[Security-sig] Unified TLS API for Python

[Cory Benfield]

> 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.

Essentially, the logic in the callback would be:

def sni_callback(buffer, hostname, context):
    # This creates a writable copy of the configuration: it does not
    # mutate the original.
    configuration = context.configuration
    configuration.certificates = certs_for_hostname(hostname)
    configuration.inner_protocols = [NextProtocol.H2, NextProtocol.HTTP1]
    return configuration

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

Cory