Cryptology ePrint Archive: Report 2019/987

Subverting Decryption in AEAD

Marcel Armour and Bertram Poettering

Abstract: This work introduces a new class of Algorithm Substitution Attack (ASA) on Symmetric Encryption Schemes. ASAs were introduced by Bellare, Paterson and Rogaway in light of revelations concerning mass surveillance. An ASA replaces an encryption scheme with a subverted version that aims to reveal information to an adversary engaged in mass surveillance, while remaining undetected by users. Previous work posited that a particular class of AEAD scheme (satisfying certain correctness and uniqueness properties) is resilient against subversion. Many if not all real-world constructions - such as GCM, CCM and OCB - are members of this class. Our results stand in opposition to those prior results. We present a potent ASA that generically applies to any AEAD scheme, is undetectable in all previous frameworks and which achieves successful exfiltration of user keys. We give even more efficient non-generic attacks against a selection of AEAD implementations that are most used in practice.In contrast to prior work, our new class of attack targets the decryption algorithm rather than encryption. We argue that this attack represents an attractive opportunity for a mass surveillance adversary. Our work serves to refine the ASA model and contributes to a series of papers that raises awareness and understanding about what is possible with ASAs.

Category / Keywords: secret-key cryptography / Algorithm Substitution Attacks, Privacy, Symmetric Encryption, Mass Surveillance,

Original Publication (in the same form): IMACC 2019

Date: received 30 Aug 2019, last revised 9 Jan 2020

Contact author: marcel armour 2017 at rhul ac uk

Available format(s): PDF | BibTeX Citation

Version: 20200109:145823 (All versions of this report)

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