Cryptology ePrint Archive: Report 2017/431

Understanding RUP Integrity of COLM

Nilanjan Datta and Atul Luykx and Bart Mennink and Mridul Nandi

Abstract: The authenticated encryption scheme COLM is a third-round candidate in the CAESAR competition. Much like its antecedents COPA, ELmE, and ELmD, COLM consists of two parallelizable encryption layers connected by a linear mixing function. While COPA uses plain XOR mixing, ELmE, ELmD, and COLM use a more involved invertible mixing function. In this work, we investigate the integrity of the COLM structure when unverified plaintext is released, and demonstrate that its security highly depends on the choice of mixing function. Our results are threefold. First, we discuss the practical nonce-respecting forgery by Andreeva et al. (ASIACRYPT 2014) against COPA's XOR mixing. Then we present a nonce-misusing forgery against arbitrary mixing functions with practical time complexity. Finally, by using significantly larger queries, we can extend the previous forgery to be nonce-respecting.

Category / Keywords: Integrity, Release of unverified plaintext, COLM, COPA, ELmD, ELmE

Original Publication (in the same form): IACR-TOSC ISSUE 2-2017

Date: received 18 May 2017

Contact author: nilanjan_isi_jrf at yahoo com, atul luykx@esat kuleuven be, b mennink@cs ru nl, mridul nandi@gmail com

Available format(s): PDF | BibTeX Citation

Version: 20170522:213822 (All versions of this report)

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