Intro to Software Reverse Engineering

What is reverse engineering?
Today the market of software is covered by an incredible number of protected
applications, which don't allow you to use all features of programs if you
aren't a registered user of these. Reverse engineering is simply the art of understanding a program by analyzing the low leve assembly instruction”.but there are some people who uses it to remove the licesning schema which is called "cracking".

In Some other words reverse engineering is described as follows: -

“When you create a program you engineer it, in fact you build the executable from
the source-code. The reverse engineering is simply the art of generate a
source-code from an executable. Reverse engineering is used to understand
how a program does an action.but it is also used to bypass protection etc. Reverse engineering used by cracker to understand the protection scheme and to break it.

“Cracking is a method of making a software program function other than it was
Originally intended by means of investigating the code, and, if necessary, patching
It.”

A Little bit of history
Reveres engg. Most probably started with the DOS based computer games. The aim is that a player has full life and armed in the final stage of the game. So what a reverse egg. Do is just find the memory location where the life and number of weapons are store and then modify this values. They used memory-cheating tools such as game hack etc. So that they have full life and armed in the last stage of the program. But in today’s world with the advent of the shareware concept more and more software author releases the shareware versions. Hence with this reverse engineering become more tadious, more complex, and trickier.
Today to protect the software a programmer use various kind of technique, some of them are old, bad repetitive techniques but some are new. We will discuss them in next section.

Various Protection schemas
Following are the most commonly used schemas
1) Hard coded serial
2) Serial number, name protection
3) Nag screen
4) Time trial
5) Dongle (hardware protection)
6) Commercial protection
7) Other (cd rom check, keyfiles, disabled function etc.)
Lets study this in detail
1) Hard coded serial: -This is the simple protection as compared to other. In this kind of protection we have to enter only a serial number and this serial number is same for all the user. Serial no. We entered is compared to original serial through an algorithm and if a user entered correct serial then the software get registered.

2) Serial number - name protection:-In this kind of protection we have to enter a name and a serial number. Then our serial no is compared with the original serial, no which is derived from our name using some algorithm. This protection is some time easy and some times hard, based on the algorithm a programmer use. Example of this type protection is most widely used software "WinZip."

3) Nag screen :-In this kind of protection a screen come each time a user start the application, to remained such that how many days are left or your software are unregistered or any other message. This is a littlie hard to remove. And most of the newcomer find it difficult as a new programmer find difficult to understand pointers.exa: - WinZip
But if a reveres has enough knowledge of windows API then he can easily remove the nag screen.

4) Time Trial: - According to +ORC This kind of protection has any of following protection or combination of following protection schema: -
a) To a predetermined amount of days, say 30 days, starting with the first day of installation. This is referred as "CINDERELLA protection".
b) To a predetermined period of time (ending at a specific fixed date) independently from the start date... 'BEST_BEFORE a given date' protection.
c) To a predetermined amount of minutes and/or seconds each time you fire them... 'COUNTDOWN' TIME PROTECTIONS' example of this kind of programs are some games and audio video player which allows an unregistered user to play game for some amount of time say 5 minutes etc.
d) To a predetermined amount of 'times' you use them, say 30 times. Strictly speaking these protections are not 'time'dependent. But they depend only on thing "HOW MANY TIMES YOU EXICUTE THEM"

5) Dongle Protection: - this kind of protection is supposed to be toughest protection to crack. This protection is consist of an EPROM, which was connected with a port on computer. The program which is protected by this is first cheeks the presence of this and then cheeks that the program is registered or not all though it implementation is too hard and hence this kind of protection is not very widely used. This is used in Big Protected shareware’s. This protection is used by a I/O LPT port (hardware) You will need the registration Card attached To your PC's parraral port Or other in order to make The program fully work, otherwise it will be Expired after xxDays / xxUses /rippled or wont work at all. Dongles such as: HASP / Sentinel are most commonly used. Dongles uses DLLs/VxD to check the "is registered"
Dongle API is also used for some checks.
Example of programs, which uses this kind of protection, included some version of CAD etc.

6) Commercial protection: - Most of the software programmer don’t want to spend there precious time in deciding which kind of protection they used to protect there software. Because they think that instead of the spending there time on designing the security algorithm of there programs, why not they spend time to improving the functionality of there program??? And here comes the concept of commercial protection. Today some software company’s designs only security algorithm for various sofware. Also they provide a general software, which converts a fully functional software in to unregistered version and after paying the registration
These software get converted back in to the fully functional registered software after entering the registration details. some of the companies which uses commercial protection for there software are macromedia, Symantec etc and some compnies which provides this type of protection are privewsystems(vbox protection) etc..
Allthough this kind of protection have high security because they are professionally desined but they also have some disadvantages. One major disadvantage is that "if a person cracks only one program which is protected using this protection, then he has cracked all the program which uses this kind of protection!!!!"
. For example if a cracker has cracked the flash mx (which is protected by vbox) then he was able to crack easily all the macromedia software such as dream waver mx etc., because all these programs are based on only one kind of protection!
And in the real world there is no protection, which is still uncracked.

7) Other protections: - There are many other techniques which are used to protect software. These are generally used in computer games. Such as cd rom protection, diasabled function etc. I think most of computer user are familiar with this protection and already seen this kind of protection. For example: - If a user doesn’t have cd for a particular game then he cannot be able to play the game directly from harddisk. Because when he run the program then the program checks for the cdrom.
Also some protection have disabled functions such as you cannot save your work or you cannot use any particular function etc.

So I hope now you understand all the protection schemas, which used to protect software.
Ok let’s study how reverse engineering is done. The first thing to keep in mind that cracker always works with the disassembly and they are familiar with the windows API.
Now all of us computer user knows that computer only understands binary nothing else. So first we create a program and then compile it now what compiler do is check for syntax, any error and then he generate the .obj file. As in high level language some function are prewritten which are stored in library file hence after this we used linker which links the programs with the library file and then after linking we get an exe file hence exe file we use is nothing but the collection of instruction in binary formats.
Now to reverse engineer there are different tools avaliable.