> The easiest way to defeat this attack is, instead of directly comparing two strings, compare their mappings under a collision-resistant hash function.
Is this really the best way to compare strings without giving away timing info?
Is this really the best way to compare strings without giving away timing info?
If you do that in a language with an optimizing compiler or JIT runtime you'd better look at the generated code and/or do your own timing measurements. A sufficiently-smart optimizer could recognize that the first difference encountered allows for early loop termination.
I see how the xor method works, but not why it's superior.
Also, it would seem that you'd be leaking information about the size of the other string with either method (whether you exit early or not when you run past the length of one of the strings) - not a problem when comparing hashes presumably, but is it never a concern?
I don't think you'd be leaking that information if you hashed both strings and compared the hash, because it wouldn't stop getting faster when the shorter string gets shorter.
This is just one of the easier ways. It's effective and easy to code.