Module Seven

Module Seven

Computer Software

Computer Software

Computer Software• How applications work.

Computer Software

Computer Software• A sequence of instructions to do a task

is called a program.– Also known as software, or sometimes a

script.

Computer Software

Computer Software• Some Examples -

Computer Software

Computer Software• Word

Computer Software

Computer Software• Excel

Computer Software

Computer Software• Outlook

Computer Software

Computer Software• Internet

Explorer

Computer Software

Computer Software• Internet

Explorer6

Computer Software

Computer Software• Firefox

Computer Software

Computer Software• Safari

Computer Software

Computer Software• Adobe

Acrobat

Computer Software

Computer Software• Skype

Computer Software

Computer Software• World

ofWarcraft

Computer Software

Computer Software• All of these use

– the execution of instructions, and– The manipulation of data

that we saw in the last section.

Computer Software

Computer Software• Now, we’ll examine the concept of

computer programs in a little more detail.

Machine Language

Computer Software Binary Codes

• Inside the computer,every piece of information is either a 0 or a 1.

• These can be represented electrically or magnetically.


• A single 0/1 value is a bit.• Every byte in memory is a group of eight bits.

– Remember, that a computer’s memory is measured as megabytes or gigabytes.


• Everything in memory is encoded using collections of bits.


• The letter “a” is represented in memory as 0110 0001.– This is an example of a bit string.– Note that it uses eight bits - a byte.


• The number 7 is represented as 0000 0111.


• Even the instructions the computer uses are bit strings. – The instruction to add two numbers for a particular

machine might be 1011 1011.

Computer Software Machine Language

• To program the first computers, programmers had to enter the binary strings themselves. – Programs in this form are called machine

code.


• Here is a sample of machine code:

01001010010011100110011100000011111101010110101000011000100100010000111111001010001111010011110111110000011110010100011110100111101111011000000000011111101011100100100101110010000000111111010111001001001100100110011011011100111011110111110000000


• For anything but tiny programs, this was a terrible burden.

• It was easy to make typos• References to data in memory had to be to

the numeric address of that memory location (i.e. the piece of information in location 1001 0101 1111 0100)

Assembly Language

Computer Software Assembly Language

• Shortly thereafter, someone wrote a program called an assembler.


• An assembler takes a file that contains names for the machine instructions.

• It then translates this into the machine code.• For example,

– Instead of 1011 1011 the programmer would write ADD for the add instruction.

– Programmers could refer to data locations by names (labels, e.g. SUM), instead of by numeric address


• Here is a sample of assembly language code:

CLR SUMMOV #TAB, R4

LOOP: TST (R4)BEQ ENDADD (R4), SUMADD #2, R4BR LOOP

END: HALT


• Easier than machine language, because it uses names for instructions and data locations.


• But, because computers’ instructions are so simple, it takes large numbers of them to do even a simple operation.


• This is especially true if they involve reading information from the keyboard or disk or writing it out to the monitor or disk.

• So, even simple assembly language programs are ridiculously long.

High-level Languages

Computer Software High-Level Languages

• High level languages (HLLs) were invented that substitute English-like commands for tens or even hundreds of machine instructions.


• HLLs make the computer much easier to program. – Statements can be made much more

humanly intuitive than the machine's native instructions.


• Also, the same HLL program can be used on different machines, with different machines languages.

• This is known as portability.


• There are hundreds of HLLs. – Thousands?

• Some of the first were – FORTRAN, – COBOL– LISP

All of these are still used today.


• Other popular languages include – ALICE– BASIC– Pascal– Modula– C– Prolog– Java

– C++– Perl– Python– Ruby– Macro languages – Scripting languages– And many, many more

Computer Software Computer Software

• High-level languages make the programmer’s life easier.– Relatively speaking

Computer Software Computer Software

• A sample HLL (Perl) program:#!/usr/bin/perl -w

# Input the valueprint "Enter a numeric value: ";$value = <>;chomp $value;

# Is it positive, negative or zero?if ($value > 0) {

print "$value is positive.\n";} elsif ($value < 0) {

print "$value is negative.\n";} else {

print "$value is zero.";}

Module Eight

Attacking Software

Attacking Software Attacking Software

• How applications work, and how they can be attacked.


• The fundamental point is that a malicious piece of software does the same kinds of things that a legitimate one does.

• The only difference is that its purpose is usually harmful to the computer or its user.

Attacking Software

Application-Based Security Attacks

• If a malicious piece of software gets on your computer, it can use the computer’s own instructions and resources against you.

• Many familiar types of computer attacks do just this:– Trojan horses– Viruses


• How malicious software uses your own computer for its purposes can be shown in a series of examples.

Search

Attacking Software

Search• A helpful feature on modern computers

is the search function.• It allows you to type in a word or

phrase.• The computer can then check to see if

that word or phrase is – part of a file name, or – part of the contents inside one of your files.

Attacking Software

Search• How it works -• Suppose you asked the computer to

search for a specific word (e.g. iPod) in your “My Documents” folder.

• The computer will search all of the data in all of those files for values matching “iPod” and will show you all files with that word in them.

Attacking Software

Search• Even though modern disk drives can

store hundreds of gigabytes of information, computers are fast enough to make even comprehensive searches of a computer’s files practical.– This is helped by the fact that computers

can keep track of (index) this information to make search even faster.

Attacking Software

Search• The ability to search your own files is

definitely something positive.

Attacking Software

Search• However, a malicious piece of software can

also search through your files.• It can look for pieces of information that look

like – Email addresses– Credit card numbers– passwords

• Once found, those data can then be sent back to the attackers.

Attacking Software

Search• A malicious search works the same way as a

legitimate one.• There is no difference from the point of view

of the computer.

• The only difference is whether the search result is beneficial or harmful for you.