path: root/doc/taler-exchange.texi

\input texinfo   @c -*-texinfo-*-
@setfilename taler-exchange.info
@documentencoding UTF-8
@include version.texi
@settitle The GNU Taler Exchange Operator Manual
@paragraphindent 0
@exampleindent 4

@include syntax.texi

@copying
This manual is for the GNU Taler Exchange
(version @value{VERSION}, @value{UPDATED}),
a payment service provider for GNU Taler.

Copyright @copyright{} 2014-2018 Taler Systems SA

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts.  A copy of the license is included in the section entitled "GNU
Free Documentation License".
@end quotation
@end copying

@dircategory Network applications
@direntry
* GNU Taler Exchange: (taler-exchange).       Electronic payment system.
@end direntry


@titlepage
@title The GNU Taler Exchange Operator Manual
@subtitle Version @value{VERSION}
@subtitle @value{UPDATED}
@author Christian Grothoff (@email{grothoff@@taler.net})
@author Marcello Stanisci (@email{stanisci@@taler.net})
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@summarycontents
@contents

@ifnottex
@node Top
@top The GNU Taler Exchange Operator Manual
@insertcopying
@end ifnottex


@menu
* Introduction::
* Installation::
* Configuration::
* Deployment::
* Diagnostics::



Appendices

* GNU-AGPL::                    The GNU Affero General Public License says how you
                                can copy and share the code of the `GNU Taler Exchange'.
* GNU-FDL::                     The GNU Free Documentation License says how you
                                can copy and share the documentation of `GNU Taler'.

Indices

* Concept Index::               Index of concepts and programs.
@end menu

@node Introduction
@chapter Introduction

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


@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).

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

@section About this manual

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

@section Organizational prerequisites

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

@cindex availability
@cindex backup
@cindex replication
GNU Taler payment service providers generally need to ensure high
availability and have @emph{really} good backups (synchronous
replication, asynchronous remote replication, off-site backup, 24/7
monitoring, etc.).@footnote{Naturally, you could operate a Taler
exchange for a toy currency without any real value on low-cost setups
like a Raspberry Pi, but we urge you to limit the use of such setups
to research and education as with GNU Taler data loss instantly
results in financial losses.}  This manual will not cover these
aspects of operating a payment service provider.

@cindex HSM
@cindex offline
@cindex database
@cindex operational security
We will assume that you can operate a (high-availability,
high-assurance) Postgres database.  Furthermore, we expect some
moderate familiarity with the compilation and installation of free
software packages.  You need to understand the cryptographic concepts
of private and public keys and must be able to protect private keys
stored in files on disk.  An exchange uses an @emph{offline} master
key as well as @emph{online} keys.  You are advised to secure your
private master key and any copies on encrypted, always-offline
computers.  Again, we assume that you are familiar with good best
practices in operational security, including securing key
material.@footnote{The current implementation does not make provisions
for secret splitting.  Still, the use of a hardware security module
(HSM) for protecting private keys is adviseable, so please contact the
developers for HSM integration support.}


@section Architecture overview

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

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

@cindex auditor
@cindex accounting
@emph{Auditors} are third parties, for example financial regulators,
that verify that the exchange operates correctly.  The same software
is also used to calculate the exchange's profits, risk and liabilities
by the accountants of the exchange.

