A security system includes a physical unclonable function circuit, a write-in protection circuit, a memory, and a readout decryption circuit. The physical unclonable function circuit provides a plurality of random bit strings. The write-in protection circuit receives a write-in address and original data, and includes an address scrambling unit. The address scrambling unit generates a scrambled address by scrambling a write-in address according to a random bit string provided by the physical unclonable function circuit. The memory stores the storage data corresponding to the original data according to the scrambled address. The readout decryption circuit reads out the storage data from the memory according to the write-in address to derive the original data.