A communication system that obtains a key includes: a server that confirms support of Identity Based Encryption (IBE) authentication; the server obtains public parameters and a private key for IBE; and the server receives a PreMasterSecret key encrypted through the IBE, and obtains a plain text of the PreMasterSecret key according to the public parameters and the private key. The system includes a client and a server. The client includes an IBE negotiating module, a public parameter obtaining module, a server identifier obtaining module, and a processing module. The server includes an IBE negotiating module, a public parameter obtaining module, a private key obtaining module, and a processing module. Through combination of the IBE technology and the SSL/TLS technology, the modes of encrypting a PreMasterSecret key in the existing SSL/TLS protocol are diversified, and the use scope of the existing SSL/TLS protocol is extended substantially.