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

@itemize
@cindex HTTP frontend
@item The HTTP frontend interacts with Taler wallets and
  merchant backends.  It is used to withdraw coins, deposit
  coins, refresh coins, issue refunds, map wire transfers to
  Taler transactions, inquire about the exchange's bank account
  details, signing keys and fee structure.
  The binary is the @code{taler-exchange-httpd}.
@cindex Aggregator
@item The aggregator combines multiple deposits made by
  the same merchant and (eventually) triggers wire transfers for the
  aggregate amount.  The merchant can control how quickly wire
  transfers are made. The exchange may be charge a fee per wire transfer
  to discourage excessively frequent transfers.  The binary
  is the @code{taler-exchange-aggregator}.
@cindex Auditor
@item The auditor verifies that the transactions performed by
  the exchange were done properly. It checks the various signatures,
  totals up the amounts and alerts the operator to any inconsistencies.
  It also computes the expected bank balance, revenue and risk exposure
  of the exchange operator.  The main binary is the
  @code{taler-auditor}.
@cindex Wire plugin
@item A wire plugin enables the HTTP frontend to talk to the
  bank. Its role is to allow the exchange to validate bank
  addresses (i.e. IBAN numbers), for the aggregator to execute
  wire transfers and for the auditor to query bank transaction
  histories.  Wire plugins are @emph{plugins} as there can be
  many different implementations to deal with different
  banking standards.  Wire plugins are automatically located
  and used by the exchange, aggregator and auditor.
@cindex DBMS
@cindex Postgres
@item The exchange requires a DBMS to stores the transaction history for
  the Taler exchange and aggregator, and a (typically separate) DBMS for
  the Taler auditor.
  For now, the GNU Taler reference implemenation only supports Postgres,
  but the code could be easily extended to support another DBMS.
@end itemize

@c The following image illustrates the various interactions of these
@c key components:

@c @center @image{arch, 3in, 4in}


@node Installation
@chapter Installation

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

@itemize

@item
GNU autoconf @geq{} 2.69

@item
GNU automake @geq{} 1.14

@item
GNU libtool @geq{} 2.4

@item
GNU autopoint @geq{} 0.19

@item
GNU libltdl @geq{} 2.4

@item
GNU libunistring @geq{} 0.9.3

@item
libcurl @geq{} 7.26 (or libgnurl @geq{} 7.26)

@item
GNU libmicrohttpd @geq{} 0.9.59

@item
GNU libgcrypt @geq{} 1.6

@item
libjansson @geq{} 2.7

@item
Postgres @geq{} 9.6, including libpq

@item
libgnunetutil (from Git)

@item
GNU Taler exchange (from Git)
@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 GNU Taler exchange.

Before you install libgnunetutil, you must download and install the
dependencies mentioned above, otherwise the build may succeed but fail
to export some of the tooling required by Taler.

To download and install libgnunetutil, proceed as follows:

@example
$ git clone https://gnunet.org/git/gnunet/
$ cd gnunet/
$ ./bootstrap
$ ./configure [--prefix=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, GNUnet will install to
@code{/usr/local}, which requires you to run the last step as
@code{root}.

To download and install the GNU Taler exchange, proceeds as follows:

@setsyntax shell
@example
$ git clone git://taler.net/exchange
$ cd exchange
$ ./bootstrap
$ ./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}.  Note that you have to specify @code{--with-gnunet=/usr/local}
if you installed GNUnet to @code{/usr/local} in the previous step.




@node Configuration
@chapter Configuration

@c In this document, we assume that @code{$HOME/.config/taler.conf} is being customized.

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

@menu
* Configuration format::
* Using taler-config::
* Keying::
* Serving::
* Currency::
* Bank account::
* Database::
* Coins (denomination keys): Coins denomination keys.
* Keys duration::

@end menu

@include configuration-format.texi
@include taler-config.texi




@node Keying
@section Keying

The exchange works with three types of keys:

@c FIXME: explain better!

@itemize

@item
@cite{master key}

@item
@cite{sign keys}

@item
@cite{denomination keys} (see section @cite{Coins})
@end itemize

@c FIXME: text here.

@itemize
@item
@cite{MASTER_PRIV_FILE}: Path to the exchange's master private file.

@item
@cite{MASTER_PUBLIC_KEY}: Must specify the exchange's master public key.
@end itemize





@node Serving
@section Serving


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

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


@itemize

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

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

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

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


@node Currency
@section Currency


The exchange supports only one currency. This data is set under the respective
option @cite{currency} in section @cite{[taler]}.

