Crypters are computer applications which are solely used to bypass the antivirus detection of malwares. Hackers use crypters to hide viruses, Trojans, RATS, keyloggers and other hacktools into a new executable, whose sole purpose is to bypass the detection of the same from antivirus. Crypters are basically dead programs which does not affect the actual functionality of the program, they just spoof the actual program behind their encryption and make antivirus fool. Most antivirus detects viruses on basis of heuristics and normal string based detection. Since we have spoofed the original program, so antivirus stand lame and does not detect it as virus.

Common terms related to crypters:

For understanding and designing crypters, hackers must be aware of certain terms, most of you already know these terms, but as i am writing this tutorial starting from novice level and take it to elite level at the end. So if you knowthese terms just read them one more time, as that might help you to clear some of your doubts.

1. FUD or UD : Fully undetectable(FUD) means that your virus is not detected by any of the existing antiviruses while undetectable(UD) means detectable by few antiviruses. FUD is our only goal and elite hackers always rely on that. 

Note: Crypter will remain FUD until you have openly shared on internet. Public crypters remains FUD up to maximum 2 to 3 days then they become UD. So if you want to use crypter for long time so never publish and share that on internet. Use it anonymously.

2. STUB : A stub is a small piece of code which contains certain basic functionality which is used again and again. It is similar to package in Java or simply like header files in C ( which already has certain standard functions defined in it). A stub basically simulates the functionality of existing codes similarly like procedures on remote machines or simply PC's. In crypters, client side server is validated using stubs, so never delete stub file from your crypter. Stubs adds portability to crypter code, so that it can be used on any machine without requiring much procedures and resources on other machines.

Let me explain with small example:

Suppose you are writing a code that converts bytes to bits, so we know formula or method for converting bytes to bits will remain same and it will be independent of machine. So our stub (or method stub or procedure) will contain something like this:

BEGIN
    totalBits = calculateBits(inputBytes)
    Compute totalBits = inputBytes * 8
END

Now what we will pass is only number of bytes to this stub. And it will return the resulting bits. Similarly, we include some common machine independent checks and functions in our stub, and in main code we only passes linkage and inputs to these stubs, which in return provides suitable results.

Note: Most of times it happens, suppose you downloaded some keylogger and you complain to provider its not working, only reason for that is stub. Also always kept in your mind, if you are downloading any keylogger or crypter  always check stub is present in it. If not, don't download it, its just a piece of waste and for sure hacker is spreading his virus using that. I recommend that never download any hacking tool on your real machine, always use virtual machine or sandbox to test hack tools.

3. USV: Unique stub version or simply USV is a part of crypter that generates a unique version of stub which differentiates it from its previous stub, thus makes it more undetectable against antiviruses. For detecting this antivirus companies has to reverse engineer your crypter stub, that is not that easy to do, so it will remain undetectable for long time. This consist of one most important component USG ( unique stub generation) which is the actual part of crypter that encrypts and decrypts the original file means its the heart of your algorithm and i will recommend never write this part in stub, rather include this part in main code. Why i am saying this, stub is part of code which is shared with victim, so it will become public and hence your Crypter will not remain FUD for much long time.

Different types of crypters:

1. External Stub based crypters : This category consists of public crypters (those you have downloaded till date :P (noobish one's) and you complains to provider that its detectable by antiviruses. That really foolish complaint, if crypter is public then it can never remain FUD. So don't ever complain to me also after my next article for such noobish things. Ahahah.. i got deviated for real thing.

External Stub based crypters are those crypters in which most of the functionality of the crypter depends of external stub, if your delete that stub file, your crypter is useless. :P Most antivirus only do that. These type of crypters contains two files one is client.exe and other is stub.exe . Stub contains the main procedures and client contains the global functions that call those procedures.

2. Internal or Inbuilt stub based crypters: The crypters that contains only one exe file (i.e client) fall under this category. This client file has inbuilt stub in it. You can separate stub and client part here too using RCE (reverse code engineering) but it is not recommended.

Note: External or Internal stub doesn't make much difference as antivirus detects files on the basis of strings related to offsets. Whenever you reverse engineer any application or program, the program execution flow will gonna remain the same but offsets may change. USV will come into picture at this point. If you include your encryption algorithm separately then it will be more harder for antivirus to detect your crypter.

3. Run time crypters: Run time crypters are those crypters which remain undetected in memory during their execution. We are looking for these type of crypters only. :P These can any of the two above.

4. Scan time crypters: Those crypters which will remain undetected while encrypting the files but will become detectable when resultant file is generated. :P Fking one's that wastes all effort we have put. This really annoys everything is working fine and at last you get your file being detected by noob antiviruses.