In this paper, we study three-user cognitive multiple-access channel with confidential messages (CMAC-CM) under the perfect secrecy criterion and for the overlay paradigm. In this channel model, each cognitive user has access to the knowledge of the corresponding noncognitive users’ codebooks or messages to improve the reliable communication of noncognitive users. Also, each cognitive user wishes to transmit it’s own message to the intended receiver in a perfectly secure manner from the corresponding noncognitive users that try to obtain information about this message by exploiting outputs from the underlying channel. For the discrete alphabet and memoryless model of this channel known as three-user DM-CMAC-CM, first, we obtain an inner bound and an outer bound on the secrecy capacity region, and second, prove that these bounds meet for a class of less noisy eavesdropping channels. Third, an inner bound on the secrecy capacity region of the Gaussian version of CMAC-CM known as GCMAC-CM is obtained and, fourth, it is shown that this inner bound is optimal under a special security situation for which the secrecy capacity region as well as the secrecy sum-capacity of a class of degraded three user GCMAC-CM is proposed. Finally, by numerical analysis, we validate the theoretical results and illustrate that our model and results include the previous ones. Copyright c 2016 John Wiley & Sons, Ltd.

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