[Python-ideas] Pre-PEP Adding A Secrets Module To The Standard Library
Steven D'Aprano
steve at pearwood.info
Sat Sep 19 20:16:12 CEST 2015
Following on to the discussions about changing the default random number
generator, I would like to propose an alternative: adding a secrets
module to the standard library.
Attached is a draft PEP. Feedback is requested.
(I'm going to only be intermittently at the keyboard for the next day or
so, so my responses may be rather slow.)
--
Steve
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PEP: xxx
Title: Adding A Secrets Module To The Standard Library
Version: $Revision$
Last-Modified: $Date$
Author: Steven D'Aprano <steve at pearwood.info>
Status: Draft
Type: Standards Track
Content-Type: text/plain
Created: 19-Sep-2015
Python-Version: 3.6
Post-History:
Abstract
This PEP proposes the addition of a module for common security-related
functions such as generating tokens to the Python standard library.
Definitions
Some common abbreviations used in this proposal:
PRNG:
Pseudo Random Number Generator. A deterministic algorithm used
to produce random-looking numbers with certain desirable
statistical properties.
CSPRNG:
Cryptographically Strong Pseudo Random Number Generator. An
algorithm used to produce random-looking numbers which are
resistant to prediction.
MT:
Mersenne Twister. An extensively studied PRNG which is currently
used by the ``random`` module as the default.
Rationale
This proposal is motivated by concerns that Python's standard library
makes it too easy for developers to inadvertently make serious security
errors. Theo de Raadt, the founder of OpenBSD, contacted Guido van Rossum
and expressed some concern[1] about the use of MT for generating sensitive
information such as passwords, secure tokens, session keys and similar.
Although the documentation for the random module explicitly states that
the default is not suitable for security purposes[2], it is strongly
believed that this warning may be missed, ignored or misunderstood by
many Python developers. In particular:
- developers may not have read the documentation and consequently
not seen the warning;
- they may not realise that their specific use of it has security
implications; or
- not realising that there could be a problem, they have copied code
(or learned techniques) from websites which don't offer best
practises.
The first[3] hit when searching for "python how to generate passwords" on
Google is a tutorial that uses the default functions from the ``random``
module[4]. Although it is not intended for use in web applications, it is
likely that similar techniques find themselves used in that situation.
The second hit is to a StackOverflow question about generating
passwords[5]. Most of the answers given, including the accepted one, use
the default functions. When one user warned that the default could be
easily compromised, they were told "I think you worry too much."[6]
This strongly suggests that the existing ``random`` module is an attractive
nuisance when it comes to generating (for example) passwords or secure
tokens.
Additional motivation (of a more philosophical bent) can be found in the
post which first proposed this idea[7].
Proposal
Alternative proposals have focused on the default PRNG in the ``random``
module, with the aim of providing "secure by default" cryptographically
strong primitives that developers can build upon without thinking about
security. (See Alternatives below.) This proposes a different approach:
* The standard library already provides cryptographically strong
primitives, but many users don't know they exist or when to use them.
* Instead of requiring crypto-naive users to write secure code, the
standard library should include a set of ready-to-use "batteries" for
the most common needs, such as generating secure tokens. This code
will both directly satisfy a need ("How do I generate a password reset
token?"), and act as an example of acceptable practises which
developers can learn from[8].
To do this, this PEP proposes that we add a new module to the standard
library, with the suggested name ``secrets``. This module will contain a
set of ready-to-use functions for common activities with security
implications, together with some lower-level primitives.
The suggestion is that ``secrets`` becomes the go-to module for dealing
with anything which should remain secret (passwords, tokens, etc.)
while the ``random`` module remains backward-compatible.
API and Implementation
The contents of the ``secrets`` module is expected to evolve over time, and
likely will evolve between the time of writing this PEP and actual release
in the standard library[9]. At the time of writing, the following functions
have been suggested:
* A high-level function for generating secure tokens suitable for use
in (e.g.) password recovery, as session keys, etc.
* A limited interface to the system CSPRNG, using either ``os.urandom``
directly or ``random.SystemRandom``. Unlike the ``random`` module, this
does not need to provide methods for seeding, getting or setting the
state, or any non-uniform distributions. It should provide the
following:
- A function for choosing items from a sequence, ``secrets.choice``.
- A function for generating an integer within some range, such as
``secrets.randrange`` or ``secrets.randint``.
- A function for generating a given number of random bits and/or bytes
as an integer.
- A similar function which returns the value as a hex digit string.
* ``hmac.compare_digest`` under the name ``equal``.
The consensus appears to be that there is no need to add a new CSPRNG to
the ``random`` module to support these uses, ``SystemRandom`` will be
sufficient.
Some illustrative implementations have been given by Nick Coghlan[10].
This idea has also been discussed on the issue tracker for the
"cryptography" module[11].
The ``secrets`` module itself will be pure Python, and other Python
implementations can easily make use of it unchanged, or adapt it as
necessary.
Alternatives
One alternative is to change the default PRNG provided by the ``random``
module[12]. This received considerable scepticism and outright opposition:
* There is fear that a CSPRNG may be slower than the current PRNG (which
in the case of MT is already quite slow).
* Some applications (such as scientific simulations, and replaying
gameplay) require the ability to seed the PRNG into a known state,
which a CSPRNG lacks by design.
* Another major use of the ``random`` module is for simple "guess a number"
games written by beginners, and many people are loath to make any
change to the ``random`` module which may make that harder.
* Although there is no proposal to remove MT from the ``random`` module,
there was considerable hostility to the idea of having to opt-in to
a non-CSPRNG or any backwards-incompatible changes.
