A security device generates a key based on a physically unclonable function (PUF). The security device includes a physically unclonable function (PUF) block, an integrity detector, and a post processor. The PUF block outputs a plurality of first random signals and a plurality of corresponding first inverted random signals each having a logic level opposite to that of each of the plurality of corresponding first random signals. The integrity detector determines data integrity of the plurality of first random signals by using the plurality of first random signals and the plurality of corresponding first inverted random signals. The post processor generates a first row key that includes validity signals satisfying the data integrity.