Edited to remove untrue information. Thanks for the corrections everyone.
Incorrect.
They’re designed to be resource intensive to calculate to make them harder to brute force, and impossible to reverse.
Some literally have a parameter which acts as a sliding scale for how difficult they are to calculate, so that you can increase security as hardware power advances.
I was incorrect but I still disagree with you. The hashing function is not designed to be resource intensive but to have a controlled cost. Key stretching by adding rounds repeats the controlled cost to make computing the final hash more expensive but the message length passed to the function isn’t really an issue. After the first round it doesn’t matter if the message length was 10, 128, or 1024 bytes because each round after is only getting exactly the number of bytes the one way hash outputs.
It depends on the hash. E.g., OWASP only recommends 2 iterations of Argon2id as a minimum.
Yes, a hashing function is designed to be resource intensive, since that’s what makes it hard to brute force. No, a hashing function isn’t designed to be infinitely expensive, because that would be insane. Yes, it’s still a bad thing to provide somebody with a force multiplier like that if they want to run a denial-of-service.
See “Password Hashing” here: https://en.m.wikipedia.org/wiki/Key_derivation_function
It is actually important to have a controlled cost to calculate in the forward direction too.
Not true. Password hashing algorithms should be resource intensive enough to prevent brute force calculation from being a viable route. This is why bcrypt stores a salt, a hash, and the current number of rounds. That number of rounds should increase as CPUs get faster to prevent older hashes from existing in the wild which can be more effectively broken by newer CPUs.
I was incorrect about the goal being minimal resources. I should have written that that goal was to have controlled resource usage. The salt does not increase the expense of the the hash function. Key stretching techniques like adding rounds increase the expense to reach the final hash output but does not increase the expense of the hash function. High password length allowances of several thousand characters should not lead to a denial of service attack but they don’t materially increase security after a certain length either.
I’m arguing semantics here but bcrypt is the hashing function. Per the Wikipedia article on bcrypt:
bcrypt is a password-hashing function designed by Niels Provos and David Mazières, based on the Blowfish cipher and presented at USENIX in 1999.
Blowfish being a symmetric encryption cipher, not a hashing function.
Agreed on the rest, though. The hashing cost of a long password would not lead to DOS any more than the bandwidth of accepting that password etc. It’s not the bottleneck. But also no extra security beyond a point, so might as well not bother when passwords are too long.
