Paper 2024/600

A note on -Tweakable HCTR: A BBB Secure Tweakable Enciphering Scheme-

Abstract

Tweakable HCTR is an tweakable enciphering proposed by Dutta and Nandi in Indocrypt 2018. It provides beyond birthday bound security when each tweak value is not used too frequently. More importantly for this note, its security bound degrades linearly with the maximum input length. We show in this note that this is not true by showing a single query distinguisher with advantage $O(l^2/2^n)$ where $l$ is the length of that query. The distinguisher does not break the beyond-birthday-bound claim but gives higher advantage than the claimed bound.

Note: After disclosing this flaw publicly, the authors of https://tosc.iacr.org/index.php/ToSC/article/view/9171 have pointed out that they also discovered this flaw in their paper earlier, yet their attack seems to have been unnoticed by the designers. Thus, this note should serve now as a confirmation of their analysis rather than a new observation.