Security and Privacy in a Networked World/Programming in Python

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NOTE: This topic makes use of Wikipedia articles to provide the basic understanding of the matters. These articles deal with technology and are not much disputed, having reached the common knowledge stage. They also provide some good links for further study under the reference sections.


Python?

Python is a programming language developed by Guido van Rossum in 1989. It is a full-fledged, generic programming language with many different uses, in networking security it is often used as a tool to write short scripts for managing (and sometimes also, attacking) computer networks. Other reasons for choosing Python as the example language for this course include:

  • Python is often deemed as a good choice to start learning programming with, as its syntax is relatively simple.
  • Python directs its users towards good programming style by using indenting to mark structural blocks in programs (most other well-known languages - e.g. Java, C or Pascal - allow free placement of program statements and denote blocks with keywords or symbols).
  • As fully free and open-source software, it is included by default in most modern Unix-based operating systems (including OS X as well as almost all Linux distributions and BSD Unixes).
  • Python is also used to write some system software, e.g. the graphic installers used by Ubuntu, Red Hat Enterprise Linux and Fedora.
  • It is used for writing scripts in LibreOffice (similar to the role of Visual Basic for Applications used in MS Office), GIMP graphics package and a number of graphics/animation packages like 3D Max, Maya, Lightwave and others.

Note that Python is currently available in two parallel flavours. Python 2 was released in 2000 and its current stable version is 2.7. However, in 2008 the creator of Python decided to fix some inconsistencies in the language, resulting in the new Python 3 being incompatible with Python 2. Most systems that include Python have opted for Python 2 so far - therefore, this version (2.7) is also used at our course.

READ MORE at Wikipedia: https://en.wikipedia.org/wiki/Python_%28programming_language%29

...

Basics of programming

Note: this section gives a very rudimentary overview of programming, as plenty of online materials (on Python and other languages) is available.

Languages

Programming is typically done in so-called high-level languages - they are command systems that could be described as something in between of math and human languages. First widely used languages date back to the 60s (FORTRAN, COBOL), current popular choices include C (with its derivatives like C++, C# and others), Java, Python, Ruby, BASIC (while the language has long history, its current versions like Microsoft's Visual Basic are modern languages) and others.

High-level languages are designed to be understood by people rather than computers. To be understood by the computer (or rather, the operating system), they must be translated into machine code. Based on the method of translation, programming languages can be roughly divided into two categories (today, several intemediate and hybrid methods also exist):

  • interpreted languages - the program is read and executed by special software (the language interpreter) row by row (more exactly, statement by statement).
  • compiled languages - use another kind of software (the compiler) that translates the whole program (provided it is error free), resulting in a machine-language version of the program. The compiled program is a separate entity that can be run, copied etc (the .exe files in MS Windows are a well-known example of compiled programs). However, in order to make changes, the original high-level version (called the source code) is typically needed - after making changes, the source program must be re-compiled.

Python in its basic form is an interpreted language - we can launch Python in OS X or Ubuntu, and use simple commands which are obeyed immediately. However, compiled versions of Pythons do exist as well.


READ MORE: https://en.wikipedia.org/wiki/High-level_programming_language

Main elements of a program

At the most general level, a program consists of three kinds of "building blocks":

  • sequence
  • choice
  • cycle

We may illustrate it with a simple recipe for oat porridge:

"Take a cup of oat flakes and a glass of milk. Pour milk to a pot and heat it. Put oats into the pot and stir. If the mix starts to boil, cut down on heat and let it simmer for 10 minutes. Keep stirring until the porridge is ready. Add some salt and sugar - if you plan to eat the porridge with hot butter, keep it more salty, otherwise make it sweet."

Here, most instructions are given as a sequence - "do this, then that". However there are two choices - the first is at the condition "the porridge boils" - in a programming language, it can be written like 

IF porridge boils THEN cut down on heat

Another choice is at the end - we may write it like this: 

IF I plan to eat the porridge with hot butter THEN
   Add more salt than sugar
ELSE
   Add more sugar than salt

Finally, there is also a cycle - a part of program that needs to be repeated until an ending condition appears. We may write 

DO
  let the porridge simmer
UNTIL 10 minutes have passed

(Note: this is one of the three main variants of a cycle in programming)


Basics of Python

In Unix-like systems, the interactive environment (the Python shell) is launched from a terminal (command line) with just "python". Launching a Python program (typically with .py extension) along with the environment is done with "python programname.py".

