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@setfilename taler-auditor.info
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@settitle Taler Auditor Manual
@defindex ge
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@dircategory CATEGORY
@direntry
* MENU ENTRY: (taler-auditor.info). DESCRIPTION
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@definfoenclose strong,`,'
@definfoenclose emph,`,'
@c %**end of header

@copying
@quotation
GNU Taler 0.8.0pre0, Jan 21, 2021

GNU Taler team

Copyright @copyright{} 2014-2020 Taler Systems SA (GPLv3+ or GFDL 1.3+)
@end quotation

@end copying

@titlepage
@title Taler Auditor Manual
@insertcopying
@end titlepage
@contents

@c %** start of user preamble

@c %** end of user preamble

@ifnottex
@node Top
@top Taler Auditor Manual
@insertcopying
@end ifnottex

@c %**start of body
@anchor{taler-auditor-manual doc}@anchor{0}
@c This file is part of GNU TALER.
@c 
@c Copyright (C) 2019-2020 Taler Systems SA
@c 
@c TALER is free software; you can redistribute it and/or modify it under the
@c terms of the GNU General Public License as published by the Free Software
@c Foundation; either version 2.1, or (at your option) any later version.
@c 
@c TALER is distributed in the hope that it will be useful, but WITHOUT ANY
@c WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
@c A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
@c 
@c You should have received a copy of the GNU General Public License along with
@c TALER; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
@c 
@c @author Christian Grothoff

@menu
* Introduction:: 
* Installation:: 
* Configuration:: 
* Deployment:: 
* Operation:: 
* Auditor implementation guide:: 
* Index:: 

@detailmenu
 --- The Detailed Node Listing ---

Introduction

* About GNU Taler:: 
* About this manual:: 
* Organizational prerequisites:: 
* Architecture overview:: 

Installation

* Installing from source:: 
* Installing the GNU Taler binary packages on Debian:: 

Configuration

* Configuration format:: 
* Using taler-config:: 
* Keys:: 
* Serving:: 
* Bank account:: 
* Database:: 

Deployment

* Exchange:: 
* Signing Denominations:: 
* Database: Database<2>. 

Database

* Ingres replication of the exchange production database:: 
* Safe replication of the ingres database into the auditor production database:: 

Operation

* Web service:: 
* Audit:: 
* Reading the report:: 
* Database upgrades:: 
* Database reset:: 
* Revocations:: 
* Failures:: 

Auditor implementation guide

* The auditor’s database:: 
* Invariants checked by the auditor:: 
* Testing the auditor:: 

Invariants checked by the auditor

* Invariants checked by the taler-helper-auditor-aggregation:: 
* Invariants checked by the taler-helper-auditor-coins:: 
* Invariants checked by the taler-helper-auditor-deposits:: 
* Invariants checked by the taler-helper-auditor-reserves:: 
* Invariants checked by the taler-helper-auditor-wire:: 

@end detailmenu
@end menu

@node Introduction,Installation,Top,Top
@anchor{taler-auditor-manual gnu-taler-auditor-operator-manual}@anchor{1}@anchor{taler-auditor-manual introduction}@anchor{2}
@chapter Introduction


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

@menu
* About GNU Taler:: 
* About this manual:: 
* Organizational prerequisites:: 
* Architecture overview:: 

@end menu

@node About GNU Taler,About this manual,,Introduction
@anchor{taler-auditor-manual about-gnu-taler}@anchor{3}
@section 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).

@node About this manual,Organizational prerequisites,About GNU Taler,Introduction
@anchor{taler-auditor-manual about-this-manual}@anchor{4}
@section About this manual


This tutorial targets exchange operators, auditors and governments
who want to run the auditor to verify that a GNU Taler exchange is
operating correctly.

@node Organizational prerequisites,Architecture overview,About this manual,Introduction
@anchor{taler-auditor-manual organizational-prerequisites}@anchor{5}
@section Organizational prerequisites


Operating a GNU Taler auditor means that you (henceforth: auditor) have a
business relationship with (or regulatory authority over) a GNU Taler exchange
operator (henceforth: exchange).  Your objective is to verify that the
exchange is operating correctly, and if not to alert the exchange, the
state or even the public about any misbehavior to limit financial losses
to other parties.

To perform this duty, you will need at least (read-only) access to the bank
transactions of the exchange, as well as a continuously synchronized replica
of the exchange’s database.  The general assumption for running the auditor
is that this is done on a separate system controlled by the auditor. After
all, the goal is to detect nerfarious activity of the exchange operator,
which cannot be effectively done on a machine controlled by the exchange
operator.

For this, every auditor needs to operate a Postgres database.  The data
collected will include sensitive information about Taler users, including
withdrawals made by consumers and income received by merchants.  As a result,
the auditor is expected to provide high confidentiality for the database.  In
general, the auditor does not have to offer high-availability: the exchange
operator can continue operations without the auditor, and the auditor can
catch up with it later when the auditor’s systems are restored. However, of
course any downtime would provide a window of opportunity for fraud and should
thus be minimized.  Finally, the auditor’s copy of the exchange’s database can
be useful as a backup to the exchange in case the exchange experiences a loss
of its own copies. Thus, business agreements between auditor and exchanges may
include availability requirements as well.

