Why should checking a wrong password take longer than checking the right one?

Question

This question has always troubled me.

On Linux, when asked for a password, if your input is the correct one, it checks right away, with almost no delay. But, on the other hand, if you type the wrong password, it takes longer to check. Why is that?

I observed this in all Linux distributions I've ever tried.

Answer 1

It's actually to prevent brute force attacks from trying millions of passwords per second. The idea is to limit how fast passwords can be checked and there are a number of rules that should be followed.

• A successful user/password pair should succeed immediately.
• There should be no discernible difference in reasons for failure that can be detected.

That last one is particularly important. It means no helpful messages like:

Your user name is correct but your password is wrong, please try again 

or:

Sorry, password wasn't long enough 

Not even a time difference in response between the "invalid user and password" and "valid user but invalid password" failure reasons.

Every failure should deliver exactly the same information, textual and otherwise.

Some systems take it even further, increasing the delay with each failure, or only allowing three failures then having a massive delay before allowing a retry.