@node Bank account
@section Bank account

To configure a bank account in Taler, we need to furnish four
pieces of information:
@itemize
@item
  The @code{payto://} URL of the bank account, which uniquely idenfies
  the account. Examples for such URLs include
  @code{payto://sepa/CH9300762011623852957} for a bank account
  in the single European payment area (SEPA) or
  @code{payto://x-taler-bank/localhost:8080/2} for the 2nd bank
  account a the Taler bank demonstrator running at @code{localhost}
  on port 8080.  The first part of the URL following @code{payto://}
  (``sepa'' or ``x-taler-bank'') is called the wire method.
@item
  A matching wire plugin that implements a protocol to interact
  with the banking system.  For example, the EBICS plugin can
  be used for SEPA transfers, or the ``taler-bank'' plugin can
  interact with the Taler bank demonstrator.  A wire plugin
  only supports one particular wire method. Thus, you must make
  sure to pick a plugin that supports the wire method used in the
  URL.
@item
  A file containing the signed JSON-encoded bank account details
  for the /wire API.  This is necessary as Taler supports offline
  signing for bank accounts for additional security.
@item
  Finally, the plugin needs to be provided resources for
  authentication to the respective banking service.  The format
  in which the authentication information must be provided
  depends on the wire plugin.
@end itemize

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

@setsyntax ini
@example

[account-1]
URL = "payto://sepa/CH9300762011623852957"
WIRE_RESPONSE = $@{TALER_CONFIG_HOME@}/account-1.json
PLUGIN = ebics

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

# ... add authentication options here
@end example

The command line tool @cite{taler-exchange-wire} is used to create
the @code{account-1.json} file.
For example, the utility may be invoked as follows to create
all of the WIRE_RESPONSE files (in the locations specified by the configuration):

@example
$ taler-exchange-wire
@end example

The generated file will be echoed by the exchange when serving
/wire@footnote{https://api.taler.net/api-exchange.html#wire-req}
requests.


@menu
* Wire plugin ``taler_bank''::
* Wire plugin ``ebics''::
* Wire fee structure::
@end menu

@node Wire plugin ``taler_bank''
@subsection Wire plugin ``taler_bank''
@cindex x-taler-bank
@cindex taler_bank plugin

The @code{taler_bank} plugin implements the wire method ``x-taler-bank''.

The format of the @code{payto://} URL is @code{payto://x-taler-bank/HOSTNAME:PORT},
possibly followed by other parameters like the amount and wire transfer subject
as per the @code{payto://} standard.

For basic authentication, the @code{taler_bank} plugin only supports
simple password-based authentication.  For this, the configuration
must contain the ``USERNAME'' and ``PASSWORD'' of the respective
account at the bank.

@setsyntax ini
@example
[account-2]
URL = "payto://test/localhost:8080"
USERNAME = exchange
PASSWORD = super-secure
@end example


@node Wire plugin ``ebics''
@subsection Wire plugin ``ebics''

The ``ebics'' wire plugin is not fully implemented and today
does not support actual wire transfers.


@cartouche
@quotation Note
The rationale behind having multiple bank accounts is that the exchange operator, as a security
measure, may want to instruct the bank that the incoming bank account is only supposed to
@emph{receive} money.
@end quotation
@end cartouche


@node Wire fee structure
@subsection Wire fee structure
@cindex wire fee
@cindex fee

For each wire method (``sepa'' or ``x-taler-wire'', but not per plugin!) the
exchange configuration must specify applicable wire fees.  This is done
in configuration sections of the format @code{fee-METHOD}.  There are two
types of fees, simple wire fees and closing fees.  Wire fees apply whenever
the aggregator transfers funds to a merchant.  Closing fees apply whenever
the exchange closes a reserve (sending back funds to the customer).  The
fees must be constant for a full year, which is specified as part of the name
of the option.

