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Comment by lubutu | Hacker News Reader

My earlier comment (which was for some reason downvoted) also solves this problem: you will always sleep for half a second, timer resolution is irrelevant, and if an attacker bogs down the system such that bcrypt takes longer than half a second, when the timer finished it would notice that bcrypt wasn't done and treat it as a failure.

It would be a bit of a hack compared to just using bcrypt every time, but it would save processing power, which may be useful in some contexts, like very energy-efficient devices, etc.

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So if your system is heavily loaded, nobody will be able to login if bcrypt takes longer than half a second? If processing power is an issue, there are alternate solutions such as dynamically lowering the work factor (upon successful login, rehash the plaintext password they supplied using a decreased work factor) or offloading logins to a pool of servers.

Well, in this instance you wouldn't need any work factor, since it would always take half a second. And if a zero work factor hash is taking half a second to complete, something is very wrong... But granted, for almost all cases it would be best just to bcrypt in every circumstance.

A bcrypt work factor of 10 or 11 on a recent CPU will take about a tenth of a second to hash. Assuming little other workload, you can get about 10 valid logins/second before you'll hit a backlog. If your app is busy and the logins/second increases you'll eventually reach half a second per hash time. If you always return failure when the timer expires and bcrypt hasn't finished, you're doing a denial of service on your own users.

You seem to only be considering the case where the only logins you need to deal with are invalid logins. A busy and successful service will see the vast majority of logins being for legitimate, known users where the bcrypt check must consume CPU time. You have to design the system to be able to handle the workload from both good and bad logins without revealing information about a bad login to an attacker. Anything other than going through the same motions every time will leak information in ways you don't expect.

You can limit it to only checking X hashes per second. The attacker can, under heavy load, figure out how many people are trying to log in, but that doesn't help them figure out any passwords.

Also, there is no reason to implement an extensive backlog in the first place. It's far better for a system at 105% load to drop one in ten attempts than to have an infinitely growing queue.

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