Click here to Skip to main content
15,795,793 members
Articles / Programming Languages / Assembler

Protection & Decompiling Software - Start Point Info

Rate me:
Please Sign up or sign in to vote.
4.78/5 (104 votes)
20 Aug 2018CPOL14 min read 145.2K   224   69
Software protection & Decompiling software as a new cracking method


This article is a quick start generic info for the beginner developers to know how their software maybe cracked and what idea should try to avoid that.

Software locking methods

  • Fixed Serial Number: In this method the software will ask for a serial number on installation or during the 1st run. Serial number could be distributed easily. Indeed the serial number is just to inform the user this is none free software and he should bought it.
  • Activation Code: In this method the software will generate a machine id and required an activation code from the user which could be getting from the vendor. Cracking of the software may be performed by debugger or by partial decompiling.
  • Internet Activation: Same as activation code put the activation will run through internet. Cracking may be done by injecting wrong interties in host file to forward the activation page to local or wrong page, or even by a custom DNS forwarder
  • Dongle: the provider will provide a small hardware piece with could be detected by the software. The dongle will contains some info to be used for a special version or software. Cracking maybe done by hacking the communicating library between software and dangle.
  • Protected CD: The installation can be run only from the original protected CD which maybe protected by laser generated bad sectors or some invalid contents or writing data after CD end mark. Cracking maybe done by coping the CD using special program or by cracking installation process.
  • Flash USB memory: The software may use the physical number of the flash memory and compare it to database before run or write a part of the software code on none assigned area of the flash disk. Cracking may be done by hacking the software.
  • After sell support: Some software provider provides a very good support so the buyer will not try to get cracked software.
  • Direct installing method: some provider will install the software by a developing company employee and no install application is distributed.

Software cracking

It is the modification of software to remove or disable features which are considered undesirable especially

  • Disable copy protection features
  • Remove  limitation of use for unregistered user
  • Disable ad-ware associated with the software for unregistered user

Cracking maybe done by:

  • Redistributed serial number,
  • Key-gen,
  • Patch,
  • Loader.

The most common software crack consists of the modification of an application's binary to cause or prevent execution specific part of the program. This is accomplished by reverse engineering. This could be done by:

  • Run the program code using a debugger until the software cracker reaches the subroutine that contains the primary method of protecting the software
  • Disassembling or decompiling an executable file.
  • Cracking some time done by monitoring the registry or file system changes done by the installation and 1st run of the application.

Software developers are constantly developing techniques such as code obfuscation, encryption, and self-modifying code to make cracking difficult.

Assembly Cracking


  • Disassembly is a type of reverse engineering.
  • To disassemble is to convert a program in its executable (ready-to-run) form into assembly language
  • Assembly language is readable by a human.
  • A program used to accomplish this is called a disassembler.
  • Another program, called a de-compiler, converts object code back into the code of a higher-level language.

Assembly language

  • An assembly language is a low-level programming language for a programmable device.
  • A program written in assembly language consists of a series of (mnemonic) processor instructions and meta-statements, comments and data.
  • Assembly language instructions usually consist of an op code mnemonic followed by a list of data

Assembly Language example:

; This program displays "Hello, World!" in a windows message box and then quits.
; Written by Stewart Moss - May 2006
; Assemble using TASM 5.0 and TLINK32
         .model  flat,STDCALL
extrn            MessageBoxA:PROC
extrn            ExitProcess:PROC
HelloWorld db "Hello, world!",0
msgTitle db "Hello world program",0
         push    offset msgTitle
         push    offset HelloWorld
         push    0
         call    MessageBoxA
         push 0
         call ExitProcess
end Start

Assembly Control Flow & Cracking

The most used technique for cracking is to

  • Run the debugger such as OllyDbg
  • Open the application in the debugger and run it
  • Note the last jump before the application ended
  • Modify the interested jump function

The modification code be done by

  • Invert jump condition. Ex.: convert Ja to Jna
  • Change the jump code to number of Nops. Ex.: specify the cods in the debugger and right click and choose fill with
  • Change conditional jump to none conditional jump with the same length

