argon2_cffi

Release v18.2.0 (What’s new?).

Argon2 won the Password Hashing Competition and argon2_cffi is the simplest way to use it in Python and PyPy:

>>> from argon2 import PasswordHasher
>>> ph = PasswordHasher()
>>> hash = ph.hash("s3kr3tp4ssw0rd")
>>> hash  
'$argon2id$v=19$m=102400,t=2,p=8$tSm+JOWigOgPZx/g44K5fQ$WDyus6py50bVFIPkjA28lQ'
>>> ph.verify(hash, "s3kr3tp4ssw0rd")
True
>>> ph.check_needs_rehash(hash)
False
>>> ph.verify(hash, "t0t411ywr0ng")
Traceback (most recent call last):
  ...
argon2.exceptions.VerifyMismatchError: The password does not match the supplied hash

Note

passlib 1.7.0 and later offers Argon2 support using this library too.

argon2_cffi’s documentation lives at Read the Docs, the code on GitHub. It’s rigorously tested on Python 2.7, 3.4+, and PyPy.

User’s Guide

Argon2

Note

TL;DR: Use argon2.PasswordHasher with its default parameters to securely hash your passwords.

You do not need to read or understand anything below this box.

Argon2 is a secure password hashing algorithm. It is designed to have both a configurable runtime as well as memory consumption.

This means that you can decide how long it takes to hash a password and how much memory is required.

Argon2 comes in three variants:

Argon2d

is faster and uses data-depending memory access, which makes it less suitable for hashing secrets and more suitable for cryptocurrencies and applications with no threats from side-channel timing attacks.

Argon2i

uses data-independent memory access, which is preferred for password hashing and password-based key derivation. Argon2i is slower as it makes more passes over the memory to protect from tradeoff attacks.

Argon2id

is a hybrid of Argon2i and Argon2d, using a combination of data-depending and data-independent memory accesses, which gives some of Argon2i’s resistance to side-channel cache timing attacks and much of Argon2d’s resistance to GPU cracking attacks.

Why “just use bcrypt” Is Not the Best Answer (Anymore)

The current workhorses of password hashing are unquestionably bcrypt and PBKDF2. And while they’re still fine to use, the password cracking community embraced new technologies like GPUs and ASICs to crack password in a highly parallel fashion.

An effective measure against extreme parallelism proved making computation of password hashes also memory hard. The best known implementation of that approach is to date scrypt. However according to the Argon2 paper, page 2:

[…] the existence of a trivial time-memory tradeoff allows compact implementations with the same energy cost.

Therefore a new algorithm was needed. This time future-proof and with committee-vetting instead of single implementors.

Password Hashing Competition

The Password Hashing Competition took place between 2012 and 2015 to find a new, secure, and future-proof password hashing algorithm. Previously the NIST was in charge but after certain events and revelations their integrity has been put into question by the general public. So a group of independent cryptographers and security researchers came together.

In the end, Argon2 was announced as the winner.

Installation

Using the Vendored Argon2

python -m pip install argon2_cffi

should be all it takes.

But since argon2_cffi vendors Argon2’s C code by default, it can lead to complications depending on the platform.

The C code is known to compile and work on all common platforms (including x86, ARM, and PPC). On x86, an SSE2-optimized version is used.

If something goes wrong, please try to update your cffi, pip and setuptools first:

python -m pip install -U cffi pip setuptools

Overall this should be the safest bet because argon2_cffi has been specifically tested against the vendored version.

Wheels

Binary wheels for macOS, Windows, and Linux are provided on PyPI. With a recent-enough pip and setuptools, they should be used automatically.

Source Distribution

A working C compiler and CFFI environment are required. If you’ve been able to compile Python CFFI extensions before, argon2_cffi should install without any problems.

Using a System-wide Installation of Argon2

If you set ARGON2_CFFI_USE_SYSTEM to 1 (and only 1), argon2_cffi will not build its bindings. However binary wheels are preferred by pip and Argon2 gets installed along with argon2_cffi anyway.

Therefore you also have to instruct pip to use a source distribution:

env ARGON2_CFFI_USE_SYSTEM=1 \
  python -m pip install --no-binary=argon2_cffi argon2_cffi

This approach can lead to problems around your build chain and you can run into incompatabilities between Argon2 and argon2_cffi if the latter has been tested against a different version.

