Key agreement protocols are essential for secure communications in open and distributed environments. Recently, identity-based key agreement protocols have been increasingly researched because of the simplicity of public key management. The basic idea behind an identity-based cryptosystem is that a public key is the identity (an arbitrary string) of a user, and the corresponding private key is generated by a trusted Private Key Generator (PKG). However, it is unrealistic to assume that a single PKG will be responsible for issuing private keys to members of different organizations or a large-scale nation. Hence, it is needed to consider multiple PKG environments with different system parameters. In this paper, we propose an identity-based key agreement protocol among users of different networks with independent PKGs, which makes use of elliptic curves. We prove the security of the proposed protocol in the random oracle model and show that all security attributes are satisfied. We also demonstrate a comparison between our protocol and some related protocols in terms of the communication costs and the execution time. The results show that the execution time of our protocol is less than 10%, and its communication costs are about 50% of the competitor protocols.
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