* Demonstrated attacks against MT are typically against PHP applications.
It is believed that PHP's version of MT is a significantly softer target
than Python's version, due to a poor seeding technique[13]. Consequently,
without a proven attack against Python applications, many people object
to a backwards-incompatible change.
Nick Coghlan made an earlier suggestion for a globally configurable PRNG
which uses the system CSPRNG by default[14], but has since hinted that he
may withdraw it in favour of this proposal[15].
Comparison To Other Languages
PHP
PHP includes a function ``uniqid``[16] which by default returns a
thirteen character string based on the current time in microseconds.
Translated into Python syntax, it has the following signature:
def uniqid(prefix='', more_entropy=False)->str
The PHP documentation warns that this function is not suitable for
security purposes. Nevertheless, various mature, well-known PHP
applications use it for that purpose (citation needed).
PHP 5.3 and better also includes a function ``openssl_random_pseudo_bytes``[17].
Translated into Python syntax, it has roughly the following signature:
def openssl_random_pseudo_bytes(length:int)->Tuple[str, bool]
This function returns a pseudo-random string of bytes of the given
length, and an boolean flag giving whether the string is considered
cryptographically strong. The PHP manual suggests that returning
anything but True should be rare except for old or broken platforms.
Javascript
Based on a rather cursory search[18], there doesn't appear to be any
well-known standard functions for producing strong random values in
Javascript, although there may be good quality third-party libraries.
Standard Javascript doesn't seem to include an interface to the
system CSPRNG either, and people have extensively written about the
weaknesses of Javascript's Math.random[19].
Ruby
The Ruby standard library includes a module ``SecureRandom``[20]
which includes the following methods:
* base64 - returns a Base64 encoded random string.
* hex - returns a random hexadecimal string.
* random_bytes - returns a random byte string.
* random_number - depending on the argument, returns either a random
integer in the range(0, n), or a random float between 0.0 and 1.0.
* urlsafe_base64 - returns a random URL-safe Base64 encoded string.
* uuid - return a version 4 random Universally Unique IDentifier.
What Should Be The Name Of The Module?
There was a proposal to add a "random.safe" submodule, quoting the Zen
of Python "Namespaces are one honking great idea" koan. However, the
author of the Zen, Tim Peters, has come out against this idea[21], and
recommends a top-level module.
In discussion on the python-ideas mailing list so far, the name "secrets"
has received some approval, and no strong opposition.
Frequently Asked Questions
Q: Is this a real problem? Surely MT is random enough that nobody can
predict its output.
A: The consensus among security professionals is that MT is not safe
in security contexts. It is not difficult to reconstruct the internal
state of MT[22][23] and so predict all past and future values. There
are a number of known, practical attacks on systems using MT for
randomness[24].
While there are currently no known direct attacks on applications
written in Python due to the use of MT, there is widespread agreement
that such usage is unsafe.
Q: Is this an alternative to specialise cryptographic software such as SSL?
A: No. This is a "batteries included" solution, not a full-featured
"nuclear reactor". It is intended to mitigate against some basic
security errors, not be a solution to all security-related issues. To
quote Nick Coghlan referring to his earlier proposal:
"...folks really are better off learning to use things like
cryptography.io for security sensitive software, so this change
is just about harm mitigation given that it's inevitable that a
non-trivial proportion of the millions of current and future
Python developers won't do that."[25]
References
[1] https://mail.python.org/pipermail/python-ideas/2015-September/035820.html
[2] https://docs.python.org/3/library/random.html
[3] As of the date of writing. Also, as Google search terms may be automatically customised for the user without their knowledge, some readers may see different results.
[4] http://interactivepython.org/runestone/static/everyday/2013/01/3_password.html
[5] http://stackoverflow.com/questions/3854692/generate-password-in-python
[6] http://stackoverflow.com/questions/3854692/generate-password-in-python/3854766#3854766
[7] https://mail.python.org/pipermail/python-ideas/2015-September/036238.html
[8] At least those who are motivated to read the source code and documentation.
[9] Tim Peters suggests that bike-shedding the contents of the module will be 10000 times more time consuming than actually implementing the module. Words do not begin to express how much I am looking forward to this.
[10] https://mail.python.org/pipermail/python-ideas/2015-September/036271.html
[11] https://github.com/pyca/cryptography/issues/2347
[12] Link needed.
[13] By default PHP seeds the MT PRNG with the time (citation needed), which is exploitable by attackers, while Python seeds the PRNG with output from the system CSPRNG, which is believed to be much harder to exploit.
[14] http://legacy.python.org/dev/peps/pep-0504/
[15] https://mail.python.org/pipermail/python-ideas/2015-September/036243.html
[16] http://php.net/manual/en/function.uniqid.php
[17] http://php.net/manual/en/function.openssl-random-pseudo-bytes.php
[18] Volunteers and patches are welcome.
[19] http://ifsec.blogspot.fr/2012/05/cross-domain-mathrandom-prediction.html
[20] http://ruby-doc.org/stdlib-2.1.2/libdoc/securerandom/rdoc/SecureRandom.html
[21] https://mail.python.org/pipermail/python-ideas/2015-September/036254.html
[22] https://jazzy.id.au/2010/09/22/cracking_random_number_generators_part_3.html
[23] https://mail.python.org/pipermail/python-ideas/2015-September/036077.html
[24] https://media.blackhat.com/bh-us-12/Briefings/Argyros/BH_US_12_Argyros_PRNG_WP.pdf
[25] https://mail.python.org/pipermail/python-ideas/2015-September/036157.html
Copyright
This document has been placed in the public domain.
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