## CryptoDB

### Jacob C. N. Schuldt

#### Publications

Year
Venue
Title
2018
PKC
Motivated by the history of randomness failures in practical systems, Paterson, Schuldt, and Sibborn (PKC 2014) introduced the notion of related randomness security for public key encryption. In this paper, we firstly show an inherent limitation of this notion: if the family of related randomness functions is sufficiently rich to express the encryption function of the considered scheme, then security cannot be achieved. This suggests that achieving security for function families capable of expressing more complex operations, such as those used in random number generation, might be difficult. The current constructions of related randomness secure encryption in the standard model furthermore reflect this; full security is only achieved for function families with a convenient algebraic structure. We additionally revisit the seemingly optimal random oracle model construction by Paterson et al. and highlight its limitations.To overcome this difficulty, we propose a new notion which we denote related refreshable randomness security. This notion captures a scenario in which an adversary has limited time to attack a system before new entropy is added. More specifically, the number of encryption queries with related randomness the adversary can make before the randomness is refreshed, is bounded, but the adversary is allowed to make an unbounded total number of queries. Furthermore, the adversary is allowed to influence how entropy is added to the system. In this setting, we construct an encryption scheme which remains secure in the standard model for arbitrary function families of size $2^p$2p (where p is polynomial in the security parameter) that satisfy certain collision-resistant and output-unpredictability properties. This captures a rich class of functions, which includes, as a special case, circuits of polynomial size. Our scheme makes use of a new construction of a (bounded) related-key attack secure pseudorandom function, which in turn is based on a new flavor of the leftover hash lemma. These technical results might be of independent interest.
2016
CRYPTO
2015
EPRINT
2014
PKC
2014
EPRINT
2014
ASIACRYPT
2014
FSE
2012
CRYPTO
2012
PKC
2012
PKC
2011
JOFC
2011
PKC
2011
ASIACRYPT
2008
PKC
2006
EPRINT
The only known construction of identity-based signatures that can be proven secure in the standard model is based on the approach of attaching certificates to non-identity-based signatures. This folklore construction method leads to schemes that are somewhat inefficient and leaves open the problem of finding more efficient direct constructions. We present the first such construction. Our scheme is obtained from a modification of Waters' recently proposed identity-based encryption scheme. It is computationally efficient and the signatures are short. The scheme's security is proven in the standard model and rests on the hardness of the computational Diffie-Hellman problem in groups equipped with a pairing.
2006
EPRINT
We propose new instantiations of chosen-ciphertext secure identity-based encryption schemes with wildcards (WIBE). Our schemes outperform all existing alternatives in terms of efficiency as well as security. We achieve these results by extending the hybrid encryption (KEM--DEM) framework to the case of WIBE schemes. We propose and prove secure one generic construction in the random oracle model, and one direct construction in the standard model.

Eurocrypt 2018