We slightly modified Ketje (now v2) in a way that encourages
cryptanalysis, while we kept Keyak unchanged (still v2) but updated and
improved its documentation.
* Ketje v2 *
Compared to Ketje v1, we now specify a different placement for the outer
(input/output) part of the state. This is done by adding a change of
coordinates ("twist"), so as to put the outer part on a diagonal and to
limit its interaction with the preceding χ and following θ step mappings.
The motivation is to encourage cryptanalysis. Cryptanalysis usually
starts by reducing the number of rounds to see at which point a given
primitive becomes insecure. In the case of Ketje, one cannot decrease
the step calls further than 1 round. Instead, a cryptanalyst can
increase the rate to more than 2 lanes to determine at which point Ketje
breaks. However, the lanes of the outer part are located in the same
plane (i.e., same y coordinate) and contain the result of χ. The
knowledge of too many lanes in the same plane could mean that χ is
easily inverted on that part of the state. Also, we should not place the
outer part on a sheet (i.e., same x coordinate) as this would help the
adversary influence the parity computed in θ. Instead, the twist places
the outer part on a diagonal.
We illustrate the usage of this twist with two new instances, Ketje
Minor and Ketje Major, that have a rate of 4 lanes (instead of 2) and
larger permutations (800 and 1600 bits).
The primary recommendation remains Ketje Sr. Both Ketje Jr and Ketje Sr
keep their rate of 2 lanes and otherwise remain unchanged modulo the twist.
* Keyak v2 *
Compared to round 2, River, Lake, Sea, Ocean and Lunar Keyak remain
We nevertheless worked on improving the description of the Motorist mode
of operation by simplifying the definition of the Piston, Engine and
Motorist algorithms. We also updated the security rationale. These
changes are available in version 2.2 of the documentation (see change
log in Appendix A).
The definition of Motorist now restricts the tag length to the capacity.
As pointed out by Seth Hoffert, a legitimate adversary could, in a
session, submit a tag as next block of metadata. If the tag is as long
as the rate, this allows the adversary to force the outer part to a
constant value, hence increasing the multiplicity. This would not break
Motorist but it would prevent it to reach near-capacity generic security.
Gilles, Guido, Joan, Michaël and Ronny
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