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Is there a key derivation/hashing function (that has any real value) that fulfills both of these criteria:

  • Slow to compute the hash
  • Fast to verify the hash based on the input string (not given a second, existing copy of said hash)

I know in general key-derivation is used to prevent brute-forcing of passwords, but that's typically to protect passwords against internal users.

I'm looking to set up an authentication system where a user must provide a username, password and a Proof of Work hash of the password, except.. I'd like to not have to do the work myself to quickly check.

The hash is not being compared against the actual hash in the database (e.g., it should be a different hashing algorithm than the one in the database.. PBKDF2), this is simply to discourage brute forcing while also not creating some endpoint on my server where a malicious actor can make my server do some hard work... for nothing.

Is this even possible? I know it's not necessary, but if it's possible it seems like a simple measure I could take to secure my web servers.

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    $\begingroup$ Conflicting requirement? If you can verify fast why the attacker cannot? $\endgroup$
    – kelalaka
    Jan 4, 2022 at 0:58
  • $\begingroup$ It just feels like I should be able to use a key-derivation function to mix a private key into the key the user will use to hash/derive another key which should make it very easy to check (given the private key, which the user will not have). $\endgroup$ Jan 4, 2022 at 0:58
  • $\begingroup$ @kelalaka see my last comment; I don't know if this is necessarily a thing or not. But I mean, this is a property of PKI (e.g., given the private key, you can just decrypt the message, otherwise you must crack it). I feel like the same should be able to be somehow transmuted into PoW... $\endgroup$ Jan 4, 2022 at 0:58
  • $\begingroup$ I mean, even just a PKI setup where it is much harder to encrypt with the public key than it is to decrypt with the private key would work. $\endgroup$ Jan 4, 2022 at 1:05

1 Answer 1

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A simple Brave search for "Proof of Work hashing" answered this... https://www.npmjs.com/package/hashcash

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    $\begingroup$ SO you store the hashcash value so that you have fast verification? This is not the way Key Derivation works. If you want to have a slow one use Argon2. $\endgroup$
    – kelalaka
    Jan 4, 2022 at 6:16
  • $\begingroup$ @kelalaka e.g., it should be a different hashing algorithm than the one in the database.. PBKDF2 No. I am storing PBKDF2 in the database. The hashcash/proof of work is not stored... No where did I imply that I would be storing the hashcash. $\endgroup$ Jan 5, 2022 at 14:50
  • $\begingroup$ It would work like this.... 1.) User computes hashcash, sends username, pass, hashcash and counter value 2.) I verify hashcash with counter and inputs (lets say the username + password). 3. Only after verification do I then check the database (where I then hash the password provided by the user with PBKDF2 and compare it against the database record). The point is so that a malicious user can't force my server to repeatedly check the PBKDF2 hash without having done a ton of work (much more work than my server) up front. $\endgroup$ Jan 5, 2022 at 14:56
  • $\begingroup$ Update and clarify your answer in the answer itself $\endgroup$
    – kodlu
    Oct 1, 2022 at 14:35

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