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?