Blowfish password verifying

Question

I use Spring security in this question but its knowledge might not be required to answer this question.

I am trying to understand how Spring security's blowfish encryption class (BCrypt) verifies a given password. I tried to :

1. Generate a Salt with BCrypt.gensalt(10);
2. Encode a password using this salt with BCrypt.hashpw(clearText, salt).

It worked well.Then, I wanted to store the used salt in the database in order to be able to reuse it when the users enter a password.

I realized that BCryptPasswordEncoder does not use the salt when it checks the password. Instead, it crypts the password with its hashed password as salt :

public boolean matches(CharSequence rawPassword, String encodedPassword) {
  [...]
  return BCrypt.checkpw(rawPassword.toString(), encodedPassword);
}

So I tried this piece of code :

public static void main(String[] args) {
    String salt = BCrypt.gensalt(12);
    String clearText="test";
    String hashed = BCrypt.hashpw(clearText, salt);
    String reHashed = BCrypt.hashpw(clearText, hashed);

    System.out.println("salt : " + salt);
    System.out.println(hashed);
    System.out.println(reHashed);
}

output :

salt : $2a$12$gzUymsNBoW.f1OfkLpb2se
$2a$12$gzUymsNBoW.f1OfkLpb2seFZrniorawujSOp6Qe.PWDIHJvmYSP6y
$2a$12$gzUymsNBoW.f1OfkLpb2seFZrniorawujSOp6Qe.PWDIHJvmYSP6y

This seems to confirm that :

good password => hashed = hash(clearPassword, hashed)

Am I understanding it wrong? Does it mean that the salt is useless for password checking? Hence, does it mean that I do not need to store the salt in database?

Answer 1

BCrypt stores the salt as part of the "hash" string that it returns.
This is handled by the Bcrypt.* functions, so you don't need to do anything.