Then, with the software provided, auditors can verify the cryptographic proofs
collected by the exchange and detect if any improper bank transactions have been
made.  There are additional tasks which an auditor should perform.  While this
manual only focuses on the audit of the exchange’s database and wire transfers
with the existing tools, a proper auditor should also perform the following
tasks:


@itemize -

@item 
security audit of the source code

@item 
audit of the operational procedures of the exchange

@item 
audit of the physical security of the deployment

@item 
background check of the individuals operating the exchange

@item 
verification that the exchange properly implements the @code{/link} protocol
(feature yet to be implemented in common Taler wallets)

@item 
verification that the exchange properly reports coins issued during
the refresh protocol (by irregularly refreshing coins withdrawn by
the auditor and comparing against the exchange’s database — the
code required to support this is not yet implemented)
@end itemize

@node Architecture overview,,Organizational prerequisites,Introduction
@anchor{taler-auditor-manual architecture-overview}@anchor{6}
@section Architecture overview


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

Auditors should generally be independent third parties that verify that the
exchange operates correctly.  However, an exchange is likely to also run the
auditing logic, as it is also used to calculate the exchange’s profits, risk
and liabilities.  Furthermore, it’s usually a good idea to not only rely on
third parties to verify one’s own work.

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


@itemize -

@item 
DBMS: Postgres

The auditor requires a DBMS to store a local copy of the transaction history for
the Taler exchange, as well as for its own internal bookkeeping and checkpointing.
The DBMS is assumed to be able to assure the auditor of the database invariants (foreign
key, uniqueness, length restrictions).  Should the exported data from the exchange
fail to be imported due to constraint violations, this is an immediate serious
concern that must be addressed manually.  The software only verifies the content
of a well-formed exchange database (well-formed with respect to SQL).
For now, the GNU Taler reference implementation
only supports Postgres, but the code could be easily extended to
support another DBMS.

@item 
The auditor Web service

The auditor is expected to provide a public Web service. At this REST API,
merchants can (probabilistically) provide deposit confirmations, allowing
the auditor to detect if an exchange is underreporting deposits.

In the future, the Web service should be extended to allow customers and
merchants to automatically upload cryptographic proof of other violations
of the specification by the exchange.  However, for now it is assumed that
the respective cryptographic proofs are reported and verified manually —
as with a well-behaved exchange this should obviously be a rare event.

The main binary of this component is the @code{taler-auditor-httpd}.

@item 
The (main) auditor

The main auditor logic checks the various signatures, totals up the
amounts and checks for arithmetic inconsistencies. It also
computes the expected bank balance, revenue and risk exposure of the
exchange operator. The main script of this component is the @code{taler-auditor}.
This script invokes several helper binaries sequentially. Production
users may want to modify the script to run those binaries in parallel,
possibly using different privileges (as only the @code{taler-helper-auditor-wire}
needs access to the wire gateway).

The @code{taler-helper-auditor-wire} auditor verifies that the bank
transactions performed by the exchange
were done properly.  This component must have access to the bank account
of the exchange, as well as to a copy of the exchange’s database.

The @code{taler-auditor} script invokes the various helpers, each generating
a JSON report. It then invokes the @code{taler-helper-auditor-render.py}
script to combine those JSON files with a Jinja2 template into a
LaTeX report.  Finally, @code{pdflatex} is used to generate a PDF report.

The resulting report includes performance data, reports on hard violations
(resulting in financial losses) and reports on soft violations (such as the
exchange not performing certain operations in a timely fashion).  The
report also includes figures on the losses of violations. Careful reading
of the report is required, as not every detail in the report is necessarily
indicative of a problem.
@end itemize

@node Installation,Configuration,Introduction,Top
@anchor{taler-auditor-manual installation}@anchor{7}
@chapter Installation


@menu
* Installing from source:: 
* Installing the GNU Taler binary packages on Debian:: 

@end menu

@node Installing from source,Installing the GNU Taler binary packages on Debian,,Installation
@anchor{taler-auditor-manual installing-from-source}@anchor{8}
@section Installing from source


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


@itemize -

@item 
libsqlite3 >= 3.16.2

@item 
GNU libunistring >= 0.9.3

@item 
libcurl >= 7.26 (or libgnurl >= 7.26)

@item 
libqrencode >= 4.0.0

@item 
GNU libgcrypt >= 1.6

@item 
libsodium >= 1.0

@item 
libargon2 >= 20171227

@item 
libjansson >= 2.7

@item 
Postgres >= 9.6, including libpq

@item 
GNU libmicrohttpd >= 0.9.71

@item 
GNUnet >= 0.14.0 (from source tarball@footnote{http://ftpmirror.gnu.org/gnunet/})

@item 
GNU Taler exchange (from download directory@footnote{http://ftpmirror.gnu.org/taler/},
see release announcement@footnote{https://mail.gnu.org/archive/cgi-bin/namazu.cgi?query=taler&idxname=info-gnu&max=20&result=normal&sort=date:late})
@end itemize

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

The following instructions will show how to install libgnunetutil and
the exchange (which includes the code for the auditor).

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:

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

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

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

@example
$ ./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
@end example

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

@node Installing the GNU Taler binary packages on Debian,,Installing from source,Installation
@anchor{taler-auditor-manual installing-the-gnu-taler-binary-packages-on-debian}@anchor{9}
@section 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
Sid, which means you should use a system which at least includes
unstable packages in its source list.  We recommend using APT pinning
to limit unstable packages to those explicitly requested. To do this,
set your @code{/etc/apt/preferences} as follows:

@example
Package: *
Pin: release a=stable
Pin-Priority: 700

Package: *
Pin: release a=testing
Pin-Priority: 650

Package: *
Pin: release a=unstable
Pin-Priority: 600

Package: *
Pin: release l=Debian-Security
Pin-Priority: 1000
@end example

A typical @code{/etc/apt/sources.list} file for this setup
would look like this:

@example
deb http://ftp.ch.debian.org/debian/ buster main
deb http://security.debian.org/debian-security buster/updates main
deb http://ftp.ch.debian.org/debian/ testing main
deb http://ftp.ch.debian.org/debian/ unstable main
deb https://deb.taler.net/apt/debian sid main
@end example

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:

@example
# wget -O - https://taler.net/taler-systems.gpg.key | apt-sign add -
# apt update
@end example

@cartouche
@quotation Note 
You may want to verify the correctness of the Taler Systems key out-of-band.
@end quotation
@end cartouche

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

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

@example
# apt install taler-auditor
@end example

For the auditor, you must manually configure access to the exchange database,
the HTTP reverse proxy (typically with TLS certificates) and offline signing.

