## CryptoDB

### Kristiyan Haralambiev

#### Publications

**Year**

**Venue**

**Title**

2015

ASIACRYPT

2010

EPRINT

Simple and Efficient Public-Key Encryption from Computational Diffie-Hellman in the Standard Model
Abstract

This paper proposes practical chosen-ciphertext secure public-key encryption systems that are provably secure under the computational Diffie-Hellman assumption, in the standard model. Our schemes are conceptually simpler and more efficient than previous constructions.
We also show that in bilinear groups the size of the public-key can be shrunk from n to 2\sqrt{n} group elements, where n is the security parameter.

2010

EPRINT

Signing on Elements in Bilinear Groups for Modular Protocol Design
Abstract

This paper addresses the construction of signature schemes whose verification keys, messages, and signatures are group elements and the verification predicate is a conjunction of pairing product equations. We answer to the open problem of constructing constant-size signatures by presenting an efficient scheme. The security is proven in the standard model based on a novel non-interactive assumption called Simultaneous Flexible Pairing Assumption that can be justified and has an optimal bound in the generic bilinear group model.
We also present efficient schemes with advanced properties including signing unbounded number of group elements, allowing simulation in the common reference string model, signing messages from mixed groups in the asymmetric bilinear group setting, and strong unforgeability.
Among many applications, we show two examples; an adaptively secure round optimal blind signature scheme and a group signature scheme with efficient concurrent join.
As a bi-product, several homomorphic trapdoor commitment schemes and one-time signature schemes are presented, too. In combination with the Groth-Sahai proof system, these schemes contribute to an efficient instantiation of modular constructions of cryptographic protocols.

2010

EPRINT

Efficient Public-Key Cryptography in the Presence of Key Leakage
Abstract

We study the design of cryptographic primitives resistant to a large class of side-channel attacks, called "memory attacks", where an attacker can repeatedly and adaptively learn information about the secret key, subject *only* to the constraint that the *overall amount* of such information is bounded by some parameter $\ell$. Although the study of such primitives was initiated only recently by Akavia et al. [AGV09], subsequent work already produced many such "leakage-resilient" primitives [NS09,ADW09,KV09], including signature, encryption, identification (ID) and authenticated key agreement (AKA) schemes. Unfortunately, every existing scheme, --- for any of the four fundamental primitives above, --- fails to satisfy at least one of the following desirable properties:
- Efficiency. While the construction may be generic, it should have some *efficient* instantiations, based on standard cryptographic assumptions, and without relying on random oracles.
- Strong Security. The construction should satisfy the strongest possible definition of security (even in the presence of leakage). For example, encryption schemes should be secure against chosen *ciphertext* attack (CCA), while signatures should be *existentially* unforgeable.
- Leakage Flexibility. It should be possible to set the parameters of the schemes so that the leakage bound $\ell$ can come arbitrarily close to the size of the secret key $sk$.
In this work we design the first signature, encryption, ID and AKA schemes which overcome these limitations, and satisfy all the properties above. Moreover, all our constructions are generic, in several cases elegantly simplifying and generalizing the prior constructions (which did not have any efficient instantiations).
We also introduce several tools of independent interest, such as the abstraction (and constructions) of *simulation extractable* NIZK arguments, and a new *deniable* DH-based AKA protocol based on any CCA-secure encryption.

2010

EPRINT

Cryptography Against Continuous Memory Attacks
Abstract

We say that a cryptographic scheme is Continous Leakage-Resilient (CLR), if it allows users to refresh their secret keys, using only fresh local randomness, such that:
1. The scheme remains functional after any number of key refreshes, although the public key never changes. Thus, the outside world is neither affected by these key refreshes, nor needs to know about their frequency.
2. The scheme remains secure even if the adversary can continuously leak arbitrary information about the current secret-key of the system, as long as the amount of leaked information is bounded in between any two successive key refreshes. There is no bound on the total amount of information that can be leaked during the lifetime of the system.
In this work, we construct a variety of practical CLR schemes, including CLR one-way relations, CLR signatures, CLR identification schemes, and CLR authenticated key agreement protocols. For each of the above, we give general constructions, and then show how to instantiate them efficiently using a well established assumption on bilinear groups, called the K-Linear assumption (for any constant K >= 1).
Our constructions are highly modular, and we develop many interesting techniques and building-blocks along the way, including: leakage-indistinguishable re-randomizable relations, homomorphic NIZKs, and leakage-of-ciphertext non-malleable encryption schemes.
Prior to our work, no truly CLR schemes were known, as previous leakage-resilient schemes suffer from one or more of the following drawbacks: (a) restrictions are placed on the type of allowed leakage, such as the axiom that only computation leaks information; (b) the overall amount of key leakage is bounded a-priori for the lifetime
of the system and there is no method for refreshing keys ; (c) the efficiency of the scheme degrades proportionally with the number of refreshes; (d) the key updates require an additional leak-free master secret key to be stored securely; (e) the scheme is only proven secure under a strong non-standard assumption.

2010

PKC

#### Coauthors

- Masayuki Abe (5)
- Jan Camenisch (3)
- Yevgeniy Dodis (3)
- Maria Dubovitskaya (2)
- Georg Fuchsbauer (2)
- Jens Groth (3)
- Tibor Jager (2)
- Eike Kiltz (2)
- Markulf Kohlweiss (3)
- Jorn Lapon (1)
- Adriana López-Alt (3)
- Vincent Naessens (1)
- Miyako Ohkubo (5)
- Victor Shoup (2)
- Daniel Wichs (3)