A method by one or more computing devices for obfuscating challenge code. The method includes obtaining challenge code for interrogating a client, inserting, into the challenge code, code for obfuscating outputs that are to be generated by the client, where the code for obfuscating the outputs includes code for applying a first chain of reversible transformations to the outputs using client-generated random values, interning strings appearing in the challenge code with obfuscated strings, inserting code for deobfuscating the obfuscated strings into the challenge code, inlining function calls in the challenge code, removing function definitions that are unused in the challenge code due to the inlining, reordering the challenge code without changing the functionality of the challenge code, and providing the challenge code for execution by the client.