Attack on LAC Key Exchange in Misuse Situation

Aurelien Greuet; Simon Montoya; Guenael Renault

Paper 2020/063

Attack on LAC Key Exchange in Misuse Situation

Aurelien Greuet, Simon Montoya, and Guenael Renault

Abstract

LAC is a Ring Learning With Error based cryptosystem that has been proposed to the NIST call for post-quantum standardization and passed the first round of the submission process. The particularity of LAC is to use an error-correction code ensuring a high security level with small key sizes and small ciphertext sizes. LAC team proposes a CPA secure cryptosystem, LAC.CPA, and a CCA secure one, LAC.CCA, obtained by applying the Fujisaki-Okamoto transformation on LAC.CPA. In this paper, we study the security of LAC Key Exchange (KE) mechanism, using LAC.CPA, in a misuse context: when the same secret key is reused for several key exchanges and an active adversary has access to a mismatch oracle. This oracle indicates information on the possible mismatch at the end of the KE protocol. In this context, we show that an attacker needs at most queries to the oracle to retrieve one coefficient of a static secret key. This result has been experimentally confirmed using the reference and optimized implementations of LAC. Since our attack can break the CPA version in a misuse context, the Authenticated KE protocol, based on the CCA version, is not impacted. However, this research provides a tight estimation of LAC resilience against this type of attacks.

Metadata

Available format(s): PDF
Category: Public-key cryptography
Publication info: Preprint. MINOR revision.
Keywords: lattice based cryptography post-quantum cryptography active attack key reuse
Contact author(s): aurelien greuet @ idemia com
simon montoya @ idemia com
guenael renault @ lix polytechnique fr
History: 2020-01-21: received
Short URL: https://ia.cr/2020/063
License: CC BY

BibTeX

@misc{cryptoeprint:2020/063,
      author = {Aurelien Greuet and Simon Montoya and Guenael Renault},
      title = {Attack on {LAC} Key Exchange in Misuse Situation},
      howpublished = {Cryptology {ePrint} Archive, Paper 2020/063},
      year = {2020},
      url = {https://eprint.iacr.org/2020/063}
}