Hashing

A one-way function that takes a string input and outputs a string of X length.

Comic about Hashing

(Source: https://xkcd.com/936/)

Problem: sha256 can calculate billions of hashes quickly. Solution: use a slow hash like BCrypt.
Problem: People reuse or share common passwords. Solution: use salting to add another string.

Fast Hashes

Takes a short time to hash, is portable, doesn't lead to collisions. Commonly used for data integrity. However:

Rainbow Tables exist: precomputed and compressed tables from hash -> passwords
Seclists exist: lists of commonly used keys, e.g. passwords, usernames, etc.

Password Seclist

With this information, we can create lists of all commonly used passwords and their variants within seconds.

Review Question: What is a hash collision? (Hint: the answer is the same as for hash tables.)

A collision is when two inputs lead to the same generated hash. In hash tables, we typically handle this by generating a linked list for a given hash table cell.

Slow Hashes

Takes longer to hash, and significantly longer to test all combinations.

Why are we saying "test" instead of "break"?

Hashes are not "broken," as they are designed to be one-way functions that cannot be reversed. Instead, we must brute-force all possibilities.

BCrypt

BCrypt is a slow hash. Process:

Ask for a cost parameter
Ask for a password
Generate a random salt
Generate key (2^cost key expansion rounds)
Prepend version number, cost, salt, and hashed password

BCrypt Disclaimer

BCrypt only handles up to 72 bytes, limiting password length. We can use hash and compress the password to handle this prior. For example: convert password to SHA256 -> convert to base64 -> pass to BCrypt.

Example Output

$2a$10$N9qo8uLOickgx2ZMRZoMyeIjZAgcfl7p92ldGxad68LJZdL17lhWy

2a is the version number
10 is the cost
N9qo8uLOickgx2ZMRZoMye is the salt
The rest of the string is the hashed password

BCrypt in Flask

Install with pip3 install flask-bcrypt and import BCrypt class:

from flask import Flask
from flask_bcrypt import BCrypt

app = Flask(__name__)
bcrypt = BCrypt(app)

Generating Hashes

Create a hash with generate_password_hash() and decode():

hash = bcrypt.generate_password_hash("a_password")
# This creates a byte string with version 2b and cost 12:
# b'$2b$12$nz0fVuySYJBzJ.sXocYsuuHfUw5weyLCzOEJHjjLwvf1u/hChnam2'

hash.decode()
# This converts the byte string into a normal string:
# '$2b$12$nz0fVuySYJBzJ.sXocYsuuHfUw5weyLCzOEJHjjLwvf1u/hChnam2'

We can rerun our code a few times to see that BCrypt hashes the password differntly on each call:

>>> bcrypt.generate_password_hash('a_password').decode()
'$2b$12$nz0fVuySYJBzJ.sXocYsuuHfUw5weyLCzOEJHjjLwvf1u/hChnam2'
>>> bcrypt.generate_password_hash('a_password').decode()
'$2b$12$TPu20eIGwAl050BcZfd60uFDNpiiob3CYr9lKSuAPCJ0k/MuXML1e'
>>> bcrypt.generate_password_hash('a_password').decode()
'$2b$12$Q0c.ZCab7QqPjQ3znxXqK.1xtuUYUhhMhy0GYj8I1r14823D/ZupK'
>>> bcrypt.generate_password_hash('a_password').decode()
'$2b$12$HfAZT2hsoY4frIl2R6qDn.HAs6A2VUyplkQs6O9D3ont2t9dS9bEe'
>>> bcrypt.generate_password_hash('a_password').decode()
'$2b$12$eIQ8MELmSkZW5kmF1MG/5uz8ufit6KrFXwRGStIM1FkUUxVbFIbMO'
>>> bcrypt.generate_password_hash('a_password').decode()
'$2b$12$vGWOxXd/Ee00ubeglwxy7eSY7ryMyGm/1k9MDejvixYkixcChLqe2'
>>> bcrypt.generate_password_hash('a_password').decode()
'$2b$12$9AnFKXs66A86ZQUknV4/m.w7zCZB3rLpfZgT7vJnLAQ7D/sl12wHa'

Checking Hashes

Compare a generated hash to an input password:

>>> pass_hash = bcrypt.generate_password_hash('a_password')
>>> pass_hash
b'$2b$12$exCUucO2pcFhvahPR/mDNeM7CJhuoj7cMJ4s1CxHZdzHApsQqbwYq'
>>> bcrypt.check_password_hash(pass_hash, 'a_password')
True
>>> bcrypt.check_password_hash(hash, 'another_password')
False

Hashing

Password Seclist

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