A method of generating a response to a physically unclonable function, said response being uniquely representative of the identity of a device having challengeable memory, the memory comprising a plurality of logical locations each having at least two possible logical states, the method comprising applying a challenge signal to an input of said memory so as to cause each of said logical locations to enter one of said two possible logical states and thereby generate a response pattern of logical states, said response pattern being dependent on said physically unclonable function which is defined by, the physical characteristics of said memory, the method further comprising reading out said response pattern.