A data structure to be used for representing the problem is selected. An instance of the problem is represented with the data structure. The data is encrypted with a sequence which is obtained by solving the problem. Alternatively, in an alternative embodiment of the present invention, a program for heuristically solving an NP-hard problem is received. A data structure that represents an instance of the problem is received. The program is applied to the instance of the problem to obtain a sequence. The data is decrypted with the sequence.