Manually downloading and installing python-cryptography and python-openssl from the Ubuntu package repository might be a safer alternative to adding the repository for a future distribution to your sources list.

I achieved this by downloading the appropriate packages from here and here, then installing them using dpkg :

sudo dpkg -i python-cryptography_1.7.1-2_amd64.deb sudo dpkg -i python-openssl_16.2.0-1_all.deb sudo apt-get -f install

Important: Read the output of the last command carefully. It threatened to remove a load of packages when I ran it, because dependencies weren't met - I initially tried to install python-openssl without python-cryptography .

Read more about manually installing packages: How do I install a .deb file via the command line?

After a reboot my Unity log file is no longer increasing in size at a rate of 10 MB per second.