Not sure if you read all of the thread, but no, you cannot use kivy, or cryptography, or any non pure pscksge (there are some which have non pure parts that are non essential, in which case you can usually install manually if pip fails)
On the question of cryptography specifically, therr are some alternatives.
pycrypto comes with pythonista, and ecsda can be installed with pip. You can also install jwt.
oauthlib uses jwt, which you can apparantly force to use pycrypto / ecsda instead of cryptography.
also, you might be interested in https://github.com/tomato42/tlslite-ng for