It is your own responsibility to deal with these risks if you choose this path.

API Reference

argon2_cffi comes with an high-level API and hopefully reasonable defaults for Argon2 parameters that result in a verification time of 40–50ms on recent-ish hardware.

Unless you have any special needs, all you need to know is:

>>> from argon2 import PasswordHasher
>>> ph = PasswordHasher()
>>> hash = ph.hash("s3kr3tp4ssw0rd")
>>> hash  
'$argon2id$v=19$m=102400,t=2,p=8$tSm+JOWigOgPZx/g44K5fQ$WDyus6py50bVFIPkjA28lQ'
>>> ph.verify(hash, "s3kr3tp4ssw0rd")
True
>>> ph.check_needs_rehash(hash)
False
>>> ph.verify(hash, "t0t411ywr0ng")
Traceback (most recent call last):
  ...
argon2.exceptions.VerifyMismatchError: The password does not match the supplied hash

A login function could thus look like this:

import argon2


ph = argon2.PasswordHasher()


def login(db, user, password):
    hash = db.get_password_hash_for_user(user)

    # Verify password, raises exception if wrong.
    ph.verify(hash, password)

    # Now that we have the cleartext password,
    # check the hash's parameters and if outdated,
    # rehash the user's password in the database.
    if ph.check_needs_rehash(hash):
        db.set_password_hash_for_user(user, ph.hash(password))

While the PasswordHasher class has the aspiration to be good to use out of the box, it has all the parametrization you’ll need:

If you don’t specify any parameters, the following constants are used:

argon2.DEFAULT_RANDOM_SALT_LENGTH
argon2.DEFAULT_HASH_LENGTH
argon2.DEFAULT_TIME_COST
argon2.DEFAULT_MEMORY_COST
argon2.DEFAULT_PARALLELISM

You can see their values in PasswordHasher.

Exceptions

exception argon2.exceptions.VerificationError

Verification failed.

You can find the original error message from Argon2 in args[0].

exception argon2.exceptions.VerifyMismatchError

The secret does not match the hash.

Subclass of argon2.exceptions.VerificationError.

New in version 16.1.0.

exception argon2.exceptions.HashingError

Raised if hashing failed.

You can find the original error message from Argon2 in args[0].

exception argon2.exceptions.InvalidHash

Raised if the hash is invalid before passing it to Argon2.

New in version 18.2.0.

Utilities

Low Level

>>> import argon2
>>> argon2.low_level.hash_secret(
...     b"secret", b"somesalt",
...     time_cost=1, memory_cost=8, parallelism=1, hash_len=64, type=argon2.low_level.Type.D
... )
b'$argon2d$v=19$m=8,t=1,p=1$c29tZXNhbHQ$ba2qC75j0+JAunZZ/L0hZdQgCv+tOieBuKKXSrQiWm7nlkRcK+YqWr0i0m0WABJKelU8qHJp0SZzH0b1Z+ITvQ'

The raw hash can also be computed:

>>> argon2.low_level.hash_secret_raw(
...     b"secret", b"somesalt",
...     time_cost=1, memory_cost=8, parallelism=1, hash_len=8, type=argon2.low_level.Type.D
... )
b'\xe4n\xf5\xc8|\xa3>\x1d'

The super low-level argon2_core() function is exposed too if you need access to very specific options:

In order to use core(), you need access to argon2_cffi’s FFI objects. Therefore it is OK to use argon2.low_level.ffi and argon2.low_level.lib when working with it:

>>> from argon2.low_level import ARGON2_VERSION, Type, core, ffi, lib
>>> pwd = b"secret"
>>> salt = b"12345678"
>>> hash_len = 8
>>> # Make sure you keep FFI objects alive until *after* the core call!
>>> cout = ffi.new("uint8_t[]", hash_len)
>>> cpwd = ffi.new("uint8_t[]", pwd)
>>> csalt = ffi.new("uint8_t[]", salt)
>>> ctx = ffi.new(
...     "argon2_context *", dict(
...         version=ARGON2_VERSION,
...         out=cout, outlen=hash_len,
...         pwd=cpwd, pwdlen=len(pwd),
...         salt=csalt, saltlen=len(salt),
...         secret=ffi.NULL, secretlen=0,
...         ad=ffi.NULL, adlen=0,
...         t_cost=1,
...         m_cost=8,
...         lanes=1, threads=1,
...         allocate_cbk=ffi.NULL, free_cbk=ffi.NULL,
...         flags=lib.ARGON2_DEFAULT_FLAGS,
...     )
... )
>>> ctx
<cdata 'struct Argon2_Context *' owning 120 bytes>
>>> core(ctx, Type.D.value)
0
>>> out = bytes(ffi.buffer(ctx.out, ctx.outlen))
>>> out
b'\xb4\xe2HjO\x14d\x9b'
>>> out == argon2.low_level.hash_secret_raw(pwd, salt, 1, 8, 1, 8, Type.D)
True

