Support cipher / key-agreement preferences

[colmmacc]

At present s2n has a single fixed ordering for cipher suites. These defaults should suit most users, but some flexibility is probably necessary.

Tentative proposal for an API:

s2n_config_set_cipher_preferences(config, "AES128 > 3DES > RC4", &err);
s2n_config_set_exchange_preferences(config, "DHE > RSA", &err);

[colmmacc]

After some thought and much discussion, this might be a bad idea. Most customers, administrators and so on do not have an in-depth understanding of SSL/TLS cipher suites; and so asking them to configure an order is a bit like passing the buck. It would be better if s2n chose sane defaults.

Since library users must maintain good deployment practices for software bugs, it seems sensible to use this same process to handle updates in the "safe" cipher suites too.

At the same time, users should be free to validate and test new defaults, as well as insulate themselves from change. To try to get both it's proposed that s2n support checkpointed defaults. What that means is:

• If a user does nothing, they will use the latest default cipher suites, in the order shipped with s2n.
• If a user wants to stay on a fixed version of the cipher suite preferences they can set this explicitly, e.g. s2n_config_set_cipher_preferences("20150202") . The responsibility is on them to update.