International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Lihua Wang

Publications

Year
Venue
Title
2015
EPRINT
2015
EPRINT
2010
EPRINT
Solving a 676-bit Discrete Logarithm Problem in $GF(3^{6n})$
Pairings on elliptic curves over finite fields are crucial for constructing various cryptographic schemes. The \eta_T pairing on supersingular curves over GF(3^n) is particularly popular since it is efficiently implementable. Taking into account the Menezes-Okamoto-Vanstone (MOV) attack, the discrete logarithm problem (DLP) in GF(3^{6n}) becomes a concern for the security of cryptosystems using \eta_T pairings in this case. In 2006, Joux and Lercier proposed a new variant of the function field sieve in the medium prime case, named JL06-FFS. We have, however, not yet found any practical implementations on JL06-FFS over GF(3^{6n}). Therefore, we first fulfilled such an implementation and we successfully set a new record for solving the DLP in GF(3^{6n}), the DLP in GF(3^{6 \cdot 71}) of 676-bit size. In addition, we also compared JL06-FFS and an earlier version, named JL02-FFS, with practical experiments. Our results confirm that the former is several times faster than the latter under certain conditions.
2010
PKC
2008
EPRINT
Security Proof for the Improved Ryu-Yoon-Yoo Identity-Based Key Agreement Protocol
Key agreement protocols are essential for secure communications in open and distributed environments. The protocol design is, however, extremely error-prone as evidenced by the iterative process of fixing discovered attacks on published protocols. We revisit an efficient identity-based (ID-based) key agreement protocol due to Ryu, Yoon and Yoo. The protocol is highly efficient and suitable for real-world applications despite offering no resilience against key-compromise impersonation (K-CI). We then show that the protocol is, in fact, insecure against reflection attacks. A slight modification to the protocol is proposed, which results in significant benefits for the security of the protocol without compromising on its efficiency. Finally, we prove the improved protocol secure in a widely accepted model.
2007
EPRINT
Cryptanalysis and Improvement of an Elliptic Curve Diffie-Hellman Key Agreement Protocol
In SAC'05, Strangio proposed protocol ECKE-1 as an efficient elliptic curve Diffie-Hellman two-party key agreement protocol using public key authentication. In this letter, we show that despite the author's claims protocol ECKE-1 is vulnerable to key-compromise impersonation attacks. We also present an improved protocol --- ECKE-1N, which can withstand such attacks. The improved protocol's performance is comparable to the well-known MQV protocol and maintains the same remarkable list of security properties.
2006
EPRINT
A Subject-Delegated Decryption Scheme with ``Tightly" Limited Authority
In this paper, we present a new proxy cryptosystem named subject-delegated decryption scheme, in which the original decryptor delegates decryption authority to multiple proxies according to different subjects. The advantage of our scheme is that the proxy authorities are tightly limited (``Tightly" Limited Authority). This means that the proxy authority can be temporarily aborted even if the validity period of the proxy key does not expire. Consequently, our protocol is more practical than the existential protocols because the secrecy of the original decryptor can be protected efficiently from his proxy, especially when the proxy becomes corrupted. Our scheme is efficient because the encryption method in our scheme is based on a hybrid of symmetric key and public key cryptographic techniques. We give the provable security using a variant decisional Bilinear Diffie-Hellman (BDH) assumption.