[yakak]

There is a framework for this:

https://en.wikipedia.org/wiki/Evaluation_Assurance_Level

Proton is claiming something similar to EAL4, which is not secure, there is an assurance that not all trained reviewers can find a vulnerability. Openssh is a little less secure than that formally, but has more trained reviewers informally, which probably cover some parts extremely well and other parts sparsely.