International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Qi Cheng

Publications

Year
Venue
Title
2021
EUROCRYPT
On the ideal shortest vector problem over random rational primes
Any non-zero ideal in a number field can be factored into a product of prime ideals. In this paper we report a surprising connection between the complexity of the shortest vector problem (SVP) of prime ideals in number fields and their decomposition groups. When applying the result to number fields popular in lattice based cryptosystems, such as power-of-two cyclotomic fields, we show that a majority of rational primes lie under prime ideals admitting a polynomial time algorithm for SVP. Although the shortest vector problem of ideal lattices underpins the security of the Ring-LWE cryptosystem, this work does not break Ring-LWE, since the security reduction is from the worst case ideal SVP to the average case Ring-LWE, and it is one-way.
2015
EPRINT
2015
EPRINT
2010
EPRINT
On Representable Matroids and Ideal Secret Sharing
Chingfang Hsu Qi Cheng
In secret sharing, the exact characterization of ideal access structures is a longstanding open problem. Brickell and Davenport (J. of Cryptology, 1991) proved that ideal access structures are induced by matroids. Subsequently, ideal access structures and access structures induced by matroids have attracted a lot of attention. Due to the difficulty of finding general results, the characterization of ideal access structures has been studied for several particular families of access structures. In all these families, all the matroids that are related to access structures in the family are representable and, then, the matroid-related access structures coincide with the ideal ones. In this paper, we study the characterization of representable matroids. By using the well known connection between ideal secret sharing and matroids and, in particular, the recent results on ideal multipartite access structures and the connection between multipartite matroids and discrete polymatroids, we obtain a characterization of a family of representable multipartite matroids, which implies a sufficient condition for an access structure to be ideal. By using this result and further introducing the reduced discrete polymatroids, we provide a complete characterization of quadripartite representable matroids, which was until now an open problem, and hence, all access structures related to quadripartite representable matroids are the ideal ones. By the way, using our results, we give a new and simple proof that all access structures related to unipartite, bipartite and tripartite matroids coincide with the ideal ones.
2007
JOFC
2004
CRYPTO
2003
CRYPTO
2002
EPRINT
A New Class of Unsafe Primes
Qi Cheng
In this paper, a new special-purpose factorization algorithm is presented, which finds a prime factor $p$ of an integer $n $ in polynomial time, if $4p-1$ has the form $d b^2$ where $d \in \{3, 11, 19, 43, 67, 163\}$ and $b$ is an integer. Hence such primes should be avoided when we select the RSA secret keys. Some generalizations of the algorithm are discussed in the paper as well.