XKX Does Not Preserve Unpredictability

In this paper, the security of the XKX tweakable blockcipher construction is revisited. Proposed at FSE 2018, the security of the XKX was proved to be a tweakable pseudorandom permutation, which assumed the underlying blockcipher a strong pseudorandom random permutation (SPRP). Though, it shows that XKX does not preserve unpredictability, i.e., it could lead itself to attacks when instantiated with unpredictable blockciphers (a weaker assumption than SPRP). This shows XKX instantiated with unpredictable blockciphers cannot be used to construct secure message authentication codes (MACs).