Sample configuration files for the HTTP reverse proxy can be found in
@code{/etc/taler-exchange/}.

@node Configuration,Deployment,Installation,Top
@anchor{taler-auditor-manual configuration}@anchor{a}
@chapter Configuration


The auditor’s configuration works the same way as the configuration of other
Taler components.
This section discusses configuration options related to the auditor.

@menu
* Configuration format:: 
* Using taler-config:: 
* Keys:: 
* Serving:: 
* Bank account:: 
* Database:: 

@end menu

@node Configuration format,Using taler-config,,Configuration
@anchor{taler-auditor-manual configuration-format}@anchor{b}
@section Configuration format


In Taler realm, any component obeys to the same pattern to get
configuration values. According to this pattern, once the component has
been installed, the installation deploys default values in
$@{prefix@}/share/taler/config.d/, in .conf files. In order to override
these defaults, the user can write a custom .conf file and either pass
it to the component at execution time, or name it taler.conf and place
it under $HOME/.config/.

A config file is a text file containing sections, and each section
contains its values. The right format follows:

@example
[section1]
value1 = string
value2 = 23

[section2]
value21 = string
value22 = /path22
@end example

Throughout any configuration file, it is possible to use @code{$}-prefixed
variables, like @code{$VAR}, especially when they represent filesystem
paths. It is also possible to provide defaults values for those
variables that are unset, by using the following syntax:
@code{$@{VAR:-default@}}. However, there are two ways a user can set
@code{$}-prefixable variables:

by defining them under a @code{[paths]} section, see example below,

@example
[paths]
TALER_DEPLOYMENT_SHARED = $@{HOME@}/shared-data
..
[section-x]
path-x = $@{TALER_DEPLOYMENT_SHARED@}/x
@end example

or by setting them in the environment:

@example
$ export VAR=/x
@end example

The configuration loader will give precedence to variables set under
@code{[path]}, though.

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

Note that, in this stage of development, the file
@code{$HOME/.config/taler.conf} can contain sections for @emph{all} the
component. For example, both an exchange and a bank can read values from
it.

The repository @code{git://taler.net/deployment} contains examples of
configuration file used in our demos. See under @code{deployment/config}.

@quotation

@strong{Note}

Expectably, some components will not work just by using default
values, as their work is often interdependent. For example, a
merchant needs to know an exchange URL, or a database name.
@end quotation

@node Using taler-config,Keys,Configuration format,Configuration
@anchor{taler-auditor-manual using-taler-config}@anchor{c}
@section Using taler-config


The tool @code{taler-config} can be used to extract or manipulate
configuration values; however, the configuration use the well-known INI
file format and can also be edited by hand.

Run

@example
$ taler-config -s $SECTION
@end example

to list all of the configuration values in section @code{$SECTION}.

Run

@example
$ taler-config -s $section -o $option
@end example

to extract the respective configuration value for option @code{$option} in
section @code{$section}.

Finally, to change a setting, run

@example
$ taler-config -s $section -o $option -V $value
@end example

to set the respective configuration value to @code{$value}. Note that you
have to manually restart the Taler backend after you change the
configuration to make the new configuration go into effect.

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

@example
$ taler-config -s ACCOUNT-bank \
               -o WIRE_RESPONSE
$ taler-config -f -s ACCOUNT-bank \
               -o WIRE_RESPONSE
@end example

While the configuration file is typically located at
@code{$HOME/.config/taler.conf}, an alternative location can be specified
to @code{taler-merchant-httpd} and @code{taler-config} using the @code{-c}
option.

@node Keys,Serving,Using taler-config,Configuration
@anchor{taler-auditor-manual auditorkeys}@anchor{d}@anchor{taler-auditor-manual keys}@anchor{e}
@section Keys


The auditor works with one signing key to certify that it is auditing
a particular exchange’s denomination keys.  This key can and should
be kept @emph{offline} (and backed up adequately). As with the exchange’s
offline key, it is only used for a few cryptographic signatures and
thus the respective code can be run on modest hardware, like a
Raspberry Pi.

The following values are to be configured in the section @code{[auditor]}:


@itemize -

@item 
@code{AUDITOR_PRIV_FILE}: Path to the auditor’s private key file.

@item 
@code{PUBLIC_KEY}: Public key of the auditor, in Base32 encoding.
Set from value printed by @code{gnunet-ecc -p $AUDITOR_PRIV_FILE}.
@end itemize

@node Serving,Bank account,Keys,Configuration
@anchor{taler-auditor-manual auditorserving}@anchor{f}@anchor{taler-auditor-manual serving}@anchor{10}
@section Serving


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

The following values are to be configured in the section @code{[auditor]}:


@itemize -

@item 
@code{serve}: must be set to @code{tcp} to serve HTTP over TCP, or @code{unix} to serve
HTTP over a UNIX domain socket

@item 
@code{port}: Set to the TCP port to listen on if @code{serve} is @code{tcp}.