@setsyntax ini
@example
[fee-iban]
WIRE-FEE-2018 = EUR:0.01
WIRE-FEE-2019 = EUR:0.01
CLOSING-FEE-2018 = EUR:0.01
CLOSING-FEE-2019 = EUR:0.01

[fee-x-taler-bank]
WIRE-FEE-2018 = KUDOS:0.01
WIRE-FEE-2019 = KUDOS:0.01
CLOSING-FEE-2018 = KUDOS:0.01
CLOSING-FEE-2019 = KUDOS:0.01
@end example


@node Database
@section Database


The option @cite{db} under section @cite{[exchange]} gets the DB backend's name the exchange
is going to use. So far, only @cite{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: @cite{TALER_EXCHANGEDB_POSTGRES_CONFIG}.

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

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

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

@node Coins denomination keys
@section Coins (denomination keys)


Sections specifying denomination (coin) information start with
"coin_".  By convention, the name continues with
"$CURRENCY_[$SUBUNIT]_$VALUE", i.e. @cite{[coin_eur_ct_10]} for a 10
cent piece.  However, only the "coin_" prefix is mandatory.  Each
"coin_"-section must then have the following options:


@itemize

@item
@cite{value}: How much is the coin worth, the format is
CURRENCY:VALUE.FRACTION.  For example, a 10 cent piece is "EUR:0.10".

@item
@cite{duration_withdraw}: How long can a coin of this type be
withdrawn?  This limits the losses incurred by the exchange when a
denomination key is compromised.

@item
@cite{duration_overlap}: What is the overlap of the withdrawal
timespan for this coin type?

@item
@cite{duration_spend}: How long is a coin of the given type valid?
Smaller values result in lower storage costs for the exchange.

@item
@cite{fee_withdraw}: What does it cost to withdraw this coin?
Specified using the same format as @cite{value}.

@item
@cite{fee_deposit}: What does it cost to deposit this coin? Specified
using the same format as @cite{value}.

@item
@cite{fee_refresh}: What does it cost to refresh this coin? Specified
using the same format as @cite{value}.

@item
@cite{rsa_keysize}: How many bits should the RSA modulus (product of
the two primes) have for this type of coin.
@end itemize


@node Keys duration
@section Keys duration


Both @cite{signkeys} and @cite{denom keys} have a starting date. The
option @cite{lookahead_provide}, under section @cite{[exchange_keys]},
is such that only keys whose starting date is younger than
@cite{lookahead_provide} will be issued by the exchange.

@cite{signkeys}. The option @cite{lookahead_sign} is such that, being
@cite{t} the time when @cite{taler-exchange-keyup} is run,
@cite{taler-exchange-keyup} will generate @cite{n} @cite{signkeys},
where @cite{t + (n * signkey_duration) = t + lookahead_sign}. In other
words, we generate a number of keys which is sufficient to cover a
period of @cite{lookahead_sign}. As for the starting date, the first
generated key will get a starting time of @cite{t}, and the
@cite{j}-th key will get a starting time of @cite{x +
signkey_duration}, where @cite{x} is the starting time of the
@cite{(j-1)}-th key.

@cite{denom keys}. The option @cite{lookahead_sign} is such that,
being @cite{t} the time when @cite{taler-exchange-keyup} is run,
@cite{taler-exchange-keyup} will generate @cite{n} @cite{denom keys}
for each denomination, where @cite{t + (n * duration_withdraw) = t +
lookahead_sign}. In other words, for each denomination, we generate a
number of keys which is sufficient to cover a period of
@cite{lookahead_sign}. As for the starting date, the first generated
key will get a starting time of @cite{t}, and the @cite{j}-th key will
get a starting time of @cite{x + duration_withdraw}, where @cite{x} is
the starting time of the @cite{(j-1)}-th key.

To change these settings, edit the following
values in section @cite{[exchange]}:

@itemize
@item
@cite{SIGNKEY_DURATION}: How long should one signing key be used?

