In September last year, Jean-Philippe Aumasson and Willi Meier introduced zero-sum distinguishers, a method to generate zero-sum structures for reduced-round versions of Keccak-f up to 16 rounds. Recently, Christina Boura and Anne Canteaut extended this to 18 rounds. (See the page on third-party cryptanalyis for references and more details.)
We publish a note, in which we give technical details and put these distinguishers into perspective. We also relate their existence to our decision to increase the number of rounds to 24, in line with the hermetic sponge strategy, in which we tolerate no structural distinguisher for the permutation used in the sponge construction.