## CryptoDB

### Shouhuai Xu

#### Publications

Year
Venue
Title
2014
EPRINT
2005
EPRINT
In the society increasingly concerned with the erosion of privacy, privacy-preserving techniques are becoming very important. This motivates research in cryptographic techniques offering built-in privacy. A secret handshake is a protocol whereby participants establish a secure, anonymous and unobservable communication channel only if they are members of the same group. This type of private" authentication is a valuable tool in the arsenal of privacy-preserving cryptographic techniques. Prior research focused on 2-party secret handshakes with one-time credentials. This paper breaks new ground on two accounts: (1) it shows how to obtain secure and efficient secret handshakes with reusable credentials, and (2) it represents the first treatment of group (or {\em multi-party}) secret handshakes, thus providing a natural extension to the secret handshake technology. An interesting new issue encountered in multi-party secret handshakes is the need to ensure that all parties are indeed distinct. (This is a real challenge since the parties cannot expose their identities.) We tackle this and other challenging issues in constructing GCD -- a flexible framework for secret handshakes. The proposed framework lends itself to many practical instantiations and offers several novel and appealing features such as self-distinction and strong anonymity with reusable credentials. In addition to describing the motivation and step-by-step construction of the framework, this paper provides a thorough security analysis and illustrates two concrete framework instantiations.
2003
ASIACRYPT
2003
PKC
2003
EPRINT
Constructing practical and provably secure group signature schemes has been a very active research topic in recent years. A group signature can be viewed as a digital signature with certain extra properties. Notably, anyone can verify that a signature is generated by a legitimate group member, while the actual signer can only be identified (and linked) by a designated entity called a group manager. Currently, the most efficient group signature scheme available is due to Camenisch and Lysyanskaya \cite{CL02}. It is obtained by integrating a novel dynamic accumulator with the scheme by Ateniese, et al. \cite{ACJT00}. In this paper, we construct a dynamic accumulator that accumulates \emph{composites}, as opposed to previous accumulators that accumulated \emph{primes}. We also present an efficient method for proving knowledge of factorization of a committed value. Based on these (and other) techniques we design a novel provably secure group signature scheme. It operates in the \emph{common auxiliary string} model and offers two important benefits: 1) the {\sf Join} process is very efficient: a new member computes only a single exponentiation, and 2) the (unoptimized) cost of generating a group signature is 17 exponentiations which is appreciably less than the state-of-the-art.
2002
EUROCRYPT
2002
EPRINT
Cryptographic computations (decryption, signature generation, etc.) are often performed on a relatively insecure device (e.g., a mobile device or an Internet-connected host) which cannot be trusted to maintain secrecy of the private key. We propose and investigate the notion of \emph{key-insulated security} whose goal is to minimize the damage caused by secret-key exposures. In our model, the secret key(s) stored on the insecure device are refreshed at discrete time periods via interaction with a physically-secure --- but computationally-limited --- device which stores a master key''. All cryptographic computations are still done on the insecure device, and the public key remains unchanged. In a (t, N)-key-insulated scheme, an adversary who compromises the insecure device and obtains secret keys for up to t periods of his choice is unable to violate the security of the cryptosystem for \emph{any} of the remaining N-t periods. Furthermore, the scheme remains secure (for \emph{all} time periods) against an adversary who compromises \emph{only} the physically-secure device. We notice that key-insulated schemes significantly improve the security guarantee of forward-secure schemes [A97,BM99], in which exposure of the secret key at even a single time period (necessarily) compromises the security of the system for all future time periods. This improvement is achieved with minimal cost: infrequent key updates with a (possibly untrusted) secure device. We focus primarily on key-insulated public-key encryption. We construct a (t,N)-key-insulated encryption scheme based on any (standard) public-key encryption scheme, and give a more efficient construction based on the DDH assumption. The latter construction is then extended to achieve chosen-ciphertext security.

#### Coauthors

Yevgeniy Dodis (3)
Jonathan Katz (3)
Gene Tsudik (3)
Moti Yung (3)
Qingji Zheng (1)