Investigate default TLS ciphers for pyOpenSSL

[sethmlarson]

#2082 makes system ciphers the default with stdlib SSLContext implementation. Can we take a similar approach with pyOpenSSL or should we still rely on urllib3's default list?

If we're relying on urllib3's default list we only need to make pyopenssl.inject_into_urllib3() set USE_SYSTEM_SSL_CIPHERS to False unconditionally.

cc @tiran @alex

[alex]

pyOpenSSL doesn't provide its own default list, so it's a matter of whether you're happy with OpenSSL's defaults (across a wide variety of OpenSSL, naturally).

My memory of "which OpenSSL versions have acceptable defaults" is kind of hazy, so I don't have a better suggestion than to flip the setting, and hit https://www.howsmyssl.com/s/api.html from a few different linux distros with different OpenSSL versions and see if the settings look ok.

[sethmlarson]

@alex Thanks for the reply, it looks like the best way to determine the OpenSSL version of pyOpenSSL to use cryptography.hazmat.backends.openssl.backend.backend.openssl_version_number().

We're planning on using OpenSSL-configured defaults on OpenSSL 1.1.1+, so would be >= 0x10101000

[tiran]

Caution, >= 0x10101000 is no good with LibreSSL. It always reports OpenSSL version 0x20000000.

[sethmlarson]

@tiran Yikes, is there a canonical way to detect OpenSSL & v1.1.1+?

[tiran]

IS_LIBRESSL = ssl.OPENSSL_VERSION.startswith('LibreSSL')
IS_OPENSSL_1_1_1 = not IS_LIBRESSL and ssl.OPENSSL_VERSION_INFO >= (1, 1, 1)

[sethmlarson]

@tiran Instead of filtering out LibreSSL should we instead only use OpenSSL? ssl.OPENSSL_VERSION.startswith("OpenSSL")?

[tiran]

IMO it makes more sense to special case LibreSSL as they decided to redefine OPENSSL_VERSION_NUMBER. CPython code and PyCA cryptography code have been checking for either LIBRESSL_VERSION_NUMBER C macro or ssl.OPENSSL_VERSION.startswith('LibreSSL') for years, too.