No legal basis for voting machines in Germany?

According to press coverage, in todays parliament elections (Bundestagswahl) some 5% of German voters will be forced to cast their vote on electronic voting machines.
However, those voting machines have no paper audit trail, and in fact seem to have no audit trail at all. The ministry of interior does not want to disclose the certification procedures or certification reports of those machines, allegedly to accommodate the trade secrets of the vendors.

Since when has a trade secret (if there is any involved, I doubt it) become more important than the citizens' right to a transparent election process?

After a quick read through the respective laws such as the Election Verification Act (Wahlprüfungsgesetz) and the Federal Election Act (Bundeswahlordnung), there is not a single mention of any kind of electronic voting machines. To the opposite, they go into every tiny detail of how the ballots have to be formatted, what color of paper they are printed on, etc.

Apparently there is already at least one person who wants to challenge the election results in those counties where electronic voting machines are used. I'm more than motivated to join such action and/or start an initiative for transparency of electronic voting. Stay tuned.