How to install cryptography on ubuntu?

The answer is on the docs of cryptography‘s installation section which pretty much reflects Angelos’ answer:

Quoting it:

For Debian and Ubuntu, the following command will ensure that the
required dependencies are installed:

$ sudo apt-get install build-essential libssl-dev libffi-dev python-dev

For Fedora and RHEL-derivatives, the following command will ensure
that the required dependencies are installed:

$ sudo yum install gcc libffi-devel python-devel openssl-devel

You should now be able to build and install cryptography with the
usual

$ pip install cryptography

If you’re using Python 3, please use python3-dev instead of python-dev in the first command. (thanks to @chasmani)

If you’re installing this on Ubuntu 18.04, please use libssl1.0 instead of libssl-dev in the first command. (thanks to @pobe)