@item 
@code{unixpath}: set to the UNIX domain socket path to listen on if @code{serve} is
@code{unix}

@item 
@code{unixpath_mode}: number giving the mode with the access permission MASK
for @code{unixpath} (i.e. 660 = @code{rw-rw----}).
@end itemize

@node Bank account,Database,Serving,Configuration
@anchor{taler-auditor-manual auditorbank-account}@anchor{11}@anchor{taler-auditor-manual bank-account}@anchor{12}
@section Bank account


Bank accounts for the auditor are configured in exactly the
same way as bank accounts for the exchange. See the exchange
documentation for details.

@node Database,,Bank account,Configuration
@anchor{taler-auditor-manual auditordatabaseconfiguration}@anchor{13}@anchor{taler-auditor-manual database}@anchor{14}
@section Database


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


@itemize -

@item 
via an environment variable: @code{TALER_AUDITORDB_POSTGRES_CONFIG}.

@item 
via configuration option @code{CONFIG}, under section @code{[auditordb-BACKEND]}.
For example, the demo exchange is configured as follows:

@example
[auditor]
...
DB = postgres
...

[auditordb-postgres]
CONFIG = postgres:///auditordemo
@end example
@end itemize

If an exchange runs its own auditor, it may use the same database for
the auditor and the exchange itself.

The @code{taler-auditor-dbinit} tool is used to initialize the auditor’s
tables. After running this tool, the rights to CREATE or DROP tables
are no longer required and should be removed.

Both the @code{taler-auditor-httpd} and the @code{taler-auditor} (and its helpers)
also need (read-only) access to a (recent, current, synchronized) copy of the
exchange’s database.  The configuration options are the same that are also
used when configuring the exchange’ database:

@quotation

@example
[exchange]
...
DB = postgres
...

[exchangedb-postgres]
CONFIG = postgres:///exchangedemo
@end example
@end quotation

@node Deployment,Operation,Configuration,Top
@anchor{taler-auditor-manual auditordeployment}@anchor{15}@anchor{taler-auditor-manual deployment}@anchor{16}
@chapter Deployment

@anchor{taler-auditor-manual wallets}@anchor{17}
Before GNU Taler wallets will happily interact with an exchange,
the respective auditor’s public key (to be obtained via @code{gnunet-ecc -p})
must be added under the respective currency to the wallet.  This
is usually expected to be hard-coded into the Taler wallet.

Users can also manually add auditors for a particular currency via a
Web page offering the respective pairing.

FIXME-DOLD: explain how that Web page works, once it works…

@menu
* Exchange:: 
* Signing Denominations:: 
* Database: Database<2>. 

@end menu

@node Exchange,Signing Denominations,,Deployment
@anchor{taler-auditor-manual auditorexchange}@anchor{18}@anchor{taler-auditor-manual exchange}@anchor{19}
@section Exchange


The next step is to add the exchange’s master public key and the base
URL of the exchange to the list of exchanges audited by the auditor.
This is done using the @code{taler-auditor-exchange} tool.  The tool
basically creates the respective record in the auditor’s database.

If this step is skipped, the auditor will malfunction at all future
stages with a foreign key violation, as it doesn’t know the exchange’s
master public key.

@example
$ taler-auditor-exchange -m $MASTER_PUB -u $EXCHANGE_BASE_URL
@end example

The equivalent step must be performed by the exchange operator.
Here, the exchange operator must use the @code{taler-exchange-offline}
tool to add the auditor’s public key, base URL and (business) name
to the list of approved auditors of the exchange. For details,
see Auditor-configuration in the exchange operator manual.

@node Signing Denominations,Database<2>,Exchange,Deployment
@anchor{taler-auditor-manual signing-denominations}@anchor{1a}@anchor{taler-auditor-manual signingdenominations}@anchor{1b}
@section Signing Denominations


@geindex maintenance

This step must be performed regularly whenever the exchange is
deploying new denomination keys. After the exchange operator
has signed new keys using the @code{taler-exchange-offline} tool,
each auditor should run:

@example
$ taler-auditor-offline download > input.json
@end example

to import the latest set of denomination keys. The key data
should then be inspected using:

@example
$ taler-auditor-offline show < input.json
@end example

and compared with the data the exchange operator saw when doing the offline
signature. This process should involve directly the humans operating both
systems and may require them to establish a trustworthy connection. The
details how the auditor communicates with the exchange operator are a business
process that is outside of the scope of this document.

Note that the @code{input.json} does not contain any confidential data.  However,
signing the wrong keys would be fatal in that it may allow an illegitimate
exchange to convince users that it is a trustworthy operator and subsequently
betray the user’s trust that is anchored in the existence of a trustworthy
auditor.

Given the verified JSON input, the auditor can then sign it (typically
on its offline system) using:

@example
$ taler-auditor-offline sign < input.json > output.json
@end example

The resulting @code{output.json} should then be copied to an online system,
and from there uploaded to the exchange using:

@example
$ taler-auditor-offline upload < output.json
@end example

The contents of @code{output.json} can again be public and require no special
handling.

If the auditor has been correctly added, the exchange’s @code{/keys}
response will contain an entry in the @code{auditors} array mentioning the
auditor’s URL.

Commands, like @code{taler-auditor-offline}, that support the @code{-l LOGFILE}
command-line option, send logging output to standard error by default.

