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Tales from the Bcrypt

So, I had to write a little piece of code to authorize users into my web app. I know enough that you really don't want to store passwords in plain text. But simply hashing a password leaves it vulnerable to dictionary lookup attacks. You need to spice up the hash a bit with some salt. You could just generate a GUID for your salt and that would work.

You could also use the random module to generate a random number, or perhaps use a datetime value somehow as a salt. Of course you'd need to pick a hashing algorithm, and one that was not too fast mind (the faster the hashing algorithm is, the more vulnerable it is to brute force attacks), and you'd also need to store the salt on your database, along with the hashed password.

Well, you could do all that.

But why do that when the hard work's been done for you?

That's where the wonderful bcrypt module comes in. Bcrypt is a little different in that it's a hashing algorithm that stores the salt with the hashed value. So you don't need to store the salt separately. That's good. It's also incredibly easy to use in Python too.

You have a little function that hashes up the plain text password. The hashed password can get stored in your database with the user's credentials. When the user logs in you need to retrieve the hashed password. You then need to hash the plain text password the user has just given you, and here's the mind-blowing bit, using the hashed password you've just retrieved. The salt will be extracted from it and used to hash the plaintext password again, to give you a hashed password generated from the given plaintext password the user just gave you.

You can now check the hashed password you've just generated against the one you've fetched from the database. A match? Great! Don't match? Wrong password!

The code will hopefully help explain it:

import bcrypt

# hashes a plaintext password using bcrypt with bcrypt the hash
# contains the salt, so no need to store separately.

def hash_password (password):

    salt = bcrypt.gensalt()
    hash = bcrypt.hashpw(password, salt)

    return hash

def check_password (password, hash):

    hashpw = bcrypt.hashpw(password, hash)

    if hashpw == hash:
        return True
        return False

password = "secret".encode('utf-8')

hash = hash_password (password)
print (hash)

new_pword = "secret!!!".encode('utf-8')

if check_password(new_pword, hash):
    print ("passwords match!")
    print ("passwords DON'T match!")

Pretty simple heh? I love Python!

The only other thing worth pointing out there is that the bcrypt function expects a sequence of bytes, and as Python stores strings internally as Unicode, actually as UCS-2 or UCS-4 characters (probably UCS-4), so you need to encode the Unicode as UTF-8, hence the call to the .encode('utf-8') method.

Actually the default for the encode method is UTF-8 anyway, but it doesn't hurt to be explicit, especially in this area of Python (strings, encodings) where there is ample scope for SNAFUs to happen!