Password hashing (crackstation.net)

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

Store the salt then add the salt to the password then turn it into a hash then store it.

Then to compare the passwords you retrieve the salt and add it to he password, convert it into a hash and then compare the two hashes.

(python example, the indentation is messed up when I copied the code from my code)

def Createpassword():
import random
import hashlib
username = input("type your username")
newpassword = input("type your new password? ")
#adds some salt
b = random.randint(1, 2000000000000)
randomsalt = str(b)

saltedpassword = (newpassword + randomsalt)

#hashes the newpassword

saltedpassword = saltedpassword.encode('utf-8')   
h = hashlib.sha256(saltedpassword)
finsh = h.hexdigest()

#creates file names
saltname = username + "salt.txt"
username = username + ".txt"


#saves the hashedpassword and the salt
text_file = open(username, "w")    
text_file.write(finsh)
text_file.close()
text_file = open(saltname, "w")    
text_file.write(randomsalt)
text_file.close()
print("your new password has been saved")

def EnterPassword():
    trys = 0
    while (trys < 3):
            username = input("type your username: ")
            #takes password input
            password = input("type your password: ")

            #gets file names
            saltname = username + "salt.txt"
            username = username + ".txt"

            #opens salt file
            text_file = open(saltname, "r")
            salt1 = text_file.read(50)
            text_file.close()
            #opens the hashed passwords file
            text_file = open(username, "r")
            correctpassword = text_file.read(500)
            text_file.close()

            #adds the salt to the password input
            saltedpassword = password + salt1


            #hashes the saltedpassword
            saltedpassword = saltedpassword.encode('utf-8') 
            m = hashlib.sha256(saltedpassword)
            hashedpassword = m.hexdigest()

            #compears hashed passwords

            if hashedpassword != correctpassword:
                    trys += 1
                    print("your password or username is incorrect")
            if trys == 3:
                    print("you have been locked out for using to many failed trys")
            if hashedpassword == correctpassword:
                    print ("Password Correct ")
                    #done give access
                    trys = 0
                    break

(I'm new to programming so feel free to correct me)

Answer 2

The reason two hashes of the same password don't match for you is because the result of create_hash() includes the salt, which is randomly generated.

To get the same hash, you have to provide the salt as you validate. The code CrackStation.net provides makes this super simple - just store the result of create_hash() in your database, exactly as-is. To confirm a user entered the correct password, use

validate_password('provided-password','hash-from-database');

This will return a boolean of whether the password was correct. Using this method is better than writing your own, because it is specifically designed to prevent certain hints at what the correct password might be based on the time it takes to return an answer. This is a minor weakness, but better to avoid it anyway, especially since the provided function takes care of it for you.

The provided functions give you a solid implementation, so don't mess with it too much. The hash includes those extra details - sha256:1000 - in order to allow future changes to your hashing without affecting the ability of existing users to login; it will simply use the method provided in the hash, no matter what the current method is.

As far as showing a potential attacker how the hash was created: the format does show everything about how, but this doesn't matter very much at all. An attacker can easily figure out the method used from one or two known password:hash pairs. Far more important than this is using a good algorithm. The goal is to make it difficult to impossible to crack the password even with all the details, as security by obscurity is poor security.

Finally, just to reiterate, you do not need to store the salt in a separate column, just leave the big 70-80 character chunk it returns alone as your hash and let the functions take care of everything for you - anything else is a waste of time and resources

Answer 3

To add a bit more to the excellent accepted answer:

The point about salting the hash is to make it impossible for someone to come along with a huge pile of pre-encrypted hashes and try them one by one. Even with the salt in your database, they'd have to hash every possible dictionary entry against that salt to successfully match a hash. And the salt needs to be different every time to prevent duplicate entries (they hack one password, they have that same password that other users have entered).

As for not being able to compare the login hash to the stored hash, that is on purpose: you use a validation function to check the login password against the stored password, not a string comparison function.

Answer 4

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 ...