Algorithm Substitution Attacks: State Reset Detection and Asymmetric Modifications

Hodges, Philip; Stebila, Douglas

doi:10.46586/tosc.v2021.i2.389-422

Cited by 5 publications

(2 citation statements)

References 16 publications

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“…While previous attacks [BPR14,BJK15] targeted the encryption algorithm, Armour and Poettering proposed an attack [AP19b] by subverting the decryption algorithm. Hodges and Stebila explored the detectability of ASAs via state resetting [HS21]. Apart from these attacks on (authenticated) encryption schemes, ASAs have also been proposed on message authentication code [AP19a], signature schemes [AMV15, BSKC19,LCWW18], and key encapsulation mechanisms [CHY20].…”

Section: Subversion Attacksmentioning

confidence: 99%

Subverting Telegram’s End-to-End Encryption

Cogliati

Ethan²,

Jha³

2023

ToSC

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Telegram is a popular secure messaging service with third biggest user base as of 2021. In this paper, we analyze the security of Telegram’s end-to-end encryption (E2EE) protocol in presence of mass-surveillance. Specifically, we show >that Telegram’s E2EE protocol is susceptible to fairly efficient algorithm substitution attacks. While official Telegram clients should be protected against this type of attack due their open-source nature and reproducible builds, this could potentially lead to a very efficient state sponsored surveillance of private communications over Telegram, either on individuals through a targeted attack or massively through some compromised third-party clients. We provide an efficient algorithm substitution attack against MTProto2.0 — the underlying authenticated encryption scheme — that recovers significant amount of encryption key material with a very high probability with few queries and fairly low latency. This could potentially lead to a very efficient state sponsored surveillance of private communications over Telegram, either through a targeted attack or a compromised third-party app. Our attack exploits MTProto2.0’s degree of freedom in choosing the random padding length and padding value. Accordingly, we strongly recommend that Telegram should revise MTProto2.0’s padding methodology. In particular, we show that a minor change in the padding description of MTProto2.0 makes it subversion-resistant in most of the practical scenarios. As a side-effect, we generalize the underlying mode of operation in MTProto2.0, as MTProto-G, and show that this generalization is a multi-user secure deterministic authenticated encryption scheme.

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Section: Subversion Attacksmentioning

confidence: 99%

Subverting Telegram’s End-to-End Encryption

Cogliati

Ethan²,

Jha³

2023

ToSC

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“…Bellare and Hoang [10] give PKE schemes that defend against the subversion of random number generators. The use of state reset to detect ASAs is studied by Hodges and Stebila [41]. Berndt and Liśkiewicz [17] reunite the fields of cryptography and steganography.…”

Section: Further Workmentioning

confidence: 99%

Algorithm substitution attacks against receivers

Armour

Poettering

2022

Int. J. Inf. Secur.

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This work describes a class of Algorithm Substitution Attack (ASA) generically targeting the receiver of a communication between two parties. Our work provides a unified framework that applies to any scheme where a secret key is held by the receiver; in particular, message authentication schemes (MACs), authenticated encryption (AEAD) and public key encryption (PKE). Our unified framework brings together prior work targeting MAC schemes (FSE’19) and AEAD schemes (IMACC’19); we extend prior work by showing that public key encryption may also be targeted. ASAs were initially introduced by Bellare, Paterson and Rogaway in light of revelations concerning mass surveillance, as a novel attack class against the confidentiality of encryption schemes. Such an attack replaces one or more of the regular scheme algorithms with a subverted version that aims to reveal information to an adversary (engaged in mass surveillance), while remaining undetected by users. Previous work looking at ASAs against encryption schemes can be divided into two groups. ASAs against PKE schemes target key generation by creating subverted public keys that allow an adversary to recover the secret key. ASAs against symmetric encryption target the encryption algorithm and leak information through a subliminal channel in the ciphertexts. We present a new class of attack that targets the decryption algorithm of an encryption scheme for symmetric encryption and public key encryption, or the verification algorithm for an authentication scheme. We present a generic framework for subverting a cryptographic scheme between a sender and receiver, and show how a decryption oracle allows a subverter to create a subliminal channel which can be used to leak secret keys. We then show that the generic framework can be applied to authenticated encryption with associated data, message authentication schemes, public key encryption and KEM/DEM constructions. We consider practical considerations and specific conditions that apply for particular schemes, strengthening the generic approach. Furthermore, we show how the hybrid subversion of key generation and decryption algorithms can be used to amplify the effectiveness of our decryption attack. 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.

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