@node Database<2>,,Signing Denominations,Deployment
@anchor{taler-auditor-manual auditordatabaseinitialization}@anchor{1c}@anchor{taler-auditor-manual id1}@anchor{1d}
@section Database


The next key step for the auditor is to configure replication of the
@emph{exchange}’s database in-house. This should be performed in two steps
as illustrated in the following figure:

@image{taler-auditor-figures/replication,,,,png}

First, the exchange should use standard Postgres replication features to
enable the auditor to obtain a full copy of the exchange’s database.
Second, the auditor should make a “trusted” local copy, ensuring that it
never replicates malicious changes using @code{taler-auditor-sync}. Both
of these steps are described in more detail below.

We note that as a result of these steps, the auditor will have three
databases: its own production primary database (as configured in
@code{auditordb-postgres}), its on production copy of the exchange’s database
(@code{exchangedb-postgress}), and a third, untrusted “ingres” copy of the
exchange database.  The untrusted database should run as a separate Postgres
instance and is only accessed via @code{taler-auditor-sync} and the replication
mechanism driven by the exchange operator.

@menu
* Ingres replication of the exchange production database:: 
* Safe replication of the ingres database into the auditor production database:: 

@end menu

@node Ingres replication of the exchange production database,Safe replication of the ingres database into the auditor production database,,Database<2>
@anchor{taler-auditor-manual ingres-replication-of-the-exchange-production-database}@anchor{1e}
@subsection Ingres replication of the exchange production database


The full copy can be obtained in various ways with Postgres.  It is
possible to use log shipping with streaming replication as described
in @indicateurl{https://www.postgresql.org/docs/13/warm-standby.html}, or to use
logical replication, as described in
@indicateurl{https://www.postgresql.org/docs/13/logical-replication.html}. We note
that asynchronous replication should suffice.

The resulting auditor database should be treated as read-only on the auditor
side.  The @code{taler-exchange-dbinit} tool can be used to setup the schema, or
the schema can be replicated using Postgres’s standard mechanisms. The same
applies for schema upgrades: if logical replication is used (which does not
replicate schema changes), @code{taler-exchange-dbinit} can be used to migrate
the schema(s) in both the ingres and production copies of the exchange’s
database as well.

For details, we refer to the Postgres manual.

@cartouche
@quotation Note 
Depending on the replication method used, the exchange may perform
unexpected changes to the schema or perform @code{UPDATE}, @code{DELETE} or
@code{DROP} operations on the tables.  Hence, the auditor cannot rely upon the
exchange’s primary copy to respect schema constraints, especially as we
have to presume that the exchange could act maliciously.  Furthermore, it
is unclear to what degree Postgres database replication mechanisms are
robust against a malicious master database.  Thus, the auditor should
isolate its primary copy of the exchange database, including the Postgres
process, from its actual operational data.
@end quotation
@end cartouche

@node Safe replication of the ingres database into the auditor production database,,Ingres replication of the exchange production database,Database<2>
@anchor{taler-auditor-manual safe-replication-of-the-ingres-database-into-the-auditor-production-database}@anchor{1f}
@subsection Safe replication of the ingres database into the auditor production database


Using @code{taler-auditor-sync}, the auditor should make a second “safe” copy of
the exchange’s ingres database.  @code{taler-auditor-sync} basically reads from one
exchange database and inserts all records found into a second exchange
database. If the source database violates invariants, the tool halts with an
error. This way, records violating invariants are never even copied, and in
particular schema changes and deletions or updates are not propagated into the
auditor’s production database.

While @code{taler-auditor-sync} could in theory be run directly against the
exchange’s production system, this is likely a bad idea due to the high
latency from the network between auditor and exchange operator. Thus, we
recommend first making an “untrusted” ingress copy of the exchange’s
production database using standard Postgres tooling, and then using
@code{taler-auditor-sync} to create a second “safe” copy.  The “safe” copy used
by the production system should also run under a different UID.

Before @code{taler-auditor-sync} can be used, the target database must be
initialized with the exchange schema using @code{taler-exchange-dbinit}.
Note that running @code{taler-auditor-sync} requires the use of two
configuration files, one specifying the options for accessing the source
database, and a second with the options for accessing the destination
database.  In both cases, likely only the @code{[exchangedb]/CONFIG} option
needs to be changed.

When the exchange performs garbage collection to @code{DELETE} obsolete records,
this change should be automatically replicated to the auditors untrusted
ingress database.  However, as @code{taler-auditor-sync} tries to be “safe”,
it will not replicate those deletions to the auditor’s production database.
Thus, it is necessary to (occasonally) run @code{taler-exchange-dbinit -g} on
the auditor’s production database to garbage collect old data in the
auditor’s production copy.  We note that this does not have to be done
at the same time when the exchange runs its garbage collection.

@node Operation,Auditor implementation guide,Deployment,Top
@anchor{taler-auditor-manual id2}@anchor{20}@anchor{taler-auditor-manual operation}@anchor{21}
@chapter Operation


@menu
* Web service:: 
* Audit:: 
* Reading the report:: 
* Database upgrades:: 
* Database reset:: 
* Revocations:: 
* Failures:: 

@end menu

@node Web service,Audit,,Operation
@anchor{taler-auditor-manual id3}@anchor{22}@anchor{taler-auditor-manual web-service}@anchor{23}
@section Web service


The @code{taler-auditor-httpd} runs the required REST API for the auditor.  The
service must have @code{INSERT} (and @code{SELECT}) rights on the
@code{deposit_confirmations} table in the auditor’s database.  We expect that in
future versions additional rights may be required.

As the @code{taler-auditor-httpd} does not include HTTPS-support, it is
advisable to run it behind a reverse proxy that offers TLS termination.