All constants and types on argon2.low_level.lib are guaranteed to stay as long they are not altered by Argon2 itself.

Deprecated APIs

These APIs are from the first release of argon2_cffi and proved to live in an unfortunate mid-level. On one hand they have defaults and check parameters but on the other hand they only consume byte strings.

Therefore the decision has been made to replace them by a high-level (argon2.PasswordHasher) and a low-level (argon2.low_level) solution. There are no immediate plans to remove them though.

Choosing Parameters

Note

You can probably just use argon2.PasswordHasher with its default values and be fine. But it’s good to double check using argon2_cffi’s CLI client, whether its defaults are too slow or too fast for your use case.

Finding the right parameters for a password hashing algorithm is a daunting task. The authors of Argon2 specified a method in their paper, however some parts of it have been revised in the RFC draft for Argon2 that is currently being written.

The current recommended best practice is as follow:

  1. Choose whether you want Argon2i, Argon2d, or Argon2id (type). If you don’t know what that means, choose Argon2id (argon2.Type.ID).

  2. Figure out how many threads can be used on each call to Argon2 (parallelism, called “lanes” in the RFC). They recommend twice as many as the number of cores dedicated to hashing passwords. PasswordHasher will not determine this for you and use a default value that you can find in the linked API docs.

  3. Figure out how much memory each call can afford (memory_cost). The RFC recommends 4 GB for backend authentication and 1 GB for frontend authentication. The APIs use Kibibytes (1024 bytes) as base unit.

  4. Select the salt length. 16 bytes is sufficient for all applications, but can be reduced to 8 bytes in the case of space constraints.

  5. Choose a hash length (hash_len, called “tag length” in the documentation). 16 bytes is sufficient for password verification.

  6. Figure out how long each call can take. One recommendation for concurent user logins is to keep it under 0.5 ms. The RFC recommends under 500 ms. The truth is somewhere between those two values: more is more secure, less is a better user experience. argon2_cffi’s defaults try to land somewhere in the middle and aim for ~50ms, but the actual time depends on your hardware.

    Please note though, that even a verification time of 1 second won’t protect you against bad passwords from the “top 10,000 passwords” lists that you can find online.

  7. Measure the time for hashing using your chosen parameters. Find a time_cost that is within your accounted time. If time_cost=1 takes too long, lower memory_cost.

argon2_cffi’s CLI will help you with this process.

CLI

To aid you with finding the parameters, argon2_cffi offers a CLI interface that can be accessed using python -m argon2. It will benchmark Argon2’s password verification in the current environment. You can use command line arguments to set hashing parameters:

$ python -m argon2
Running Argon2id 100 times with:
hash_len: 16 bytes
memory_cost: 102400 KiB
parallelism: 8 threads
time_cost: 2 iterations

Measuring...

45.3ms per password verification

This should make it much easier to determine the right parameters for your use case and your environment.

Frequently Asked Questions

I’m using bcrypt/PBKDF2/scrypt/yescrypt, do I need to migrate?

Using password hashes that aren’t memory hard carries a certain risk but there’s no immediate danger or need for action. If however you are deciding how to hash password today, Argon2 is the superior, future-proof choice.

But if you already use one of the hashes mentioned in the question, you should be fine for the foreseeable future. If you’re using scrypt or yescrypt, you will be probably fine for good.

Why do the verify() methods raise an Exception instead of returning False?

  1. The Argon2 library had no concept of a “wrong password” error in the beginning. Therefore when writing these bindings, an exception with the full error had to be raised so you could inspect what went actually wrong.

    It goes without saying that it’s impossible to switch now for backward-compatibility reasons.