Assembly Instructions links

Assembly unconditional jump Instructions

Function OP Code Hex

Creates a stack frame


C8 i0 i1 i0

Halts the processor



Destroys the current stack



Lock prefix on next instruction



Waits for the FPU to finish its last calculation



Jump sl Jmp EB r0
JMP np Jmp E9 o0 o1
JMP rmw Jmp FF /4 d0 d1
JMP DWORD PTR [rmw] Jmp FF /5 d0 d1
JMP FAR PTR fp Jmp EA o0 o1 s0 s1
CALL np Call E8 o0 o1
CALL rw Call FF /2 d0 d1
CALL DWORD PTR[rw] Call FF /3 d0 d1
CALL FAR PTR fp Call 9A o0 o1 sl sh
RET Ret C3
RET iw Ret C2 i0 i1
RETF iw Ret CA i0 i1

Conditional Jumps

Function OP Code Hex &
2Byte Address
Hex &
1Byte Address

Jump if Above, unsigned comparison


0F 87 r0 r1

77 r0

Jump if not above


0f 86 r0 r1

76 r0

Jump if Above or equal, unsigned comparison


0F 83 r0 r1

73 r0

Jump if not above or equal


0f 82 r0 r1

72 r0

Jump if bellow, unsigned comparison


0f 82 r0 r1

72 r0

Jump if not bellow, unsigned comparison


0F 83 r0 r1

73 r0

Jump if bellow or equal, unsigned comparison


0F 86 r0 r1

76 r0

Jump if not bellow or equal, unsigned comparison


0f 87 r0 r1

77 r0

Jump on Equality


0f 84 r0 r1

74 r0

Jump on Inequality


0f 85 r0 r1

75 r0

Jump if greater


0f 8f r0 r1

7F r0

Jump if not greater


0f 8e r0 r1

7E r0

Jump if greater or equal


0f 8d r0 r1

7D r0

Jump if not greater or equal


0f 8c r0 r1

7C r0

Jump if less


0f 8c r0 r1

7C r0

Jump if not less


0f 8d r0 r1

7D r0

Jump if less or equal


0f 8e r0 r1

7E r0
Jump if not less or equal Jnle 0f 8f r0 r1 7F r0

Jump on overflow


0F 80 r0 r1

70 r0

Jump on not overflow


0F 81 r0 r1

71 r0

Jump on sign


0F 88 r0 r1

78 r0

Jump on no sign


0F 89 r0 r1

79 r0

Jump on zero


0F 84 r0 r1

74 r0

Jump on not zero


0F 85 r0 r1

75 r0




E2 r0

Loop if equal



E1 r0

loop if not equal



E0 r0

loop if zero



E1 r0

loop if not zero



E0 r0


  • To decompile is to convert executable (ready-to-run) program code into some form of higher-level programming language.
  • The resulting code can be read by a human.
  • De-compilation is a type of reverse engineering that does the opposite of what a compiler does.
  • The tool that accomplishes this is called a de-compiler.
  • A similar tool, called a disassembler, translates object code into assembler language.
  • De-compilation was first used in the 1960s to facilitate the migration of a program from one platform to another.
  • De-compilation is not always successful.
  • De-compilation is sometimes used unethically, to reproduce source code for reuse or adaptation without permission of the copyright holder.
  • Programs can be designed to be resistant to de-compilation through protective means such as obfuscation.

Other Uses for de-compilation and disassembly

  • Understanding a program,
  • Recovering the source code for purposes of archiving or updating,
  • Finding virus,
  • Debugging programs,
  • Translating obsolete code.


De-compilation fail rezones

  • It is not possible to decompile all programs,
  • Data and code are difficult to separate,
  • Both data and code are represented similarly in most current computer systems.
  • The meaningful names that programmers give variables and functions (to make them more easily identifiable) are not always stored in an executable file, so they are not always recovered in de-compiling.


