Set OP_NO_COMPRESSION & OP_CIPHER_SERVER_PREFERENCE on TLS contexts

[Hynek Schlawack]

There is a broad consensus that there are two options that should always be set:

• OP_NO_COMPRESSION (CRIME attacks, seems like most OpenSSLs set that as default nowadays but still)
• OP_CIPHER_SERVER_PREFERENCE (if we push for secure ciphers, we should make sure they get used too)

Both are missing in pyOpenSSL ​at the moment but we can easily wing it for now.

Confusingly, OP_NO_COMPRESSION is documented in my self-built pyOpenSSL docs ( ​http://glui.me/?i=3b37w0j9wemcrsl/2013-10-25_at_14.30.png – maybe I’ve built them from a bzr checkout? been a while) and my OpenSSL should have support for it but I can’t find it neither in the official docs at ​http://pythonhosted.org/pyOpenSSL/openssl-ssl.html nor in the actual library. In any case we’d have to take into account that the underlying (Py)OpenSSL may not support either.

Implementation is trivial, I’d just wait for #6772 to land in trunk because it’s laying groundwork for setting options.

[Tobias Oberstein]

Probably these 2 should be set also:

SSL.OP_SINGLE_ECDH_USE = 0x00080000L
SSL.OP_SINGLE_DH_USE = 0x00100000L

[Hynek Schlawack]

(In [41219]) Branching to harden-tls-options-6801.

[Hynek Schlawack]

I’ve done the following:

1. Consolidated options handling by baking them into self._options on __init__ to avoid repeated calls to set_options in _makeContext.
2. Set OP_NO_COMPRESSION and OP_CIPHER_SERVER_PREFERENCE unconditionally.
3. Bonus: Added tests that OP_SINGLE_DH_USE is set if enableSingleUseKeys is active and also added OP_SINGLE_ECDH_USEin the same case so we don't forget it once we actually add ECDH support.

Buildbots are green, please let me know what you think.

[Jean-Paul Calderone]

1. The docstring for test_singleUseKeys says what the test is doing rather than what the implementation is expected to do (though it does so without using the word should for which I guess you get a few points :).
2. The news fragment says "and makes servers choose the cipher". I think this might be a little ambiguous wrt CertificateOptions usage for clients. I think that OP_CIPHER_SERVER_PREFERENCE is a no-op on clients? So maybe "and, for servers, uses server preference to choose the cipher." That's a little awkward due to the repeated word "server" but maybe it's an improvement anyway; I'll leave it up to you.

Otherwise looks great, please address those issues and merge. Thanks a lot!

[Hynek Schlawack]

(In [41286]) Merge harden-tls-options-6801

Author: hynek Reviewer: exarkun Fixes: #6801

twisted.internet.ssl.CertificateOptions now disables TLS compression to avoid CRIME attacks and, for servers, uses server preference to choose the cipher.