  2. In my opinion, a wrong password should raise an exception such that it can’t pass unnoticed by accident. See also The Zen of Python: “Errors should never pass silently.”

  3. It’s more Pythonic.

Project Information

Backward Compatibility

argon2_cffi has a very strong backward compatibility policy. Generally speaking, you shouldn’t ever be afraid of updating.

If breaking changes are needed do be done, they are:

  1. …announced in the changelog.

  2. …the old behavior raises a DeprecationWarning for a year.

  3. …are done with another announcement in the changelog.

What explicitly may change over time are the default hashing parameters and the behavior of the CLI.

How To Contribute

Every open source project lives from the generous help by contributors that sacrifice their time and argon2_cffi is no different.

Here are a few guidelines to get you started:

  • If you want to install a development version of argon2_cffi into your current virtualenv, you have to remember to:

    1. git submodule init (to initialize git submodule mechanics)

    2. git submodule update (to update the vendored Argon2 C library to the version argon2_cffi is currently packaging)

    3. python setup.py build (to build the CFFI module)

    4. pip install -e .[dev] (to [re-]install it along with the Python code and test dependencies)

    You have to perform steps 2, 3, and 4 whenever something changes in the Argon2 C code (e.g. if the vendored code has been updated).

  • Try to limit each pull request to one change only.

  • To run the test suite, all you need is a recent tox. It will ensure the test suite runs with all dependencies against all Python versions just as it will on Travis CI. If you lack some Python versions, you can can make it a non-failure using tox --skip-missing-interpreters (in that case you may want to look into pyenv that makes it very easy to install many different Python versions in parallel).

    One of the environments requires a system-wide installation of Argon2. On macOS, it’s available in Homebrew and recent Ubuntus (zesty and later) ship it too.

  • Make sure your changes pass our CI. You won’t get any feedback until it’s green unless you ask for it.

  • Once you’ve addressed review feedback, make sure to bump the pull request with a short note, so we know you’re done.

  • If your change is noteworthy, add an entry to the changelog. Use semantic newlines and add a link to your pull request.

  • No contribution is too small; please submit as many fixes for typos and grammar bloopers as you can!

  • Don’t break backward compatibility.

  • Always add tests and docs for your code. This is a hard rule; patches with missing tests or documentation won’t be merged.

  • Write good test docstrings.

  • Obey PEP 8 and PEP 257.

Please note that this project is released with a Contributor Code of Conduct. By participating in this project you agree to abide by its terms. Please report any harm to Hynek Schlawack in any way you find appropriate.

Thank you for considering to contribute!

Changelog

Versions are year-based with a strict backward compatibility policy. The third digit is only for regressions.

18.2.0 (2018-08-19)

Vendoring Argon2 @ 670229c (20171227)

Changes:

  • The hash type for argon2.PasswordHasher is Argon2id now.

    This decision has been made based on the recommendations in the latest Argon2 RFC draft. #33 #34

  • To make the change of hash type backward compatible, argon2.PasswordHasher.verify() now determines the type of the hash and verifies it accordingly.

  • Some of the hash parameters have been made stricter to be closer to said recommendations. The current goal for a hash verification times is around 50ms. #41

  • To allow for bespoke decisions about upgrading Argon2 parameters, it’s now possible to extract them from a hash via the argon2.extract_parameters() function. #41

  • Additionally argon2.PasswordHasher now has a check_needs_rehash() method that allows to verify whether a hash has been created with the instance’s parameters or whether it should be rehashed. #41

18.1.0 (2018-01-06)

Vendoring Argon2 @ 670229c (20171227)

Changes:

  • It is now possible to use the argon2_cffi bindings against an Argon2 library that is provided by the system.

16.3.0 (2016-11-10)

Vendoring Argon2 @ 1c4fc41f81f358283755eea88d4ecd05e43b7fd3 (20161029)

Changes:

  • Prevent side-effects like the installation of cffi if setup.py is called with a command that doesn’t require it. #20

  • Fix a bunch of warnings with new cffi versions and Python 3.6. #14 #16

  • Add low-level bindings for Argon2id functions.

16.2.0 (2016-09-10)

Vendoring Argon2 @ 4844d2fee15d44cb19296ddf36029326d17c5aa3

Changes:

  • Fix compilation on debian jessie. #13

16.1.0 (2016-04-19)

Vendoring Argon2 @ 00aaa6604501fade85853a4b2f5695611ff6e7c5.

