Parameter extraction functions

[Vlix]

Might be nice to also expose the functions that take a PasswordHash a and return the parameters for that hash. (in case the hash is parsable)
e.g. extractParams :: PasswordHash Scrypt -> Maybe ScryptParams

Which later on might be extended to extractParamsWithFormat :: ScryptFormat -> PasswordHash Scrypt -> Maybe ScryptParams when we also support different hashing formats.

Kind of part of the #22 enhancement, so users can check hashes. e.g. when filtering hashes from a DB.

[blackheaven]

I was going to tackle it, but I had hard time figuring out how to extract the salt, should I use the default to fake it?

[Vlix]

@blackheaven Thank you for taking a stab at this.

Every checkPassword function parses the necessary parameters and salt from the hash, so I imagine you can factor out those parts as extractParams 🤔

[blackheaven]

After a second read you're right but (except Bcrypt's checkPassword which does not extract the salt at all):
All checkPassword uses/extracts Salt a and default their salt parameter (which is the salt size, not the salt itself).

So, just to be sure: we want to have functions which extracts *Params with a correct salt size?

[Vlix]

Be aware that the parameters for passwords never contain salts. Salts are supposed to be random for security's sake.

The checkPassword functions default the salt parameter just to not have any undefined record fields, since the salt parameter isn't used when checking the password. The extractPassword functions would do well to check the salt's length to construct an accurate salt size parameter.

Also, bcrypt as an algorithm requires the salt to be 16 bytes, so that is not part of the adjustable parameters.

[blackheaven]

I've made an attempt in #61.

There's two ways to mitigate de default value issue coming to my mind:

• Create dedicated types (eg. Argon2ParamsParsed)
• Having a Phantom type and a type family (see trees that grow)

Let me know what you think, and whether I should handle it it in my PR, another one, or not at all.

[Vlix]

I'm pretty sure that the following holds:

-- This doesn't actually work, since 'newSalt' is in IO, but you get the idea
testProperty "length of salt = size of salt" $ \i ->
  let Salt salt = Data.Password.Internal.newSalt i
  in Data.ByteString.length salt === i

So you can just get the salt size from the salt:

-- e.g. the Argon2 one
extractParams passHash = do
  (params, Salt salt, _) <- parseArgon2PasswordHashParams passHash
  let saltSize = Data.ByteString.length salt
  pure params{argon2Salt = saltSize}

[Vlix]

#61 implemented this