De-compiling as cracking method

  • Although disassembler is well known as a cracking method but decompiling is much power.
  • Disassembler normally attacks the program that is running on the system.
  • Decompiling is mostly done on none running application, it could cover the whole application,
  • It will convert the program to its own programming language,
  • The resulting code is easy to be edit and patched.
  • Cracker search for the error message that may your application show where pad registration or activation code.
  • Then they will search for the code use this string and try to understand it and edit as required.
  • Decompiling take much time and work than other cracking methods, but it give more flexible result.


Protecting from unlocking codes

Bad Protection Codes

Direct input compare

If input_serial_nimber = real_serial_number Then 
	MsgBox("Wrong serial number")
End if

To bypass this code

  • Run the debugger such as OllyDbg
  • Open the program to cracked in the debugger
  • Rub code until the serial is requested.
  • Put a wrong serial
  • Step with the code until you find the assembly compare code that take the serial you entered
  • Reverse the comparison

Software debugging prevention

A lot of new software have a debugger detection techniques and will act by deferent way if the debugger is detected. Debuggers are the most used technique for cracking; many crackers will not try to de-compile your code. Putting anti debug code will increase your software protection even if you don't want to use obfuscation. Combining obfuscation and debugging prevention the right work. Many crackers use de-compiling before debugging. Without obfuscation they can jump to another place in code and go around the debugger check.

[DllImport("kernel32.dll", SetLastError = true, ExactSpelling = true)]
static extern bool CheckRemoteDebuggerPresent(IntPtr hProcess, ref bool isDebuggerPresent);
public static void Test()
    bool isDebuggerPresent = false;
    CheckRemoteDebuggerPresent(Process.GetCurrentProcess().Handle, ref isDebuggerPresent);
    //if isDebuggerPresent then do something or raise an error

Software packing

Much software are packing the software code in special format and the application will unpack the code in the memory to prevent decompiling or debugging. Cracking will unpack the software 1st or debug or decompile the code in memory.

Obfuscation (Compiled code obfuscation)

Obfuscation Method

  • You develop your application as normal.
  • Run and debug your application
  • Build it with your compiler
  • Run a post compile tool to convert your redistributable release into obfuscation mode

Obfuscation aims

  • It make the de-compiling fail difficult and many de-compiler will fail to de-compile the application
  • Many de-compilers will refuse to decompile this file at all. To do it.
  • If the de-compilers succeed the resulting code will be harder to understand or analyzed

Obfuscation can be done or more of the following

  • Name obfuscation
    • This will delete the names of all functions, forms, modules and objects and all kinds of signatures from the EXE file.
    • The new name will be random and include nonprinting characters and symbols that can't be used in the programing language
  • String Encryption
  • Control Flow Obfuscation is about modifying the program so that it yields the same result when run, but is different flow to prevent decompiling into a well-structured source code.
  • Code Encryption and adding loader of your codes. The loader will decrypt the MSIL as needed.
  • Code Virtualization converts your MSIL code into Virtual Op codes that will only be understood by a Virtual machine that Obfuscator adds to your code. Code Virtualization can significantly degrade performance if used unwisely.
  • See List of obfuscators for .NET for more info.
  • For an of compiled none .Net code (ex. C++) obfuscator see Obfuscator for the x86 assembler

Example of de-compile code after obfuscation

loc_B6FE91: MemVar_F903A0 = &HFF
            If (MemVar_F9039E = &HFF) Then 'B70001
                  If MemVar_F903A0 Then 'B6FFDD
loc_B6FFD5:           Function_A15FF8()
loc_B6FFDA:     	    GoTo loc_B70001
                  End If
                  If Not(Function_A83D7C(1)) Then 'B70001
loc_B6FFED:            Function_ADC870(0)
                       If arg_8(206) Then 'B6FFFF
loc_B6FFFA:                Function_B07AC8()
                       End If
loc_B6FFFF:            End 'Application End
                  End If
loc_B70001:       ' Referenced from: B6FFDA
            End If
  • Cracker will look in the code and will see the application end code which may indicate test for unlocking
  • Cracker try to modify the code by replace End with nop on the address loc_B6FFFF
  • One of if statements may be changed to avoid the application end
  • De-compiler will generate random name for variables and function because obfuscation put non printing chars in names.

