Provided are a flash memory device, a flash memory system, and methods of operating the same. A method of operating a flash memory system includes selecting memory cells of a flash memory in response to an authentication challenge, programming pieces of input data into the selected memory cells, respectively, reading the selected memory cells and generating and storing control information, dividing the selected memory cells into at least one first region memory cell and at least one second region memory cell based on the control information, and setting read values of the at least one first region memory cell and the at least one second region memory cell as a first value and a second value, respectively, and generating an authentication response in the response to the authentication challenge.