A program is obfuscated by reordering its instructions. Original instruction addresses are mapped to target addresses in an irregular way, with position dependent address steps between the addresses of logically successive instructions. Preferably pseudo-random address steps are used, for example with address steps that have mutually opposite sign with equal frequency. The data processing device has an instruction flow control unit that updates instruction addresses according the position dependent address steps. The instruction flow control unit may comprise a circuit that contains secret information, which is not normally accessible from the outside, to control the updates. A lookup table may be used for example, with address steps, successor addresses or mapped address values. In an embodiment the mapping of original instruction addresses to target addresses may be visualized by means of a path (36) along points in an n-dimensional array, where n is greater than one. Successive original instruction addresses are mapped to successive locations in the path, and locations along respective rows of the array are mapped to respective mutually disjoint ranges (33a-d). Because an n-dimensional path is used a form of locality is preserved that provides for cache efficiency, whereas the multi-dimensional nature of the path makes it possible to combine this locality with pseudo-random steps.