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Description:

PasswordValidation 0.1.1

Password Validation
Aim
This library aims to allow the programmer to simply validate passwords according to their desired policy.
Flow
How does it work:

You create password policy
You test passwords against that policy
The passwords that abide by the policy are valid, those that don't abide are invalid

Simple.
Password Guidelines
I recommend reading the 2017 NIST guidelines:

Verifiers should not impose composition rules e.g., requiring mixtures of different character types or prohibiting consecutively repeated characters
Verifiers should not require passwords to be changed arbitrarily or regularly e.g. the previous 90 day rule
Passwords must be at least 8 characters in length
Password systems should permit subscriber-chosen passwords at least 64 characters in length.
All printing ASCII characters, the space character, and Unicode characters should be acceptable in passwords
When establishing or changing passwords, the verifier shall advise the subscriber that they need to select a different password if they have chosen a weak or compromised password
Verifiers should offer guidance such as a password-strength meter, to assist the user in choosing a strong password
Verifiers shall store passwords in a form that is resistant to offline attacks. Passwords shall be salted and hashed using a suitable one-way key derivation function. Key derivation functions take a password, a salt, and a cost factor as inputs then generate a password hash. Their purpose is to make each password guessing trial by an attacker who has obtained a password hash file expensive and therefore the cost of a guessing attack high or prohibitive.

Personal headline points:

Don't enforce bizarre convention e.g. 1 lowercase, 1 uppercase, 1 number, 1 symbol, etc
Don't make make users change them regularly
Make it at least 12 characters long
AND SALT AND HASH THEM WHEN PERSISTING

The XKCD comic puts it best: https://xkcd.com/936/
How to use
Install
Install it:
pip install password_validation

How
Example:
>>> from password_validation import PasswordPolicy
>>> policy = PasswordPolicy()
>>> policy.validate("hello-this-is-quite-a-good-password")
True

>>> policy.validate("password")
False

>>> test = policy.test_password("hello")
>>> test
[<RequirementUnfulfilled('the minimum password length', statement=(5 >= 12))>,
<RequirementUnfulfilled('entropy', statement=(23.50219859070546 >= 32))>]

In the above example "password" is not valid, because it's not more than 12 characters (which is a default requirement). And "hello" isn't valid either because it's too short and too low in entropy.
You can, when using test_password get back a list of unfulfilled requirements.
Or you can see all the requirements:
>>> from password_validation import PasswordPolicy
>>> policy = PasswordPolicy()
>>> policy.test_password("goodbye", failure_only=False)
[<RequirementFulfilled('the minimum number of lowercase characters', statement=(7 >= 0))>,
<RequirementFulfilled('the minimum number of uppercase characters', statement=(0 >= 0))>,
<RequirementFulfilled('the minimum number of number characters', statement=(0 >= 0))>,
<RequirementFulfilled('the minimum number of symbol characters', statement=(0 >= 0))>,
<RequirementFulfilled('the minimum number of whitespace characters', statement=(0 >= 0))>,
<RequirementFulfilled('the minimum number of other characters', statement=(0 >= 0))>,
<RequirementUnfulfilled('the minimum password length', statement=(7 >= 12))>,
<RequirementFulfilled('the maximum password length', statement=(7 <= 128))>,
<RequirementFulfilled('entropy', statement=(32.90307802698764 >= 32))>,
<RequirementFulfilled('forbidden words', statement=("goodbye" not in []))>]

The __init__ looks something like this:
class PasswordPolicy:
def __init__(
self,
lowercase: int = 0,
uppercase: int = 0,
symbols: int = 0,
numbers: int = 0,
whitespace: int = 0,
other: int = 0,
min_length: int = 12,
max_length: int = 128,
entropy: typing.Union[int, float] = 32,
forbidden_words: list = None
)

Features in your policy can include:

number of lowercase characters (default 0) PasswordPolicy(lowercase=1)
number of uppercase characters (default 0) PasswordPolicy(uppercase=1)
number of symbols characters (default 0) PasswordPolicy(symbols=1)
number of number characters (default 0) PasswordPolicy(numbers=1)
number of whitespace characters (default 0) PasswordPolicy(whitespace=1)
number of other characters (default 0) PasswordPolicy(other=1)
minimum password length (default 12) PasswordPolicy(min_length=1)
maximum password length (default 128)PasswordPolicy(max_length=1)
minimum password entropy (default 32) PasswordPolicy(entropy=1)
a list of forbidden words PasswordPolicy(forbidden_words=['password'])

FYI other characters is if you wanted to add non-ascii characters
Flask example
from password_validation import PasswordPolicy

policy = PasswordPolicy()

@app.route("/register")
def register():
password = request.form.get("password")
if policy.validate(password):
# create user
else:
for requirement in policy.test_password(password):
alert = f"{requirement.name} not satisfied: expected: {requirement.requirement}, got: {requirement.actual}"
flash(alert)
return render_template("register.html")

You can also get your fields using policy.to_dict()
Character Pool
If you don't like the default characters (ascii) you can make your own CharacterPool:
hex_pool = CharacterPool(
lowercase="",
uppercase="ABCDEF",
numbers="0123456789",
symbols="",
whitespace="",
other="",
)

or
random_pool = CharacterPool(
lowercase="åéîøü",
uppercase="XYZ",
numbers="123",
symbols=".",
whitespace="",
other="",
)

You can then pass to the policy:
policy = PasswordPolicy(character_pool=hex_pool)

and the same logic will apply but for that pool