Fun with Timing Attacks

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2025-01-15 07:30:09

You’ll be exposing this checkSecret function to external users so you want to make sure it’s safe to use without leaking the secret. As long as your secret is long enough, it’s unlikely to be brute-forced. You’re feeling pretty confident that this simple function that does nothing but check equality doesn’t have any glaring security flaws.

Anyway, an adversary who can call this function repeatedly can derive a 10-character secret in just a few thousand calls to checkSecret.

Builtins checking equality are implemented in native code that may differ per runtime, but it’s straightforward to imagine how anybody would implement it. === has some details regarding string interning that make analysis a lot more complicated, so we’ll use startsWith for this post. Ignoring details, startsWith might look something like this:

Just iterate through the input and check if the character at each position matches. If not, break early and return false. If so, return true. While the spec doesn’t technically require early exit, most implementations use something like the above. (See V8’s implementation here.)