@node Audit,Reading the report,Web service,Operation
@anchor{taler-auditor-manual audit}@anchor{24}@anchor{taler-auditor-manual id4}@anchor{25}
@section Audit


Performing an audit is done by invoking the @code{taler-auditor} shell script.
The shell script invokes the various helper processes. For additional
performance and security, one may want to run the various helpers individually
and with the respective minimal set of access rights (only
@code{taler-helper-auditor-wire} needs the credentials to query the bank for wire
transfers).  The shell script combines the final JSON outputs of the various
helpers using the @code{taler-helper-auditor-render.py} script into the TeX
report.  Regardless, the simplest way to obtain a report is to run:

@example
$ taler-audit
@end example

This generates a file @code{auditor-report.pdf} (in a temporary directory created
for this purpose) with all of the issues found and the financial assessment of
the exchange.  The exact filename will be output to the console upon
completion.

We note that @code{taler-audit} by default runs in incremental mode. As a result,
running the commands again will only check the database entries that have been
added since the last run.

You can use @code{taler-auditor-dbinit -r} to force a full check since the
beginning of time. However, as this may require excessive time and
interactions with the bank (which may not even have the wire transfer records
anymore), this is not recommended in a production setup.

@node Reading the report,Database upgrades,Audit,Operation
@anchor{taler-auditor-manual reading-the-report}@anchor{26}
@section Reading the report


The auditor’s report needs to be read carefully, as it includes
several categories of failures of different severity:


@itemize -

@item 
Delayed operations, where an operation was expected to have
happened, but did not happen yet, possibly because of a
disagreement in system time or overloading of the system.
These failures only require action if the delays are
significant.

@item 
Inconsistencies in the data that have no clear financial
impact.

@item 
Inconsistencies in the data that show that the exchange
experienced an unexpected financial loss (such as accepting a coin for
deposit with an invalid signature).

@item 
Inconsistencies in the data that show that the exchange
caused some other party to experience a financial loss (such as not wiring
the correct amount to a merchant).

@item 
Configuration issues (such was wire fees unavailable).
@end itemize

@node Database upgrades,Database reset,Reading the report,Operation
@anchor{taler-auditor-manual auditordatabaseupgrades}@anchor{27}@anchor{taler-auditor-manual database-upgrades}@anchor{28}
@section Database upgrades


To upgrade the database between Taler versions can be done by running:

@example
$ taler-auditor-dbinit
$ taler-exchange-dbinit
@end example

In any case, it is recommended that exchange and auditor coordinate closely
during schema-changing database upgrades as without coordination the database
replication or @code{taler-auditor-sync} will likely experience problematic
failures.  In general, we recommend:

@quotation


@itemize *

@item 
halting the exchange business logic,

@item 
allowing the replication and @code{taler-auditor-sync} to complete
(see also the @strong{-t} option of @code{taler-auditor-sync})

@item 
completing a @code{taler-audit} run against the old schema

@item 
migrating the exchange schema (@code{taler-exchange-dbinit}) of
the master database, possibly the ingres database and the
auditor’s production copy

@item 
migrating the auditor database (@code{taler-auditor-dbinit})

@item 
resuming database replication between the exchange’s master
database and the auditor’s ingres copy

@item 
resuming @code{taler-auditor-sync}

@item 
resuming the regular exchange and auditor business logic
@end itemize
@end quotation

Regardless, the above is merely the general rule. Please review the specific
release notes to ensure this procedure is correct for the specific upgrade.

@node Database reset,Revocations,Database upgrades,Operation
@anchor{taler-auditor-manual database-reset}@anchor{29}
@section Database reset


The auditor database can be reset using:

@example
$ taler-auditor-dbinit -R
@end example

However, running this command will result in all data in the database being
@emph{lost}, including steps like enabling an exchange using
@code{taler-auditor-exchange}. Thus, doing so may result in significant
commputation (and bandwidth consumption with the bank) when the auditor is
next launched, as it will re-download and re-verify all historic transactions.
Hence this should not be done in a production system.

@node Revocations,Failures,Database reset,Operation
@anchor{taler-auditor-manual auditorrevocations}@anchor{2a}@anchor{taler-auditor-manual revocations}@anchor{2b}
@section Revocations


When an auditor detects that the private key of a denomination key pair has
been compromised, one important step is to revoke the denomination key.  The
exchange operator includes the details on how to revoke a denomination key, so
the auditor should only have to report (and possibly enforce) this step.
For more information, see Revocations in the exchange operator manual.

If all denominations of an exchange are revoked, the exchange includes logic
to wire back all returned funds to the bank accounts from which they
originate.  If some denominations remain operational, wallets will generally
exchange old coins of revoked denominations for new coins – while providing
additional information to demonstrate that these coins were not forged from
the compromised private key but obtained via a legitimate withdraw operation.

@node Failures,,Revocations,Operation
@anchor{taler-auditor-manual failures}@anchor{2c}
@section Failures


Most audit failures are handled by the auditor’s regular reporting functionality,
creating a (hopefully descriptive) PDF report detailing the problems found.

However, there is one category of errors where this is not possible, which
concerns arithmetic overflows for amounts. Taler’s specification limits amount
values to at most 2^52. If, during the auditor’s calculations, amounts are
encountered that exceed this threshold, the auditor will not generate a regular
report, but instead write a log statement explaining where the problem happened
and exit with a status code of @emph{42}.

The most common expected case when this happens is a corrupted database. This
could be because the exchange is actively malicious, or more likely due to
some data corruption.  The audit cannot continue until the corruption has been
addressed. If it is not possible to restore a fully @emph{correct} version of the
database, the suggestion is to replace the corrupted (and likely very large)
amounts with zero (Note: this does not apply to the value of denominations or
fees, here it is crucial that the correct amounts are restored). While an
amount of zero would be incorrect, the auditing logic should be able to do its
calculations with zero instead.

