The inventive concept provides a security device capable of reducing an area of a die required for implementation of a stable PUF by increasing the value of entropy from a predefined number of entropy sources and/or minimizing a blind zone of a validity checking module. The security device uses an asynchronous configuration to minimize a blind zone. In various embodiments of the inventive concept, the blind zone is generated only in a period when a reset signal is at a first logic level. Therefore, it is possible to minimize the blind zone by minimizing a period in which the reset signal is at such logic level. A semiconductor device, semiconductor package, and/or smart card can be provided with such security device, as well as a method for determining a validity of a random signal using a semiconductor security device.