Cryptology ePrint Archive: Report 2015/1184

Extension Field Cancellation: a New Central Trapdoor for Multivariate Quadratic Systems

Alan Szepieniec and Jintai Ding and Bart Preneel

Abstract: This paper introduces a new central trapdoor for multivariate quadratic (MQ) public-key cryptosystems that allows for encryption, in contrast to time-tested MQ primitives such as Unbalanced Oil and Vinegar or Hidden Field Equations which only allow for signatures. Our construction is a mixed-field scheme that exploits the commutativity of the extension field to dramatically reduce the complexity of the extension field polynomial implicitly present in the public key. However, this reduction can only be performed by the user who knows concise descriptions of two simple polynomials, which constitute the private key. After applying this transformation, the plaintext can be recovered by solving a linear system. We use the minus and projection modifiers to inoculate our scheme against known attacks. A straightforward C++ implementation confirms the efficient operation of the public key algorithms.

Category / Keywords: public-key cryptography / MQ, multivariate, quadratic, public-key, post-quantum, encryption, mixed-field, trapdoor

Original Publication (with minor differences): PQCrypto 2016

Date: received 11 Dec 2015

Contact author: alan szepieniec at esat kuleuven be

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Version: 20151213:041325 (All versions of this report)

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