Paper 2021/849

Curse of Re-encryption: A Generic Power/EM Analysis on Post-Quantum KEMs

Rei Ueno, Keita Xagawa, Yutaro Tanaka, Akira Ito, Junko Takahashi, and Naofumi Homma


This paper presents a side-channel analysis (SCA) on key encapsulation mechanism (KEM) based on the Fujisaki–Okamoto (FO) transformation and its variants. The FO transformation has been widely used in actively securing KEMs from passively secure public key encryption (PKE), as it is employed in most of NIST post-quantum cryptography (PQC) candidates for KEM. The proposed attack exploits side-channel leakage during execution of a psuedorandom function (PRF) in the re-encryption of KEM decapsulation as a plaintext-checking oracle that tells whether the PKE decryption result is equivalent to the reference plaintext. The generality and practicality of the plaintext-checking oracle allows the proposed attack to attain a full-key recovery of various KEMs when an active attack on the underlying PKE is known. This paper demonstrates that the proposed attack can be applied to most NIST PQC third-round KEM candidates, namely, Kyber, Saber, FrodoKEM, NTRU, NTRU Prime, HQC, BIKE, and SIKE (for BIKE, the proposed attack achieves a partial key recovery). The applicability to Classic McEliece is unclear because there is no known active attack on this cryptosystem. This paper also presents a side-channel distinguisher design based on deep learning (DL) for mounting the proposed attack on practical implementation without the use of a profiling device. The feasibility of the proposed attack is demonstrated through experimental attacks on various PRF implementations (a SHAKE software, an AES software, an AES hardware, a bit-sliced masked AES software, and a masked AES hardware based on threshold implementation). The success of the proposed attack against all of these implementations, which include masked hardware based on threshold implementation, confirms its practicality.

Available format(s)
Public-key cryptography
Publication info
Published elsewhere. IACR-TCHES-2022
Side-channel analysisFujisaki–Okamoto transformationKey encapsulation mechanismPublic key encryptionPost-quantum cryptographyand Deep learning
Contact author(s)
rei ueno a8 @ tohoku ac jp
keita xagawa zv @ hco ntt co jp
y-tanaka @ riec tohoku ac jp
ito @ riec tohoku ac jp
junko takahashi fc @ hco ntt co jp
homma @ riec tohoku ac jp
2021-10-15: last of 3 revisions
2021-06-22: received
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Creative Commons Attribution


      author = {Rei Ueno and Keita Xagawa and Yutaro Tanaka and Akira Ito and Junko Takahashi and Naofumi Homma},
      title = {Curse of Re-encryption: A Generic Power/EM Analysis on Post-Quantum KEMs},
      howpublished = {Cryptology ePrint Archive, Paper 2021/849},
      year = {2021},
      note = {\url{}},
      url = {}
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