Question

I have a question regarding password hashing. I found a great tutorial on http://crackstation.net/hashing-security.htm. This is how I create a hash from the password using this code:

$hashed_password = create_hash(form_password);

The problem with this code is that it creates a different hash for the same password every time. I guess it’s because of adding a random salt to the password:

function create_hash($password)
{
    // format: algorithm:iterations:salt:hash
    $salt = base64_encode(mcrypt_create_iv(PBKDF2_SALT_BYTES, MCRYPT_DEV_URANDOM));
    return PBKDF2_HASH_ALGORITHM . ":" . PBKDF2_ITERATIONS . ":" .  $salt . ":" .
        base64_encode(pbkdf2(
            PBKDF2_HASH_ALGORITHM,
            $password,
            $salt,
            PBKDF2_ITERATIONS,
            PBKDF2_HASH_BYTES,
            true
        ));
}

So due to this, I cannot compare the hash created from the password typed by the user at the login with the hash retrieved from the database, because that hash will never be the same with the one stored in the database. The only way I see that it can be done is to store the salt in another column, or am I wrong?

A thing also annoys me. The format of the hast is e.g. sha256:1000:u9p+OqAZgVkIBtlTBkr9ZxhFvtt+zjcA:PvhJY+oesrdBeD5pjeXMQ/3wficCU0EG. A hacker obviously knows this form of hash so the algorythm that created this hash is clear for him. Let’s suppose the database has been compromised and thousands of these hash codes were revealed. Doesn’t having this form of hash code raise security issues? I guess a simple OqAZgVkIBtlTBkr9ZxhFvtt+zjcA:PvhJY+oesrdBeD5pjeXMQ/3wfic would be better because it reveals nothing about the encryption mechanism.

Answer 1

Read this link to Wikipedia here

It explains more about the use of a random salt. The key information there is that the random salt is stored in the same database as the hashed password. It is only created randomly a few time, when the user creates or changes their password. It is used whenever authentication is required.

Leaving the info about which algorithm it used to create the password hash leaves you the option of changing the algorithm at some time in the future without interrupting your users. If you decide up upgrade the hash algorithm you wait until a user logs in. At that point you have their password in plain text. Once you authenticate against the old algorithm, you rehash and store the new hash in their database row. It may take months or years for all users to log in but once they have they will accrue the benefits of better hashing. So you would then send out an email asking them to login and read your special news about better security (and as a side effect, improve their hashed password).

Here is an interesting discussion of password hashing StackOverflow:Passwords Storage Hash …