A communication system according to an embodiment includes one or more hardware processors. The one or more hardware processors: transmit, to an external communication system, a cryptographic random number obtained by encrypting a random number; receive verification information for verifying the external communication system, the verification information being generated by the external communication system with the cryptographic random number and attribute information of the external communication system; and perform, by using a cryptographic key based on the random number, communication with the external communication system having been verified with the verification information.