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Password strength and validation

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Password Strength

Password strength and validation.

PasswordPolicy

Perform tests on a password.

Init Policy

PasswordPolicy(*tests)

Init password policy with a list of tests

Alternatively:

PasswordPolicy.from_names(**tests)

Init password policy from a dictionary of test definitions.

A test definition is simply:

{ test-name: argument } or { test-name: [arguments] }

Test name is just a lowercased class name.

Example:

PasswordPolicy.from_names(
    length=8,
    strength=(0.33, 30),
)

Bundled Tests

These objects perform individual tests on a password, and report True of False.

tests.EntropyBits(bits)

Test whether the password has >= bits entropy bits

tests.Length(length)

Tests whether password length >= length

tests.NonLetters(count)

Test whether the password has >= count non-letter characters

tests.NonLettersLc(count)

Test whether the password has >= count non-lowercase characters

tests.Numbers(count)

Test whether the password has >= count numeric characters

tests.Special(count)

Test whether the password has >= count special characters

tests.Strength(strength, weak_bits=30)

Test whether the password has >= strength strength.

A password is evaluated to the strength of 0.333 when it has weak_bits entropy bits, which is considered to be a weak password. Strong passwords start at 0.666.

tests.Uppercase(count)

Test whether the password has >= count uppercase characters

Testing

After the PasswordPolicy is initialized, there are two methods to test:

PasswordPolicy.password

password(password)

Get password stats bound to the tests declared in this policy.

If in addition to tests you need to get statistics (e.g. strength) -- use this object to double calculations.

See PasswordStats for more details.

PasswordPolicy.test

test(password)

Perform tests on a password.

Shortcut for: PasswordPolicy.password(password).test().

Custom Tests

ATest is a base class for password tests.

To create a custom test, just subclass it and implement the following methods:

  • init() that takes configuration arguments
  • test(ps) that tests a password, where ps is a PasswordStats object.

PasswordStats

PasswordStats allows to calculate statistics on a password.

It considers a password as a unicode string, and all statistics are unicode-based.

Constructor:

from password_strength import PasswordStats
PasswordStats(password)

PasswordStats.alphabet

Get alphabet: set of used characters

PasswordStats.alphabet_cardinality

Get alphabet cardinality: alphabet length

PasswordStats.char_categories

Character count per top-level category

The following top-level categories are defined:

  • L: letter
  • M: Mark
  • N: Number
  • P: Punctuation
  • S: Symbol
  • Z: Separator
  • C: Other

PasswordStats.char_categories_detailed

Character count per unicode category, detailed format.

See: http://www.unicode.org/reports/tr44/#GC_Values_Table

PasswordStats.combinations

The number of possible combinations with the current alphabet

PasswordStats.count(*categories)

Count characters of the specified classes only

PasswordStats.count_except(*categories)

Count characters of all classes except the specified ones

PasswordStats.entropy_bits

Get information entropy bits: log2 of the number of possible passwords

https://en.wikipedia.org/wiki/Password_strength

PasswordStats.entropy_density

Get information entropy density factor, ranged {0 .. 1}.

This is ratio of entropy_bits() to max bits a password of this length could have. E.g. if all characters are unique -- then it's 1.0. If half of the characters are reused once -- then it's 0.5.

PasswordStats.length

Get password length

PasswordStats.letters

Count all letters

PasswordStats.letters_lowercase

Count lowercase letters

PasswordStats.letters_uppercase

Count uppercase letters

PasswordStats.numbers

Count numbers

PasswordStats.repeated_patterns_length

Detect and return the length of repeated patterns.

You will probably be comparing it with the length of the password itself and ban if it's longer than 10%

PasswordStats.sequences_length

Detect and return the length of used sequences:

  • Alphabet letters: abcd...
  • Keyboard letters: qwerty, etc
  • Keyboard special characters in the top row: ~!@#$%^&*()_+
  • Numbers: 0123456

PasswordStats.special_characters

Count special characters

Special characters is everything that's not a letter or a number

PasswordStats.strength(weak_bits=30)

Get password strength as a number normalized to range {0 .. 1}.

Normalization is done in the following fashion:

  1. If entropy_bits <= weak_bits -- linear in range{0.0 .. 0.33} (weak)
  2. If entropy_bits <= weak_bits*2 -- almost linear in range{0.33 .. 0.66} (medium)
  3. If entropy_bits > weak_bits*3 -- asymptotic towards 1.0 (strong)

PasswordStats.test(tests)

Test the password against a list of tests

PasswordStats.weakness_factor

Get weakness factor as a float in range {0 .. 1}

This detects the portion of the string that contains:

  • repeated patterns
  • sequences

E.g. a value of 1.0 means the whole string is weak, and 0.5 means half of the string is weak.

Typical usage:

password_strength = (1 - weakness_factor) * strength

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