International Association for Cryptologic Research

International Association
for Cryptologic Research


Shengbao Wang


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.
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.
Practical Identity-Based Encryption (IBE) in Multiple PKG Environments and Its Applications
Shengbao Wang Zhenfu Cao
Identity-based encryption (IBE) schemes are usually used in multiple-PKG environments --- on the one hand, each administrative domain (e.g., a relatively small and close organization) maintains its own private key generator (PKG); on the other hand, encryption across domains becomes a prevalent requirement. In this paper, we present a new IBE scheme using bilinear pairings. Compared with the famous IBE scheme of Boneh and Franklin, we show that ours is more practical in the multiple-PKG environment. We prove that our scheme meets chosen ciphertext security in the random oracle model, assuming the intractability of the standard Bilinear Diffie-Hellman (BDH) problem. As an application of our IBE scheme, we also propose an escrowed ElGamal scheme which possesses certain good properties in practice.
Perfect Forward Secure Identity-Based Authenticated Key Agreement Protocol in the Escrow Mode
There are several essential features in key agreement protocols such as key escrow (essential when confidentiality, audit trail and legal interception are required) and perfect forward secrecy (i.e., the security of a session key established between two or more entities is guaranteed even when the private keys of the entities are compromised). Majority of the existing escrowable identity-based key agreement protocols, however, only provide partial forward secrecy. Therefore, such protocols are unsuitable for real-word applications that require a stronger sense of forward secrecy --- perfect forward secrecy. In this paper, we propose an efficient perfect forward secure identity-based key agreement protocol in the escrow mode. We prove the security of our protocol in the random oracle model, assuming the intractability of the Gap Bilinear Diffie-Hellman (GBDH) problem. Security proofs are invaluable tools in assuring protocol implementers about the security properties of protocols. We note, however, that many existing security proofs of previously published identity-based protocols entail lengthy and complicated mathematical proofs. In this paper, our proof adopts a modular approach and, hence, simpler to follow.
New Identity-Based Authenticated Key Agreement Protocols from Pairings (without Random Oracles)
We present the first provably secure ID-based key agreement protocol, inspired by the ID-based encryption scheme of Gentry, in the standard (non-random-oracle) model. We show how this key agreement can be used in either escrowed or escrowless mode. We also give a protocol which enables users of separate private key generators to agree on a shared secret key. All our proposed protocols have comparable performance to all known protocols that are proven secure in the random oracle model.