Use TLSv1.2 for fake servers in tests

[danc86]

Some Linux distros have begun disabling TLSv1.0 and TLSv1.1 by default
for security reasons, for example in Fedora 33 onwards:

https://fedoraproject.org/wiki/Changes/StrongCryptoSettings2

Use TLSv1.2 for the fake TLS servers created in the test suite, to avoid
failures due to OpenSSL disallowing TLSv1.0:

<twisted.python.failure.Failure OpenSSL.SSL.Error: [('SSL routines',
'ssl_choose_client_version', 'unsupported protocol')]>

You can reproduce the failures by setting MinProtocol = TLSv1.2 in the [crypto_policy] section of the system-wide OpenSSL config (/etc/pki/tls/openssl.cnf on Fedora, maybe elsewhere on other systems).

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
• Pull request includes a sign off
• Code style is correct (run the linters)

[clokep]

This seems reasonable to me, I'm not sure if TLS v1.0 was chosen on purpose here or was just a lowest-common denominator sort of option.

[richvdh]

can we not just use the default (PROTOCOL_TLS) ? https://docs.python.org/3/library/ssl.html#ssl.SSLContext

[anoadragon453]

can we not just use the default (PROTOCOL_TLS) ? https://docs.python.org/3/library/ssl.html#ssl.SSLContext

That is from a different library (built in ssl), whereas this is OpenSSL.SSL. PROTOCOL_TLS is incompatible here:

...
  File "/home/user/code/synapse/env/lib/python3.8/site-packages/twisted/protocols/tls.py", line 827, in _createConnection
    connection = connectionCreator.serverConnectionForTLS(tlsProtocol)
  File "/home/user/code/synapse/tests/http/__init__.py", line 149, in serverConnectionForTLS
    ctx = SSL.Context(PROTOCOL_TLS)
  File "/home/user/code/synapse/env/lib/python3.8/site-packages/OpenSSL/SSL.py", line 723, in __init__
    raise ValueError("No such protocol")
builtins.ValueError: No such protocol

Not sure if there's a simple way to translate them either - they seem to just be enums:

>>> from ssl import PROTOCOL_TLS
>>> from OpenSSL.SSL import TLSv1_2_METHOD
>>> print(TLSv1_2_METHOD)
6
>>> print(PROTOCOL_TLS)
_SSLMethod.PROTOCOL_TLS              
>>> print(PROTOCOL_TLS.value)
2

I agree that having a library choose the most up-to-date supported TLS version would be nice though.

[richvdh]

oh sorry, the thing you want in pyOpenSSL is called SSLv23_METHOD (no, really.)

For some context as to why it has a silly name:

• The name SSLv23_METHOD predates the existence of TLS, and has always meant "use the 'best' version"
• See https://www.openssl.org/docs/man1.0.2/man3/SSL_CTX_new.html which documents SSLv23_method, and also recommends against values like TLSv1_2_method and TLSv1_method
• OpenSSL 1.1.0 has improved the situation with a TLS_method (see https://www.openssl.org/docs/man1.1.0/man3/SSL_CTX_new.html), but pyOpenSSL (or rather, the cryptography.hazmat.bindings.openssl library that it wraps) has yet to catch up: FFI interface to SSL_set_min_proto_version etc pyca/cryptography#5379

[richvdh]

thanks!

[th3b0x]

@richvdh @danc86

I'm prepping my long-overdue commits to pyca-cryptography that address the issues mentioned.

That said, the desired functionality (i.e. disabling TLS_1 and TLS_1.1) can be accomplished by utilizing the SSL.OP_NO_TLSv1 and SSL.OP_NO_TLSv1_1 context options. This functionality was validated in July. I do have a string of Pull Requests that I'm preparing to submit to the pyca-cryptography project in relation to the bindings requested above (as @richvdh is already aware, I believe)