First steps in the Python shell

After lauching the Python shell, it displays a prompt. An example: 

Python 2.7.5+ (default, Feb 27 2014, 19:37:08) 
[GCC 4.8.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

Here, the ">>>" shows the system being ready to receive commands.

A simple use for the Python shell is as a calculator. We can enter a formula and it is immediately calculated by the interpreter: 

>>> 5 + 2
7
>>> 3 * 4 + 10 / 2
17
>>>

NB!!! Somewhat awkwardly, exiting the Python shell is done via the key combination of Ctrl-D. More logical commands like "exit" and "quit" also work, but need empty parentheses added - quit() and exit().

Variables

Variables are used in most programming languages. Technically, they are named locations in computer memory that can hold a value. The main purpose is the increased flexibility, as the value of an variable can be changed along the program. Variables can be of different types (e.g. integers or textual values) or contain single elements or more complex structures. The exact way of handling variables depends on the programming language used.

In Python, we can again start with the shell: 

>>> a = 3
>>> b = 6
>>> a + b
9
>>>

In this example, the variable "a" is assigned the value of 3 and the variable "b" the value of 6. Now, instead of summing the values, we can sum the variables - the interpreter sums their values and outputs the result.

If we now assign "b" a new value of 2, the result will be: >>> b = 2 >>> a + b 5 >>>

While "a" remained at 3, "b" received a new value and the resulting sum is different.


Outputting values and the first programs

The Python interpreter prints out resulting values by default. However, in programs, we need to use the "print" command. The classic first program in all programming languages is called "Hello World!" - often, it is a single-line program like in Python: 

print "Hello World!"

We can try it out by writing the line into a file named e.g. hello.py and then launcing the program along with the Python shell with "python hello.py".

For the second example, let's do the same addition as above, but now with a separate program.

Note: Python programs can be written with any clear-text editor. In Windows, Notepad is fine - however, WordPad or Word need to be told separately to save the file as pure text, as they add formatting information to their documents by default.

Open an editor of choice and create a file named sumitup.py: 

a = 3
b = 6
print a + b

Launch it with "python sumitup.py"

Note: this is the simplest form of output. In practice, the output is usually formatted using some extra settings. Also notice that textual values are in quotes while numerical ones are not.


Input

Many programs need to obtain some data from the program user and use it to calculate results. In Python, this is done with "raw_input": 

print "We will now add some numbers."
a = raw_input("Please enter the first number: ")
b = raw_input("... and now the second one: ")
total = int(a) + int(b)
print "The total of these is ", total

Notes:

  • In Python, the "raw_input" is technically a function - a subprogram that accepts parameters in parentheses and returns a value. Here, the variable "a" and "b" are assigned the values entered by the user.
  • the input is initially interpreted as textual. Therefore, to convert the values into numbers, we need the int() function (meaning "turn the value into an integer"). The conversion could have been done also on input - in this case, e.g. the second line would start as "a = int(raw_input("Please... " and the fourth would be just "total = a + b" (but if the conversion is omitted altogether, the result would be concatenation of two textual values: "3" and "6" result in "36"). The reverse - from integer to text - is obtained by str() function (in programming, textual values are commonly referred to as "strings").


Selection

In most programming languages, the simple selection

Additional reading and links

Note: there is plenty of materials online. Care must be taken though with the ones that rely on too old versions of Python.

Study and Blog

  • Pick one of the programming tasks:
    • Total beginners: write a simple game of "Sell Me an Animal!". You are a zookeeper, the program is a potential buyer of animals. It should ask you "What do you offer?" and react in three possible ways: a) exclaim "I take it!" on a certain animal, b) declare "I hate those!" on another, or c) say "Not interested!" on all the rest. HINT: Assign the two special cases (good and bad) to variables and compare the input values to these.
    • Intermediate: the same as above, plus a) keep asking (using a cycle) until either the "good" or "bad" animal is reached, b) count the total number of animals offered and output it at the end.
    • Advanced: the same as above, plus a) read the list of animals from an external text file, and b) in case of the neutral answer, ask for another animal on the list whose name begins with the same letter. For example: "What do you offer?", "Monkey", "Not interested, do you have a moose?". Note that for this to work properly, the data file must have at least two animals with all letters (no need to do all the alphabet though - e.g. two with "a", two with "k" and two with "s" should work well).

Sample data for the lazy ones: 

ant
ara
cat
cow
crocodile
dog
dolphin
donkey
elephant
elk
pig
puppy
snail
skunk
warthog
wolf
wolverine
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