future use; either the balance of a customer at the exchange ready for withdrawal, or the funds kept in the exchange;s bank account to cover obligations from coins in circulation.

step in the refresh protocol where some of the transfer private keys are revealed to prove honest behavior on the part of the wallet. In the reveal step, the exchange returns the signed fresh coins.
exceptional operation by which an exchange withdraws a denomination from circulation, either because the signing key was compromised or because the exchange is going out of operation; unspent coins of a revoked denomination are subjected to recoup.
users can share ownership of a coin by sharing access to the coin's private key, thereby allowing all co-owners to spend the coin at any time.
operation by which a customer gives a merchant the right to deposit coins in return for merchandise
the general terms of service of an operator, possibly including the privacy policy. Not to be confused with the contract terms which are about the specific purchase.
method by which ownership is exclusively transferred from one entity
wire transfer
wire transfers
method of sending funds between bank accounts
transfer key
transfer keys
special cryptographic key used in the refresh protocol, some of which are revealed during the reveal step. Note that transfer keys have, despite the name, no relationship to wire transfers. They merely help to transfer the value from a dirty coin to a fresh coin
any individual using the Taler payment system (see customer, buyer, merchant).
Taler uses various forms of versioning. There is a database schema version (stored itself in the database, see *-0000.sql) describing the state of the table structure in the database of an exchange, auditor or merchant. There is a protocol version (CURRENT:REVISION:AGE, see GNU libtool) which specifies the network protocol spoken by an exchange or merchant including backwards-compatibility. And finally there is the software release version (MAJOR.MINOR.PATCH, see https://semver.org/) of the respective code base.
software running on a customer’s computer; withdraws, stores and spends coins
Cross-browser API used to implement the GNU Taler wallet browser extension.
wire gateway
FIXME: explain
wire transfer identifier
Subject of a wire transfer from the exchange to a merchant; set by the aggregator to a random nonce which uniquely identifies the transfer.
operation by which a wallet can convert funds from a reserve to fresh coins
zombie coin
coin where the respective denomination key is past its deposit expiration time, but which is still (again) valid for an operation because it was melted while it was still valid, and then later again credited during a recoup process

12.18. Developer Tools

This section describes various internal programs to make life easier for the developer.

12.18.1. taler-config-generate

taler-config-generate - tool to simplify Taler configuration generation

taler-config-generate [-C CURRENCY | –currency=CURRENCY] [-c FILENAME | –config=FILENAME] [-e | –exchange] [-f AMOUNT | –wirefee=AMOUNT] [-h | –help] [-J JSON | –wire-json-exchange=JSON] [-j JSON | –wire-json-merchant=JSON] [-L LOGLEVEL | –loglevel=LOGLEVEL] [-m | –merchant] [-t | –trusted] [-v | –version] [-w WIREFORMAT | –wire WIREFORMAT] [–bank-uri] [–exchange-bank-account] [–merchant-bank-account]

taler-config-generate can be used to generate configuration files for the Taler exchange or Taler merchants.

Which currency should we use in the configuration.
Location where to write the generated configuration. Existing file will be updated, not overwritten.
-e | –exchange
Generate configuration for a Taler exchange.
-f AMOUNT | -wirefee=AMOUNT
Setup wire transfer fees for the next 5 years for the exchange (for all wire methods).
-h | –help
Shows this man page.
-J JSON | –wire-json-exchange=JSON
Wire configuration to use for the exchange.
-j JSON | –wire-json-merchant=JSON
Wire configuration to use for the merchant.
Use LOGLEVEL for logging. Valid values are DEBUG, INFO, WARNING and ERROR.
-m | –merchant
Generate configuration for a Taler merchant.
-t | –trusted
Setup current exchange as trusted with current merchant. Generally only useful when configuring for testcases.
-v | –version
Print version information.
Specifies which wire format to use (i.e. “test” or “sepa”)
Alternative to specify wire configuration to use for the exchange and merchant for the “test” wire method. Only useful if WIREFORMAT was set to “test”. Specifies the URI of the bank.
Alternative to specify wire configuration to use for the exchange for the “test” wire method. Only useful if WIREFORMAT was set to “test”. Specifies the bank account number of the exchange.
Alternative to specify wire configuration to use for the merchant for the “test” wire method. Only useful if WIREFORMAT was set to “test”. Specifies the bank account number of the merchant.