In the course of installing app-backup/obnam, I discovered that dev-python/cryptography-1.7.1 cannot be built against libressl, whereas version 1.7.2 can.
Created attachment 498256 [details] build.log
dev-python/cryptography-2.0.2 is already stable on every platform except sparc and alpha (where it is missing keywords). Perhaps try that first?
Thanks for the comment, dwfreed. On the last occasion I installed obnam, 1.7 was part of the depgraph. I had assumed that this was intentional but you have a point; perhaps it was simply a reflection of the keywords that were effective at that time. I'm currently working on a new install, so I shall test shortly. In the event that cryptography-2.0.2 works, I'll close this as INVALID.
Duly tested and cryptography-2.0.2 worked perfectly. How embarrassing! I don't think that I can mark this as INVALID of my own accord, so I shall wait for a gracious dev to serve me my duly-earned ignominy.