node.js - What's the difference between a secure compare and a simple ==(=)

Question

Github's securing webhooks page says:

Using a plain == operator is not advised. A method like secure_compare performs a “constant time” string comparison, which renders it safe from certain timing attacks against regular equality operators.

I use bcrypt.compare('string', 'computed hash') when comparing passwords.

What makes this a "secure compare" and can I do this using the standard crypto library in Node?

Answer 1

The point of a "constant time" string comparison is that the comparison will take the exact same amount of time no matter what the comparison target is (the unknown value). This "constant time" reveals no information to an attacker about what the unknown target value might be. The usual solution is that all characters are compared, even after a mismatch is found so no matter where a mismatch is found, the comparison runs in the same amount of time.

Other forms of comparison might return an answer in a shorter time when certain conditions are true which allows an attacker to learn what they might be missing. For example, in a typical string comparison, the comparison will return false as soon as an unequal character is found. If the first character does not match, then the comparison will return in a shorter amount of time than if it does. A diligent attacker can use this information to make a smarter brute force attack.

A "constant time" comparison eliminates this extra information because no matter how the two strings are unequal, the function will return its value in the same amount of time.

In looking at the nodejs v4 crypto library, I don't see any signs of a function to do constant time comparison and per this post, there is a discussion about the fact that the nodejs crypto library is missing this functionality.

EDIT: Node v6 now has crypto.timingSafeEqual(a, b).

There is also such a constant time comparison function available in this buffer-equal-constant-time module.