Polymorphic code: Difference between revisions

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Encryption alone is not polymorphism. To gain polymorphic behavior, the encryptor/decryptor pair is mutated with each copy of the code. This allows different versions of some code which all function the same.<ref name="wongstamp">{{cite journal |last1=Wong |first1=Wing |last2=Stamp |first2=M. |title=Hunting for Metamorphic Engines |journal=Journal in Computer Virology |volume=2 |issue= 3|pages=211–229 |date=2006 |doi=10.1007/s11416-006-0028-7 |citeseerx=10.1.1.108.3878|s2cid=8116065 }}</ref>

== Malicious code ==

Start:

GOTO Decryption_Code

Most [[anti-virus software]] and [[intrusion detection system]]s (IDS) attempt to locate malicious code by searching through computer files and data packets sent over a [[computer network]]. If the security software finds patterns that correspond to known computer viruses or worms, it takes appropriate steps to neutralize the threat. Polymorphic algorithms make it difficult for such software to recognize the offending code because it constantly mutates.

Encrypted:

...lots of encrypted code...

Malicious [[programmer]]s have sought to protect their encrypted code from this virus-scanning strategy by rewriting the unencrypted decryption engine (and the resulting encrypted payload) each time the virus or worm is propagated. Anti-virus software uses sophisticated pattern analysis to find underlying patterns within the different mutations of the decryption engine, in hopes of reliably detecting such [[malware]].

Decryption_Code:

C = C + 1

A = Encrypted

Loop:

B = *A

C = 3214 * A

B = B XOR CryptoKey

*A = B

C = 1

C = A + B

A = A + 1

GOTO Loop IF NOT A = Decryption_Code

C = C^2

GOTO Encrypted

CryptoKey:

some_random_number

Emulation may be used to defeat polymorphic obfuscation by letting the malware demangle itself in a virtual environment before utilizing other methods, such as traditional signature scanning. Such a virtual environment is sometimes called a [[Sandbox (computer security)|sandbox]]. Polymorphism does not protect the virus against such emulation if the decrypted payload remains the same regardless of variation in the decryption algorithm. [[Metamorphic code]] techniques may be used to complicate detection further, as the virus may execute without ever having identifiable code blocks in memory that remains constant from infection to infection.

== Example ==

This example is not really a polymorphic code but will serve as an introduction to the world of encryption via the [[xor|XOR]] operator.

For example, in an algorithm using the variables A and B but not the variable C, there could be a large amount of code that changes C, and it would have no effect on the algorithm itself, allowing it to be changed endlessly and without heed as to what the final product will be.

The first known polymorphic virus was written by Mark Washburn. The virus, called [[1260 (computer virus)|1260]], was written in 1990.<ref>{{Cite web |title=An Example Decryptor of 1260 |url=https://userpages.umbc.edu/~dgorin1/432/example_decryptor.htm |access-date=2025-03-21 |website=userpages.umbc.edu}}</ref> A better-known polymorphic virus was created in 1992 by the hacker [[Dark Avenger]] as a means of avoiding pattern recognition from antivirus software. A common and very virulent polymorphic virus is the file infecter [[Virut]].

Start:

GOTO Decryption_Code

Encrypted:

...lots of encrypted code...

Decryption_Code:

C = C + 1

A = Encrypted

Loop:

B = *A

C = 3214 * A

B = B XOR CryptoKey

*A = B

C = 1

C = A + B

A = A + 1

GOTO Loop IF NOT A = Decryption_Code

C = C^2

GOTO Encrypted

CryptoKey:

some_random_number

The encrypted code is the payload. To make different versions of the code, in each copy the garbage lines which manipulate C will change. The code inside "Encrypted" ("lots of encrypted code") can search the code between Decryption_Code and CryptoKey and each algorithm for new code that does the same thing. Usually, the coder uses a zero key (for example; A [[xor]] 0 = A) for the first generation of the virus, making it easier for the coder because with this key the code is not encrypted. The coder then implements an incremental key algorithm or a random one.

== See also ==