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The Centre for Telematics and Information Technology (CTIT) at the University of Twente invites applications for a 4-year PhD position in cryptographic protocol design.
In the course of the PhD project, the PhD student will deal with cryptographic concepts such as Homomorphic Encryption, Functional Encryption, and Secure Multiparty Computation. The research focus of the project is on the design and evaluation of new cryptographic protocols for specific application scenarios.
The PhD candidate will be expected to do active and internationally visible research which will be supervised by Dr. Andreas Peter from the Services, Cybersecurity and Safety Group of the University of Twente. The PhD candidate will be appointed for a period of four years, at the end of which he/she must have completed a PhD thesis. During this period, the PhD student has the opportunity to broaden his/her knowledge by joining international exchange programs, to participate in national and international conferences and workshops, and to visit other research institutes and universities worldwide.
Successful candidates must hold an outstanding M.Sc. degree (or equivalent) from the university study of Computer Science, Mathematics, or similar, obtained within the last two years. The topic of the master thesis should ideally have relevance to cryptography. Applications from students that are about to finish their master thesis will be accepted as well. Further requirements include excellent skills in the English language, firm knowledge in cryptography and basic programming skills. Early experiences with scientific publications are of advantage.
The position will be closed as soon as a suitable candidate is found. Applications must include:
In this paper, we continue this line of research and show that most existing somewhat homomorphic encryption schemes are not IND-CCA1 secure. In fact, we show that these schemes suffer from key recovery attacks (stronger than a typical IND-CCA1 attack), which allow an adversary to recover the private keys through a number of decryption oracle queries. The schemes, that we study in detail, include those by Brakerski and Vaikuntanathan at Crypto 2011 and FOCS 2011, and that by Gentry, Sahai and Waters at Crypto 2013. We also develop a key recovery attack that applies to the somewhat homomorphic encryption scheme by van Dijk et al., and our attack is more efficient and conceptually simpler than the one developed by Zhang et al.. Our key recovery attacks also apply to the scheme by Brakerski, Gentry and Vaikuntanathan at ITCS 2012, and we also describe a key recovery attack for the scheme developed by Brakerski at Crypto 2012.
After formally defining constrained VRFs, we derive instantiations from the multilinear-maps-based constrained PRFs by Boneh and Waters, yielding a VRF with constrained keys for any set that can be decided by a polynomial-size circuit. Our VRFs have the same function values as the Boneh-Waters PRFs and are proved secure under the same hardness assumption, showing that verifiability comes at no cost. Constrained (functional) VRFs were stated as an open problem by Boyle et al.