Federal Electoral Law to the Federal Republic of Germany of 1993 (Last amended 2013), Article 35
(1) In order to facilitate the casting and counting of votes, voting machines equipped with independent registers may be used instead of ballot papers, envelopes and ballot boxes.
(2) Voting machines within the meaning of Paragraph (1) above must guarantee the secrecy of voting. They must be officially licensed in their design to be used at elections to the Bundestag for individual elections or on a general scale. The Federal Ministry of the Interior shall decide on the licensing of a voting machine on the application of its manufacturer. The use of an officially licensed voting machine shall be subject to approval by the Federal Ministry of the Interior. Such approval may be pronounced in respect of individual elections or with general effect.
Link to the Constitutional Court decision on e-voting