Obfuscation & Unlocking software

  • Good obfuscation makes many de-compiler fails to de-compile your code but power de-compiler will de-compile it.
  • The de-complied code will be hard to understood and analyze. and may required very hard work to understood who to unlock your application.
  • The good obfuscation makes the cost of unlocking your code much more than buying it.

Source code obfuscation

  • Many developer call it as unreal obfuscation because it deal with source code not with the compiled code
  • Some companies that ship cross-platform software prefer to ship it in the obfuscated source code form, expecting customers to build that software on any platform they wish.
  • Some language does not have compiler and your application will shipped in source codes forms such as PHP & Javascript.
  • Obfuscator making source code difficult to understand.

Stunnix Obfuscator is an example of Source code obfuscation and it will do the following

  • Replacing symbol names with non-meaningful ones, e.g. replacing list_of_customers with zcadaa4fc81
  • Replacing numeric constants with expressions, e.g. replacing 232 with (0x14b6+2119-0x1c15)
  • Replacing characters in strings with their hex escapes, e.g. turning string "cust" into "\x63\x75\x73\x74"
  • Unique! Renaming files and directories with source code, e.g. renaming /lib/context.c into /7a84b51/b4e8c5.c
  • Removing or obfuscation of comments
  • Removing spaces and tabs in the lines of code
  • Joining all lines in your code
  • They have separate obfuscator for C++, Javascript, VB Script & Prel.

C++ obfuscated codes

main(  argc,  char * argv[ ] ) 
  int j; 
  char version[ 80] ; 
  while ( ( j =  getopt_helper( argc,  argv,  "n:o:vV:",  
  ((char)(0x10b4+390-0x11d2)),  ((char)(0xeb+9378-0x2537))) )   !=  - 1)  { 
switch ( j)  { 
case ((char)(0x607+6157-0x1da6)): 
   name_wide =  MYMIN( atoi( optarg) , 0xff) ; 

PHP Obfuscators

What tools to be used to check the crack ability of may software?

What to do to protect my software

  • Build your application as 64 bit application, 64bit application is much harder to crack.
  • Encrypt error messages: for example see this link
  • Use debugger detection technique: see this Link
  • Use Computer ID: It is simple software key useful to identify user computer to be used for give separate license for each computer see: C++ simple software key
  • Use Obfuscation which can delete the names of all functions, forms, modules and objects and all kinds of signatures from the EXE file. It will make analyzing the program with this de-compiler difficult and many de-compilers will refuse to decompile this file at all. To do it. see Eazfuscator.NET
  • Strong-Naming Your Assemblies: By strong-naming your assembly you ensure the fact that the assembly was not altered since it was built.
  • lARP64Pro: It is a 64 bit anti-piracy and anti-cracking program with implemented code compression, it make the de-compiling  much difficult.
  • Soft Activate: It is tools provide protecting with license key and software activation
  • Dongle is not always the wright idea, it maybe increase the crack ability.
  • Try to crack your software.
  • Search the net for the crack of your software and ask search engine to remove the search result.
  • Have the checking logic run at many critical times (not just startup), and using different ways
  • Have the checking logic invert the conditional in some checking and direct in another.
  • Have the checking in different ways
  • Some checks were performed ONLY on a specific day of the week. And would tell them to call support.
  • When you detect a license failure, close the application in different ways each time, you could use direct close, application exit or raise an error to crash your application, or even go to un limited looping to make your application not stable
  • Finally, we would modify the EXE itself, so at the end of the actual EXE we would store a hash of the file, and check at run time that if there is a modification done to exe file.
  • If you have any other idea please set it in the comments, I will modify the article to include it.

