International Association for Cryptologic Research

International Association
for Cryptologic Research

IACR News item: 28 November 2022

Daniele Friolo, Matteo Salvino, Daniele Venturi
ePrint Report ePrint Report
The Fujisaki-Okamoto (FO) transform (CRYPTO 1999 and JoC 2013) turns any weakly (i.e., IND-CPA) secure public-key encryption (PKE) scheme into a strongly (i.e., IND-CCA) secure key encapsulation method (KEM) in the random oracle model (ROM). Recently, the FO transform re-gained momentum as part of CRISTAL-Kyber, selected by the NIST as the PKE winner of the post-quantum cryptography standardization project.

Following Fischlin (ICALP 2005), we study the complete non-malleability of KEMs obtained via the FO transform. Intuitively, a KEM is completely non-malleable if no adversary can maul a given public key and ciphertext into a new public key and ciphertext encapsulating a related key for the underlying blockcipher.

On the negative side, we find that KEMs derived via FO are not completely non-malleable in general. On the positive side, we show that complete non-malleability holds in the ROM by assuming the underlying PKE scheme meets an additional property, or by a slight tweak of the transformation.
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