Known-Key Distinguishing and Partial-Collision Attacks on GFN-2 with SP F-Function

We study known-key distinguishing and partial-collision attacks on GFN-2 structures with various block lengths in this paper. For 4-branch GFN-2, we present 15-round known-key distinguishing attack and 11-round partial-collision attack which improve previous results. We also present 17-round known-key distinguishing attack on 6-branch GFN-2 and 27-round known-key distinguishing attack on 8-branch GFN-2 and show that several partial-collision attacks are derived from them. Additionally, some attacks are valid under special conditions for the F-function.