@item
@cite{LOOKAHEAD_SIGN}: How much time we want to cover with our
signing keys? Note that if @cite{SIGNKEY_DURATION} is bigger than
@cite{LOOKAHEAD_SIGN}, @code{taler-exchange-keyup} will generate a
quantity of signing keys which is sufficient to cover all the
gap.
@end itemize

@c FIXME: LEGAL_DURATION not covered!
@c FIXME: LOOKAHEAD_PROVIDE not covered!


@node Deployment
@chapter Deployment


@menu
* Keys generation::
* Database upgrades::
@end menu

@node Keys generation
@section Keys generation

Once the configuration is properly set up, all the keys can be generated
by the tool @code{taler-exchange-keyup}.  The following command generates denomkeys
and signkeys, plus the "blob" that is to be signed by the auditor.

@example
taler-exchange-keyup -o blob
@end example

@emph{blob} contains data about denomkeys that the exchange operator needs to
get signed by every auditor he wishes (or is forced to) work with.

In a normal scenario, an auditor must have some way of receiving the blob to
sign (Website, manual delivery, ..).  Nonetheless, the exchange admin can fake
an auditor signature --- for testing purposes --- by running the following command

@example
taler-auditor-sign -m EXCHANGE_MASTER_PUB -r BLOB -u AUDITOR_URL -o OUTPUT_FILE
@end example

Those arguments are all mandatory.

@itemize
@item @code{EXCHANGE_MASTER_PUB} the base32 Crockford-encoded exchange's master
public key.  Tipically, this value lies in the configuration option
@code{[exchange]/master_public_key}.
@item @code{BLOB} the blob generated in the previous step.
@item @code{AUDITOR_URL} the URL that identifies the auditor.
@item @code{OUTPUT_FILE} where on the disk the signed blob is to be saved.
@end itemize

@code{OUTPUT_FILE} must then be copied into the directory specified
by the option @code{AUDITOR_BASE_DIR} under the section @code{[exchangedb]}.
Assuming @code{AUDITOR_BASE_DIR = $@{HOME@}/.local/share/taler/auditors}, the
following command will "add" the auditor identified by @code{AUDITOR_URL} to
the exchange.

@example
cp OUTPUT_FILE $@{HOME@}/.local/share/taler/auditors
@end example

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


@c FIXME: reference section about where keys are stored.

@node Database upgrades
@section Database upgrades

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

The exchange database can be re-initialized using:

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

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

@node Diagnostics
@chapter Diagnostics

This chapter includes various (very unpolished) sections on specific topics
that might be helpful to understand how the exchange operates, which files
should be backed up.  The information may also be helpful for diagnostics.

@menu
* Reserve management::
* Database Scheme::
* Signing key storage::
* Denomination key storage::
* Auditor signature storage::
@end menu


@node Reserve management
@section Reserve management


Incoming transactions to the exchange's provider result in the
creation or update of reserves, identified by their reserve key.  The
command line tool @cite{taler-exchange-reservemod} allows create and
add money to reserves in the exchange's database.




@node Database Scheme
@section Database Scheme


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

The database scheme used by the exchange look as follows:

@image{exchange-db,5in,,,png}


@node Signing key storage
@section Signing key storage


The private online signing keys of the exchange are stored in a
subdirectory "signkeys/" of the "KEYDIR" which is an option in the
"[exchange]" section of the configuration file.  The filename is the
starting time at which the signing key can be used in microseconds
since the Epoch.  The file format is defined by the @cite{struct TALER_EXCHANGEDB_PrivateSigningKeyInformationP}:

@example
struct TALER_EXCHANGEDB_PrivateSigningKeyInformationP @{
   struct TALER_ExchangePrivateKeyP signkey_priv;
   struct TALER_ExchangeSigningKeyValidityPS issue;
@};
@end example


