Q:

How to install cryptography on ubuntu?

My ubuntu is 14.04 LTS.
When I install cryptography, the error is:
Installing egg-scripts.
uses namespace packages but the distribution does not require setuptools.
Getting distribution for 'cryptography==0.2.1'.

no previously-included directories found matching 'documentation/_build'
zip_safe flag not set; analyzing archive contents...
six: module references __path__

Installed /tmp/easy_install-oUz7ei/cryptography-0.2.1/.eggs/six-1.10.0-py2.7.egg
Searching for cffi>=0.8
Reading https://pypi.python.org/simple/cffi/
Best match: cffi 1.5.0
Downloading https://pypi.python.org/packages/source/c/cffi/cffi-1.5.0.tar.gz#md5=dec8441e67880494ee881305059af656
Processing cffi-1.5.0.tar.gz
Writing /tmp/easy_install-oUz7ei/cryptography-0.2.1/temp/easy_install-Yf2Yl3/cffi-1.5.0/setup.cfg
Running cffi-1.5.0/setup.py -q bdist_egg --dist-dir /tmp/easy_install-oUz7ei/cryptography-0.2.1/temp/easy_install-Yf2Yl3/cffi-1.5.0/egg-dist-tmp-A2kjMD
c/_cffi_backend.c:15:17: fatal error: ffi.h: No such file or directory
 #include <ffi.h>
                 ^
compilation terminated.
error: Setup script exited with error: command 'x86_64-linux-gnu-gcc' failed with exit status 1
An error occurred when trying to install cryptography 0.2.1. Look above this message for any errors that were output by easy_install.
While:
  Installing egg-scripts.
  Getting distribution for 'cryptography==0.2.1'.
Error: Couldn't install: cryptography 0.2.1

I don't know why it was failed. What is the reason. Is there something necessary when install it on ubuntu system?

A:

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)

A:

I had the same problem when pip installing the cryptography module on Ubuntu 14.04. I solved it by installing libffi-dev:
apt-get install -y libffi-dev

Then I got the following error:
build/temp.linux-x86_64-3.4/_openssl.c:431:25: fatal error: openssl/aes.h: No such file or directory
 #include <openssl/aes.h>
                         ^
compilation terminated.
error: command 'x86_64-linux-gnu-gcc' failed with exit status 1

Which I resolved by installing libssl-dev:
apt-get install -y libssl-dev

