Abstract. This paper introduces and makes concrete the concept of certificateless public key cryptography (CL-PKC), a model for the use of public key cryptography which avoids the inherent escrow of identitybased cryptography and yet which does not require certificates to guarantee the authenticity of public keys. The lack of certificates and the presence of an adversary who has access to a master key necessitates the careful development of a new security model. We focus on certificateless public key encryption (CL-PKE), showing that a concrete pairing-based CL-PKE scheme is secure provided that an underlying problem closely related to the Bilinear Diffie-Hellman Problem is hard.

Abstract. We present a new Certificateless Public Key Encryption (CL-PKE) scheme whose security is proven to rest on the hardness of the Bilinear Diffie-Hellman Problem (BDHP) and that is more efficient than the original scheme of Al-Riyami and Paterson. We then give an analysis of Gentry's Certificate Based Encryption (CBE) concept, repairing a number of problems with the original definition and security model for CBE. We provide a generic conversion showing that a secure CBE scheme can be constructed from any secure CL-PKE scheme. We apply this result to our new efficient CL-PKE scheme to obtain a CBE scheme that improves on the original scheme of Gentry.

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Since Boneh and Franklin published their seminal paper on identity based encryption (IBE) using the Weil pairing, there has been a great deal of interest in cryptographic primitives based on elliptic-curve pairings. One particularly interesting application has been to control access to data, via possibly complex policies. In this paper we continue the research in this vein. We present an encryption scheme such that the receiver of an encrypted message can only decrypt if it satisfies a particular policy chosen by the sender at the time of encryption. Unlike standard IBE, our encryption scheme is escrow free in that no credential-issuing authority (or colluding set of credential-issuing authorities) is able to decrypt ciphertexts itself, providing the users' public keys are properly certified. In addition we describe a security model for the scenario in question and provide proofs of security for our scheme (in the random oracle model).

This paper shows that the scheme described in Haber and Stornetta [Haber and Stornetta Jr., 1994] for extending the validity of a cryptographic timestamp for a Time Stamping Service contains security shortcomings. A modification is proposed to rectify the identified shortcomings.