TLS cipher suite compile time option for downstream

[tiran]

BPO	31429
Nosy	@tiran, @alex, @dstufft, @stratakis
PRs	bpo-31429: Define TLS cipher suite on build time  #3532

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/tiran'
closed_at = <Date 2018-01-29.13:10:35.723>
created_at = <Date 2017-09-12.13:22:45.087>
labels = ['type-security', 'expert-SSL', '3.7']
title = 'TLS cipher suite compile time option for downstream'
updated_at = <Date 2018-01-29.13:10:35.722>
user = 'https://github.com/tiran'

bugs.python.org fields:

activity = <Date 2018-01-29.13:10:35.722>
actor = 'christian.heimes'
assignee = 'christian.heimes'
closed = True
closed_date = <Date 2018-01-29.13:10:35.723>
closer = 'christian.heimes'
components = ['SSL']
creation = <Date 2017-09-12.13:22:45.087>
creator = 'christian.heimes'
dependencies = []
files = []
hgrepos = []
issue_num = 31429
keywords = ['patch']
message_count = 3.0
messages = ['301957', '310351', '311126']
nosy_count = 5.0
nosy_names = ['janssen', 'christian.heimes', 'alex', 'dstufft', 'cstratak']
pr_nums = ['3532']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'security'
url = 'https://bugs.python.org/issue31429'
versions = ['Python 3.7']

[tiran]

Python overrides OpenSSL's default cipher suites because the default selection used to be pretty bad and insecure. Python disables cipher suites with insecure algorithm such as RC4, MD5, DES, and 3DES. The SSL module has hard-coded cipher strings for SSLContext and ssl.create_default_context() in multiple places:

• https://github.com/python/cpython/blob/v3.6.2/Modules/_ssl.c#L2693
• https://github.com/python/cpython/blob/v3.6.2/Lib/ssl.py#L387
• https://github.com/python/cpython/blob/v3.6.2/Lib/ssl.py#L503

However the cipher suite overrides makes it harder for vendors and downstream to enforce consistent policies. For example the hard-coded strings disable Fedora's crypto policy, https://fedoraproject.org/wiki/Changes/CryptoPolicy . Fedora has patched OpenSSL to support a "PROFILE=SYSTEM" cipher suite string. The string causes OpenSSL to read crypto settings from a system wide configuration file.

In order to make it easier to override the default string, Python should have a configure option --with-ssl-default-suite that defines a PY_SSL_DEFAULT_SUITE macro. In the absence of the option / macro, Python shall set a sensible default suite. Application are encouraged to use this default suite list. They are still free to override the default by calling SSLContext's set_ciphers() method.

Fedora's OpenSSL patch: https://src.fedoraproject.org/cgit/rpms/openssl.git/tree/openssl-1.1.0-system-cipherlist.patch?h=f26

[tiran]

My PR is ready for review.

[tiran]

New changeset 892d66e by Christian Heimes in branch 'master':
bpo-31429: Define TLS cipher suite on build time (bpo-3532)
892d66e