@node Denomination key storage
@section Denomination key storage

The private denomination keys of the exchange are store in a
subdirectory "denomkeys/" of the "KEYDIR" which is an option in the
"[exchange]" section of the configuration file.  "denomkeys/" contains
further subdirectories, one per denomination.  The specific name of
the subdirectory under "denomkeys/" is ignored by the exchange.
However, the name is important for the "taler-exchange-keyup" tool
that generates the keys.  The tool combines a human-readable encoding
of the denomination (i.e.  for EUR:1.50 the prefix would be
"EUR_1_5-", or for EUR:0.01 the name would be "EUR_0_01-") with a
postfix that is a truncated Crockford32 encoded hash of the various
attributes of the denomination key (relative validity periods, fee
structure and key size).  Thus, if any attributes of a coin change,
the name of the subdirectory will also change, even if the
denomination remains the same.

Within this subdirectory, each file represents a particular
denomination key.  The filename is the starting time at which the
signing key can be used in microseconds since the Epoch.  The
format on disk begins with a
@cite{struct TALER_EXCHANGEDB_DenominationKeyInformationP} giving
the attributes of the denomination key and the associated
signature with the exchange's long-term offline key:

@example
struct TALER_EXCHANGEDB_DenominationKeyInformationP @{
  struct TALER_MasterSignatureP signature;
  struct TALER_DenominationKeyValidityPS properties;
@};
@end example

This is then followed by the variable-size RSA private key in
libgcrypt's S-expression format, which can be decoded using
@cite{GNUNET_CRYPTO_rsa_private_key_decode()}.

@menu
* Revocations::
@end menu

@node Revocations
@subsection Revocations

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

When a denomination key is revoked, a revocation file is placed
into the respective subdirectory of ``denomkeys/''.  The file has the
same prefix as the file that stores the
@cite{struct TALER_EXCHANGEDB_DenominationKeyInformationP} information,
but is followed by the ``.rev'' suffix.  It contains a 64-byte
EdDSA signature made with the master key of the exchange with purpose
@code{TALER_SIGNATURE_MASTER_DENOMINATION_KEY_REVOKED}.  If such a file
is present, the exchange must check the signature and if it is valid
treat the respective denomination key as revoked.

Revocation files can be generated using the
@code{taler-exchange-keyup} command-line tool using the @code{-r}
option.  The Taler auditor will instruct operators to generate
revocations if it detects a key compromise (which is possible more
coins of a particular denomination were deposited than issued).

It should be noted that denomination key revocations should only happen
under highly unusual (``emergency'') conditions and not under normal
conditions.


@node Auditor signature storage
@section Auditor signature storage


Signatures from auditors are stored in the directory specified
in the exchange configuration section "exchangedb" under the
option "AUDITOR_BASE_DIR".  The exchange does not care about
the specific names of the files in this directory.

Each file must contain a header with the public key information
of the auditor, the master public key of the exchange, and
the number of signed denomination keys:

@example
struct AuditorFileHeaderP @{
  struct TALER_AuditorPublicKeyP apub;
  struct TALER_MasterPublicKeyP mpub;
  uint32_t dki_len;
@};
@end example

This is then followed by @cite{dki_len} signatures of the auditor of type
@cite{struct TALER_AuditorSignatureP}, which are then followed by another
@cite{dki_len} blocks of type @cite{struct TALER_DenominationKeyValidityPS}.
The auditor's signatures must be signatures over the information of
the corresponding denomination key validity structures embedded in a
@cite{struct TALER_ExchangeKeyValidityPS} structure using the
@cite{TALER_SIGNATURE_AUDITOR_EXCHANGE_KEYS} purpose.



@c **********************************************************
@c *******************  Appendices  *************************
@c **********************************************************

@node GNU-AGPL
@chapter GNU Affero GPL
@cindex license
@include agpl.texi

@node GNU-FDL
@chapter GNU Free Documentation License
@cindex license
@include fdl-1.3.texi

@node Concept Index
@chapter Concept Index

@printindex cp

@bye