Does having a salted password hash make cracking an unsalted hash of the same password easier?

Question

I've been asked to implement a system to prevent reusing passwords on the same account.

The most secure way I know to do this is to compare hashes of the password + a known salt. bcrypt.hash(sha256(password), user.savedSalt))

The rest of the system uses a separate salted sha256 - bcrypt.hash(sha256(password), 10).

These two hashes each reveal different information about the original password.

Does having both make cracking the known salt hash easier?

Answer 1

If you are storing the hashes of multiple passwords with the same salt, you are defeating the purpose of using a salt. The salt must be unique for each and every password, otherwise an attacker could compute H(password || salt) just once for each candidate password, and compare the resulting digest against every hash in your database, because they all use the same salt.

Also, as mentioned in the comments, you seem to be confusing the cost parameter for something else. The "10" specifies how much computation the bcrypt operation will take. A larger value increases the cost for attackers by making the operation more computationally expensive.

If you want to check for duplicate passwords, you don't need to come up with some homebrew scheme like this. If you really want, you can take the supplied password, concatenate it with the salt of the first user and hash it, and compare it against the stored hash. If it does not match, do the same for the second user, and so on. This does mean that each new password that is set will require walking through the entire database, but it will detect passwords that are exact duplicates.

Answer 2

"Does having a salted password hash make cracking an unsalted hash of the same password easier?"

I found some places where people said this was theoretically true, but no one cited any work on it or anything in practice.

"The salt must be unique for each and every password..."

This is not true. We set a salt per user that remembered the users last few passwords. Salts are to defeat rainbow tables, and calculating a rainbow table per user for 3 passwords seem as inefficient as calculating tables for one password.