\section{GNU Taler}\label{taler}
GNU Taler is an open protocol for electronic payment system using blind
signatures to protect the privacy of the customer.
One key component of the GNU Taler payment system is the exchange which is
responsible for exchanging existing money into electronic money. Customers can
retrieve funds from the exchange to make anonymous payments. The merchant is
not anonymous and thus can not hide the income. This helps to avoid tax evasion
and money laundering \cite{Taler}.

GNU Taler uses denominations to represent the values of a coins. A denomination
contains the unit of currency and the face value of a given coin. Each
denomination contains a cryptographic public key used by the exchange to verify
the denomination. The Donau is based on the exchange and requires some of its
parts to work. The concept of the denomination was adapted into the donation
units, which are used to represent the value of a donation.

