12.5 Performing Constant Transforms on Variables

12.5.1 Problem

Variables used frequently such as in loops or counters are difficult to obfuscate without impacting the performance of the program.

12.5.2 Solution

Change variables by a constant value.

12.5.3 Discussion

Changing variables by a constant value is a trivial form of obfuscation; however, it is fast and easy to implement, and it can be combined with other obfuscation methods. Here is an example of the obfuscation:

#define SET_VAR(var) (((var) * 3) + 0x01040200) #define GET_VAR(var) (((var) - 0x01040200) / 3)

The macros can be applied to any usage of an integer:

for (i = SET_VAR(0);  GET_VAR(i) < 10;  i = SET_VAR(j + 1)) {    j = GET_VAR(i);   printf("2 + %d = %d\n", i, 2 + GET_VAR(i)); }

Constant transforms are useful only if the SET_VAR and GET_VAR macros are used far apart; otherwise, the transform is immediately obvious. Transformations that are more robust can be created that use different mathematical operations in each of the SET_VAR and GET_VAR macros so that different constants are used in the expansion of each macro. Note that the SET_VAR macro can be used in the initialization of a variable, which will obfuscate the value of the variable at compile time.