### GIFT-COFB is Tightly Birthday Secure with Encryption Queries

Akiko Inoue and Kazuhiko Minematsu

##### Abstract

GIFT-COFB is a finalist of NIST Lightweight cryptography project that aims at standardizing authenticated encryption schemes for constrained devices. It is a block cipher-based scheme and comes with a provable security result. This paper studies the tightness of the provable security bounds of GIFT-COFB, which roughly tells that, if instantiated by a secure $n$-bit block cipher, we need $2^{n/2}$ encrypted blocks or $2^{n/2}/n$ decryption queries to break the scheme. This paper shows that the former condition is indeed tight, by presenting forgery attacks that work with $2^{n/2}$ encrypted blocks with single decryption query. This fills the missing spot of previous attacks presented by Khairallah, and confirms the tightness of the security bounds with respect to encryption. We remark that our attacks work independent of the underlying block cipher.

Available format(s)
Category
Secret-key cryptography
Publication info
Preprint. MINOR revision.
Keywords
Contact author(s)
a_inoue @ nec com
k-minematsu @ nec com
History
Short URL
https://ia.cr/2021/737

CC BY

BibTeX

@misc{cryptoeprint:2021/737,
author = {Akiko Inoue and Kazuhiko Minematsu},
title = {GIFT-COFB is Tightly Birthday Secure with Encryption Queries},
howpublished = {Cryptology ePrint Archive, Paper 2021/737},
year = {2021},
note = {\url{https://eprint.iacr.org/2021/737}},
url = {https://eprint.iacr.org/2021/737}
}

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