After patching the database, the audit can
be restarted. A full reset is not required, as the audit transaction is aborted
when the auditor exits with code @emph{42}.  After restarting, the resulting audit
report is likely to indicates errors relating to the corrupted fields (such as
invalid signatures, arithmetic errors by the exchange, etc.), but at least the
loss/gain calculations will be meaningful and actually indicative of the scope
of the error created by the corrupted data.

@node Auditor implementation guide,Index,Operation,Top
@anchor{taler-auditor-manual auditor-implementation-guide}@anchor{2d}
@chapter Auditor implementation guide


The auditor implementation is split into five main processes, called
@code{taler-helper-auditor-XXX}.  The split was done to realize the principle of
least privilege and to enable independent logic to be possibly run in
parallel.  Only the taler-wire-auditor must have (read-only) access to the
exchange’s bank account, the other components only need access to the
database.

All auditor subsystems basically start their audit from a certain transaction
index (@code{BIG SERIAL}) in the auditor database which identifies where the last
audit concluded. They then check that the transactions claimed in the
exchange’s database match up internally, including the cryptographic
signatures and also with respect to amounts adding up. The auditor also
calculates the exchange’s profits and expected bank balances.  Once all
existing transactions are processed, the auditor processes store the current
checkpoint in its database and generate a JSON report.

The @code{taler-auditor} shell script calls the five helpers and then
uses Jinja2 with a TeX template to convert the five individual
JSON reports into LaTeX and then into PDF.

@menu
* The auditor’s database:: 
* Invariants checked by the auditor:: 
* Testing the auditor:: 

@end menu

@node The auditor’s database,Invariants checked by the auditor,,Auditor implementation guide
@anchor{taler-auditor-manual the-auditor-s-database}@anchor{2e}
@section The auditor’s database


The database scheme used by the exchange looks as follows:

@image{taler-auditor-figures/auditor-db,,,,png}

@node Invariants checked by the auditor,Testing the auditor,The auditor’s database,Auditor implementation guide
@anchor{taler-auditor-manual invariants-checked-by-the-auditor}@anchor{2f}
@section Invariants checked by the auditor


The auditor verifies a large number of invariants that must hold for a Taler
exchange.  One objective in the design of the auditor was to check each
invariant only once, both to minimize cost and to avoid duplicate reporting of
problems where possible. As a result, not every invariant is checked in every
pass where it might seem applicable.

@menu
* Invariants checked by the taler-helper-auditor-aggregation:: 
* Invariants checked by the taler-helper-auditor-coins:: 
* Invariants checked by the taler-helper-auditor-deposits:: 
* Invariants checked by the taler-helper-auditor-reserves:: 
* Invariants checked by the taler-helper-auditor-wire:: 

@end menu

@node Invariants checked by the taler-helper-auditor-aggregation,Invariants checked by the taler-helper-auditor-coins,,Invariants checked by the auditor
@anchor{taler-auditor-manual invariants-checked-by-the-taler-helper-auditor-aggregation}@anchor{30}
@subsection Invariants checked by the taler-helper-auditor-aggregation


This is from CodeBlau’s analysis. A proper write-up is pending.
CodeBlau reports the following checks:


@itemize -

@item 
arithmetic inconsistencies


@itemize -

@item 
disagreement in fee for deposit between auditor and exchange db

@item 
disagreement in fee for melt between auditor and exchange db

@item 
disagreement in fee for refund between auditor and exchange db

@item 
aggregation of fee is negative

@item 
aggregation (contribution): Expected coin contributions differ:
coin value without fee, total deposit without refunds

@item 
wire out fee is negative
@end itemize

@item 
coin arithmetic inconsistencies


@itemize -

@item 
refund (merchant) is negative

@item 
refund (balance) is negative

@item 
spend > value
@end itemize

@item 
coin denomination signature invalid

@item 
start date before previous end date

@item 
end date after next start date

@item 
wire out inconsistencies in amount

@item 
row inconsistencies


@itemize -

@item 
wire account given is malformed

@item 
h(wire) does not match wire

@item 
failed to compute hash of given wire data

@item 
database contains wrong hash code for wire details

@item 
no transaction history for coin claimed in aggregation

@item 
could not get coin details for coin claimed in aggregation

@item 
could not find denomination key for coin claimed in aggregation

@item 
coin denomination signature invalid

@item 
target of outgoing wire transfer do not match hash of wire from deposit

@item 
date given in aggregate does not match wire transfer date

@item 
wire fee signature invalid at given time

@item 
specified wire address lacks method

@item 
wire fee unavailable for given time
@end itemize
@end itemize

@node Invariants checked by the taler-helper-auditor-coins,Invariants checked by the taler-helper-auditor-deposits,Invariants checked by the taler-helper-auditor-aggregation,Invariants checked by the auditor
@anchor{taler-auditor-manual invariants-checked-by-the-taler-helper-auditor-coins}@anchor{31}
@subsection Invariants checked by the taler-helper-auditor-coins


This is from CodeBlau’s analysis. A proper write-up is pending.
CodeBlau reports the following checks:


@itemize -

@item 
check that all denominations used by the exchange have been signed using
this auditor’s key. All denominations encountered in the database that
this auditor did not officially sign for are reported (but still included
in the audit as they obviously may impact the exchange’s bank balance).
Depending on the business situation, this may be normal (say if an exchange
is changing auditors and newer denominations are no longer supported until
their end-of-life by the current auditor).