Why cracker attack some software and leave another one

  • Cracking is not always for money, it may be as challenge or as punishment.
  • If you have a contact email to whom cracked your software; Write a message and ask politely, if they can stop cracking your software. They may do so.
  • If the software mentioned that it is hard to cracked then the cracker will attacked it, Normally one of the will succeed.
  • If you put a very high price or put a hiden fees or ask the users to pay a new license for formatting his hard disk or any bad practice; then your software may be attacked as punishment.


Please comment any useful data you want me to add it to the article.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Written By
Lebanon Lebanon
ASP.Net Hosting on Linux server
There is a developer behind every piece of code!
DNA is too complex what about it!
No junk DNA; There is a functional role for noncoding DNA

Comments and Discussions

QuestionDo you really think your software is worth cracking ? Pin
pkfox22-Aug-18 12:15
professionalpkfox22-Aug-18 12:15 
BugWord correction Pin
Chad3F22-Aug-18 10:55
Chad3F22-Aug-18 10:55 
GeneralRe: Word correction Pin
NewPast15-Mar-19 11:05
NewPast15-Mar-19 11:05 
QuestionVery nice article Pin
Alberto Bencivenni21-Aug-18 21:04
Alberto Bencivenni21-Aug-18 21:04 
QuestionHere's a nice trick to decrease the chance that someone can crack your code... Pin
Keith Vinson21-Aug-18 11:46
Keith Vinson21-Aug-18 11:46 
AnswerRe: Here's a nice trick to decrease the chance that someone can crack your code... Pin
Alberto Bencivenni21-Aug-18 21:04
Alberto Bencivenni21-Aug-18 21:04 
SuggestionNice Pin
wvd_vegt21-Aug-18 2:29
professionalwvd_vegt21-Aug-18 2:29 
GeneralRe: Nice Pin
Nelek21-Aug-18 9:17
protectorNelek21-Aug-18 9:17 
GeneralRe: Nice Pin
wvd_vegt21-Aug-18 9:23
professionalwvd_vegt21-Aug-18 9:23 
GeneralRe: Nice Pin
Nelek21-Aug-18 9:32
protectorNelek21-Aug-18 9:32 
PraiseNice article Pin
tranthanhtu.vn4-Jun-17 14:38
professionaltranthanhtu.vn4-Jun-17 14:38 
GeneralMy vote of 5 Pin
BytePower13-Oct-15 21:43
BytePower13-Oct-15 21:43 
AnswerRe: My vote of 5 Pin
NewPast13-Oct-15 22:07
NewPast13-Oct-15 22:07 
GeneralMy vote of 5 Pin
Franc Morales10-Oct-15 0:22
Franc Morales10-Oct-15 0:22 
AnswerRe: My vote of 5 Pin
NewPast10-Oct-15 1:13
NewPast10-Oct-15 1:13 
GeneralInteresting read Pin
Slacker0079-Oct-15 8:20
professionalSlacker0079-Oct-15 8:20 
AnswerRe: Interesting read Pin
NewPast9-Oct-15 19:46
NewPast9-Oct-15 19:46 
GeneralMy vote of 5 Pin
BytePower21-Sep-15 22:02
BytePower21-Sep-15 22:02 
AnswerRe: My vote of 5 Pin
NewPast21-Sep-15 23:40
NewPast21-Sep-15 23:40 
GeneralRe: My vote of 5 Pin
BytePower29-Sep-15 18:33
BytePower29-Sep-15 18:33 
GeneralRe: My vote of 5 Pin
NewPast29-Sep-15 22:00
NewPast29-Sep-15 22:00 
GeneralMy vote of 5 Pin
Code Artist13-Sep-15 3:40
professionalCode Artist13-Sep-15 3:40 
AnswerRe: My vote of 5 Pin
NewPast15-Sep-15 0:25
NewPast15-Sep-15 0:25 
QuestionWasting time to protect your software is not worth it Pin
N.Mayer10-Sep-15 0:49
N.Mayer10-Sep-15 0:49 
AnswerRe: Wasting time to protect your software is not worth it Pin
NewPast11-Sep-15 21:25
NewPast11-Sep-15 21:25 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Praise Praise    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.