The use of Voice over Internet Protocol (VoIP) is becoming ubiquitous due to the multiple shortcomings of traditional Public Switched Telephone Network (PSTN) systems. As a result, the development of secure key establishment protocols is becoming increasingly important. The Communications-Electronics Security Group (CESG), in response to this demand, has published new key agreement protocols for the Multimedia Internet KEYing (MIKEY) protocol to provide low-cost secure VoIP communications, supported on Identity-based Public-Key Cryptography (IDPKC). In the context of IDPKC, the identity of users is used to derive their public-keys, which eliminates the expenses of maintaining a Public-Key Infrastructure (PKI). However, IDPKC systems suffer from inefficient user revocation and key renewal. In this paper, we take advantage of the fact that users need to be connected to the Internet to communicate, for introducing a SEcurity Mediator (SEM), who possesses a share of the users' private-keys, and with whom the users must cooperate, to sign and decrypt cryptograms. By taking advantage of this sharing, we introduce mechanisms to provide immediate user revocation and key renewal.
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