Backward-incompatible changes:

  • Python 3.3 and 2.6 aren’t supported anymore. They may work by chance but any support to them has been ceased.

    The last Python 2.6 release was on October 29, 2013 and isn’t supported by the CPython core team anymore. Major Python packages like Django and Twisted dropped Python 2.6 a while ago already.

    Python 3.3 never had a significant user base and wasn’t part of any distribution’s LTS release.

Changes:

  • Add VerifyMismatchError that is raised if verification fails only because of a password/hash mismatch. It’s a subclass of VerificationError therefore this change is completely backward compatible.

  • Add support for Argon2 1.3. Old hashes remain functional but opportunistic rehashing is strongly recommended.

16.0.0 (2016-01-02)

Vendoring Argon2 @ 421dafd2a8af5cbb215e16da5953663eb101d139.

Deprecations:

  • hash_password(), hash_password_raw(), and verify_password() should not be used anymore. For hashing passwords, use the new argon2.PasswordHasher. If you want to implement your own higher-level abstractions, use the new low-level APIs hash_secret(), hash_secret_raw(), and verify_secret() from the argon2.low_level module. If you want to go really low-level, core() is for you. The old functions will not raise any warnings though and there are no immediate plans to remove them.

Changes:

  • Add argon2.PasswordHasher. A higher-level class specifically for hashing passwords that also works on Unicode strings.

  • Add argon2.low_level module with low-level API bindings for building own high-level abstractions.

15.0.1 (2015-12-18)

Vendoring Argon2 @ 4fe0d8cda37691228dd5a96a310be57369403a4b.

Changes:

  • Fix long_description on PyPI.

15.0.0 (2015-12-18)

Vendoring Argon2 @ 4fe0d8cda37691228dd5a96a310be57369403a4b.

Changes:

  • verify_password() doesn’t guess the hash type if passed None anymore. Supporting this resulted in measurable overhead (~ 0.6ms vs 0.8ms on my notebook) since it had to happen in Python. That means that naïve usage of the API would give attackers an edge. The new behavior is that it has the same default value as hash_password() such that verify_password(hash_password(b"password"), b"password") still works.

  • Conditionally use the SSE2-optimized version of argon2 on x86 architectures.

  • More packaging fixes. Most notably compilation on Visual Studio 2010 for Python 3.3 and 3.4.

  • Tweaked default parameters to more reasonable values. Verification should take between 0.5ms and 1ms on recent-ish hardware.

15.0.0b5 (2015-12-10)

Vendoring Argon2 @ 4fe0d8cda37691228dd5a96a310be57369403a4b.

Initial work. Previous betas were only for fixing Windows packaging. The authors of Argon2 were kind enough to help me to get it building under Visual Studio 2008 that we’re forced to use for Python 2.7 on Windows.

Credits & License

argon2_cffi is maintained by Hynek Schlawack and released under the MIT license.

The development is kindly supported by Variomedia AG.

A full list of contributors can be found in GitHub’s overview.

Vendored Code

Argon2

The original Argon2 repo can be found at https://github.com/P-H-C/phc-winner-argon2/.

Except for the components listed below, the Argon2 code in this repository is copyright (c) 2015 Daniel Dinu, Dmitry Khovratovich (main authors), Jean-Philippe Aumasson and Samuel Neves, and under CC0 license.

The string encoding routines in src/encoding.c are copyright (c) 2015 Thomas Pornin, and under CC0 license.

The BLAKE2 code in src/blake2/ is copyright (c) Samuel Neves, 2013-2015, and under CC0 license.

The authors of Argon2 also were very helpful to get the library to compile on ancient versions of Visual Studio for ancient versions of Python.

The documentation also quotes frequently from the Argon2 paper to avoid mistakes by rephrasing.

msinttypes

In order to be able to compile on Visual Studio 2008 and Visual Studio 2010 which are required for Python 2.7 and 3.4 respectively, we also ship two C headers with integer types. They are from the msinttypes project (auto-import on GitHub) and licensed under New BSD:

Copyright (c) 2006-2013 Alexander Chemeris

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

  2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

  3. Neither the name of the product nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE AUTHOR ‘’AS IS’’ AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

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