@item 
emergency on denomination over loss


@itemize -

@item 
value of coins deposited exceed value of coins issued
@end itemize

@item 
emergency on number of coins, num mismatch

@item 
arithmetic inconsistencies


@itemize -

@item 
melt contribution vs. fee

@item 
melt (cost)

@item 
refund fee
@end itemize

@item 
row inconsistencies


@itemize -

@item 
revocation signature invalid

@item 
denomination key not found

@item 
denomination key for fresh coin unknown to auditor

@item 
denomination key for dirty coin unknown to auditor

@item 
denomination key for deposited coin unknown to auditor
@end itemize

@item 
coin validity in known_coin, by checking denomination signatures

@item 
coin validity in melt, by checking signatures

@item 
refresh hanging, zero reveals (harmless)

@item 
verify deposit signature

@item 
verify refund signature

@item 
recoup, check coin

@item 
recoup, check signature

@item 
recoup, denomination not revoked
@end itemize

@node Invariants checked by the taler-helper-auditor-deposits,Invariants checked by the taler-helper-auditor-reserves,Invariants checked by the taler-helper-auditor-coins,Invariants checked by the auditor
@anchor{taler-auditor-manual invariants-checked-by-the-taler-helper-auditor-deposits}@anchor{32}
@subsection Invariants checked by the taler-helper-auditor-deposits


This tool verifies that the deposit confirmations reported by merchants
directly to the auditor are also included in the database duplicated from the
exchange at the auditor.  This is to ensure that the exchange cannot defraud
merchants by simply not reporting deposits to the auditor.

@node Invariants checked by the taler-helper-auditor-reserves,Invariants checked by the taler-helper-auditor-wire,Invariants checked by the taler-helper-auditor-deposits,Invariants checked by the auditor
@anchor{taler-auditor-manual invariants-checked-by-the-taler-helper-auditor-reserves}@anchor{33}
@subsection Invariants checked by the taler-helper-auditor-reserves


This is from CodeBlau’s analysis. A proper write-up is pending.
CodeBlau reports the following checks:


@itemize -

@item 
report arithmetic inconsistency


@itemize -

@item 
closing aggregation fee

@item 
global escrow balance
@end itemize

@item 
denomination key validity withdraw inconsistencies

@item 
bad signature losses in withdraw

@item 
bad signature losses in recoup

@item 
bad signature losses in recoup-master

@item 
reserve balance, insufficient, losses and gains

@item 
reserve balance, summary wrong

@item 
reserve not closed after expiration time

@item 
could not determine closing fee / closing-fee unavailable

@item 
denomination key not found for withdraw

@item 
denomination key not in revocation set for recoup

@item 
target account not verified, auditor does not know reserve

@item 
target account does not match origin account
@end itemize

@node Invariants checked by the taler-helper-auditor-wire,,Invariants checked by the taler-helper-auditor-reserves,Invariants checked by the auditor
@anchor{taler-auditor-manual invariants-checked-by-the-taler-helper-auditor-wire}@anchor{34}
@subsection Invariants checked by the taler-helper-auditor-wire


This auditor is special in that it is the only pass that is required to have
@emph{read-only} access to the exchange’s bank account (privilege separation).  Its
main role is to verify that the wire transfers in the exchange’s database and
those reported by the bank are identical.

This is from CodeBlau’s analysis. A proper write-up is pending.
CodeBlau reports the following checks:


@itemize -

@item 
check pending

@item 
wire missing

@item 
execution date mismatch

@item 
wire out consistency

@item 
wire transfer not made (yet?)

@item 
receiver account mismatch

@item 
wire amount does not match

@item 
justification for wire transfer not found

@item 
duplicate subject hash

@item 
duplicate wire offset

@item 
incoming wire transfer claimed by exchange not found

@item 
wire subject does not match

@item 
wire amount does not match

@item 
debit account url does not match

@item 
execution date mismatch

@item 
closing fee above total amount
@end itemize

@node Testing the auditor,,Invariants checked by the auditor,Auditor implementation guide
@anchor{taler-auditor-manual testing-the-auditor}@anchor{35}
@section Testing the auditor


The main objective of the auditor is to detect inconsistencies. Thus, the
@code{test-auditor.sh} script deliberately introduces various inconsistencies into
a synthetic exchange database.  For this, an “normal” exchange database is
first generated using the @code{taler-wallet-cli}.  Then, various fields or rows
of that database are manipulated, and the auditor is let loose on the modified
database.  Afterwards, the test verifies that the JSON contains values
indicating that the auditor found the inconsistencies.  The script also
verifies that template expansion and LaTeX run work for the JSON output,
but it does not verify the correctness of the final PDF.

The @code{test-auditor.sh} script is written to maximize code coverage: it should
cover as many code paths as possible in both the exchange and the auditor.  It
should also ideally create all interesting possible variations of the exchange
database fields (within the constraints of the database schema).

In general, @code{test-auditor.sh} runs the tests against an “old” database where
some transactions are past the due-date (and hence the aggregator would trigger
wire transfers), as well as a freshly generated exchange database where the
auditor would not perform any transfers.  Auditor interactions can be made
before or after the aggregator, depending on what is being tested.

The current script also rudimentarily tests the auditor’s resume logic,
by re-starting the auditor once against a database that the auditor has
already seen.

The @code{test-revocation.sh} script performs tests related to the handling of
key revocations.

The @code{test-sync.sh} script performs tests related to the @code{taler-auditor-sync}
tool.

@c TODO
@c 
@c More extensive auditor testing where additional transactions
@c have been made against the database when the audit is being resumed
@c should be done in the future.

@node Index,,Auditor implementation guide,Top
@unnumbered Index


@printindex ge


@c %**end of body
@bye