To store an input string of M N-tuples of bits, a substitution transformation is selected in accordance with the input string and is applied to the input string to provide a transformed string of M N-tuples of bits. M or more memory cells are programmed to represent the transformed string and preferably also to represent a key of the transformation. Alternatively, the memory selectively programs each of M or more cells to a respective one of 2.sup.N states. A mapping that maps the binary numbers in [0,2.sup.N-1] into respective states is selected in accordance with the input string and is used to program M cells to represent the input string. Preferably, a key of the mapping is stored in the memory in association with the M cells.