[Security-sig] Unified TLS API for Python

[Nick Coghlan]

On 14 January 2017 at 01:58, Cory Benfield <cory at lukasa.co.uk> wrote:
>
>> On 13 Jan 2017, at 15:45, Nick Coghlan <ncoghlan at gmail.com> wrote:
>>
>> So the essential stack would look like:
>>
>>  TLSConfig[uration?]: implementation independent, settings only
>>  TLSClientContext: ABC to combine settings with a specific TLS implementation
>>  TLSServerContext: ABC to combine settings with a specific TLS implementation
>>  TLSSocket: ABC to combine a context with a network socket
>>  TLSBuffer: ABC to combine a context with a pair of data buffers
>>
>> And then TLSPolicy would be a potential future implementation
>> independent addition that could be used to constrain acceptable TLS
>> configurations.
>
> If we were going this way, I’d want to add one extra caveat: I think I’d want the Contexts to become immutable.
>
> The logic for the SNI callback would then become: you are called with the Context that created the socket/buffer, and you return a Configuration object that contains any changes you want to make, and the Context applies them if it can (or errors out if it cannot). A new Context is created. This relegates Context to the role of “socket/buffer factory”. The advantage of this is that we have vastly reduced the moving parts: a Context can ensure that, once initiated, the Policy that belongs to it will not change under its feet. It also allows the Context to refuse to change settings that a given concrete implementation cannot change in the SNI callback.

Having immutable-by-default config with explicitly managed state
changes in particular well-defined scenarios sounds like a big win to
me.

> This would almost certainly make Context implementation easier, as there is no longer a requirement to monitor your configuration and support live-updates.

It also drastically reduces the test matrix required, as eliminating
any potential dependence on the order in which settings are applied,
means you only need to test interesting *combinations* of settings,
and not different configuration *sequences*.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia