Cache the public keys once generated

[fousa]

The public keys are generated for the domain on every request. but I think we should cache the generated value so that the public key pinning process is optimised.

[dgyesbreghs]

We never know when they're going to update the certificates. So there is going to be a 'grey periode'. For example: when someone has stolen the private 🔑
Don't think this will improve the security/performance. Maybe I'm wrong 🤔

[fousa]

Good point.

I was only planning to cache it in memory, I think Alamofire does the same but I'm not 100% sure. Not sure if calculating the public key on every request is a performance issue neither. To be examined.

[dgyesbreghs]

I have checked the Alamofire source code and there are not caching it in memory.
https://github.com/Alamofire/Alamofire/blob/master/Source/ServerTrustEvaluation.swift
Maybe it's also a good idea to compare our implementation with the Alamofire implementation.