10. An apparatus of a speaker verification system for preventing malicious attacks on protected resources, comprising: at least one processor; and at least one memory storing computer code instructions thereon, the at least one processor and the at least one memory, with the computer code instructions, being configured to cause the apparatus to: extract a plurality of speech features from multiple segments of a speech signal, the speech signal being a synthetic speech signal generated by transforming voice or text with voice characteristics of a user, the speech signal enabling access to protected resources by a requestor; analyze the plurality of speech features to determine whether the plurality of speech features exhibit periodic variation behavior, the analyzing comparing the plurality of speech features at edges of speech frames with the plurality of speech features in middles of the speech frames, wherein, to perform the analyzing, the at least one processor and the at least one memory, with the computer code instructions, are further configured to cause the apparatus to: generate a representation of variability of the plurality of speech features; and perform a periodicity analysis of the generated representation of variability to determine that the generated representation of variability exhibits periodic variation behavior; and deny the requestor access to the protected resources based on a determination that the speech signal is a synthetic speech signal based on the exhibited periodic variation behavior.
10. An apparatus of a speaker verification system for preventing malicious attacks on protected resources, comprising: at least one processor; and at least one memory storing computer code instructions thereon, the at least one processor and the at least one memory, with the computer code instructions, being configured to cause the apparatus to: extract a plurality of speech features from multiple segments of a speech signal, the speech signal being a synthetic speech signal generated by transforming voice or text with voice characteristics of a user, the speech signal enabling access to protected resources by a requestor; analyze the plurality of speech features to determine whether the plurality of speech features exhibit periodic variation behavior, the analyzing comparing the plurality of speech features at edges of speech frames with the plurality of speech features in middles of the speech frames, wherein, to perform the analyzing, the at least one processor and the at least one memory, with the computer code instructions, are further configured to cause the apparatus to: generate a representation of variability of the plurality of speech features; and perform a periodicity analysis of the generated representation of variability to determine that the generated representation of variability exhibits periodic variation behavior; and deny the requestor access to the protected resources based on a determination that the speech signal is a synthetic speech signal based on the exhibited periodic variation behavior. 13. An apparatus according to claim 10 , wherein the at least one processor and the at least one memory, with the computer code instructions, are configured to further cause the apparatus to employ a result of a determination whether the speech signal is a synthetic speech signal or a natural speech signal in verifying or identifying a speaker associated with the speech signal.