We describe the design and implementation of GNU Taler, an
electronic payment system based on an extension of Chaum's online e-cash with
efficient change.  In addition to anonymity for customers, it provides the
novel notion of income transparency, which guarantees that merchants can
reliably receive a payment from an untrusted payer only when their income from
the payment is visible to tax authorities.

We show the provable security of our protocol, which in addition to the usual
security properties also formally models conservation of funds and income
transparency.

Lastly, we contribute a novel Byzantine consensus algorithm, which could be
applied to distributed ledgers.