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Comment by fwip | Hacker News Reader

Why not?

A year after you leave the job, your team is told to build a new auth backend against the database, using a different bcrypt implementation. They just know to use bcrypt, but not about your truncation hack. The deployment is a success.

Two weeks later, an angry user (the only one with a 100-digit password) complains that they can't log in anymore. The guy is the company's best-paying customer; the boss is furious. The whole team goes on a wild goose chase for two days and nights just to find out what happened, as clearly there's nothing wrong with their code.

A few years later, a former colleague shares the episode on HN. As you read the comments, it dawns on you that the idiot antagonist of the story is you. In this moment, you are enlightened.

The point of the upstream post was that bcrypt implementations often already truncate your passwords to 72 characters.

If you switch to a different bcrypt implementation that does/does not truncate at 72 characters, the server-side truncation keeps all those 73 character passwords working.

If the server-side truncation were not in place, you'd get angry users.

Suppose a user whose first name is Jonathan and whose wife's name is Katherine uses the password "JonathanAndKatherineLU8zWNmkimQanaSdPdqatWJEWR8goyyhdtQeqZOp2+0" and you truncate it to 20 characters.

You would truncate to 72 bytes, not 20 characters (which might be more than 72 bytes anyway).

So potentially even worse. Somebody decides they don't like variable-width encodings and uses UTF-32 and now 72 bytes doesn't even get you 20 code points, only 18.

But it's the same principle no matter where you cut. User thinks they can use an arbitrarily long password, puts a long but low entropy or easily guessable string at the front and makes up for it by having some good entropy at the end, and then you chop off the end.

Because you're misleading the user. Loudly complaining about invalid input is okay, but accepting and silently changing it in a way that affects it properties is not.

How would the user tell the 73rd byte is truncated? Might as well call bcrypt itself misleading.

In any case this doesn’t preclude client-side validation.

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