[Python-checkins] peps: Create PEP 506 by RSTifying Steven's content
chris.angelico
python-checkins at python.org
Sun Sep 20 00:59:00 CEST 2015
https://hg.python.org/peps/rev/a48965a64fd2
changeset: 6072:a48965a64fd2
user: Chris Angelico <rosuav at gmail.com>
date: Sun Sep 20 08:58:49 2015 +1000
summary:
Create PEP 506 by RSTifying Steven's content
files:
pep-0506.txt | 347 +++++++++++++++++++++++++++++++++++++++
1 files changed, 347 insertions(+), 0 deletions(-)
diff --git a/pep-0506.txt b/pep-0506.txt
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+PEP: 506
+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/x-rst
+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.
+
+
+
+..
+ Local Variables:
+ mode: indented-text
+ indent-tabs-mode: nil
+ sentence-end-double-space: t
+ fill-column: 70
+ coding: utf-8
+ End:
--
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