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In computer terminology, polymorphic code is code that mutates while keeping the original algorithm intact. This technique is sometimes used by computer viruses, shellcodes and computer worms to hide their presence.

Most anti-virus software and intrusion detection systems 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 locate the offending code as it constantly mutates.

Encryption is the most commonly used method of achieving polymorphism in code. However, not all of the code can be encrypted as it would be completely unusable. A small portion of it is left unencrypted and is used to jumpstart the encrypted software. Anti-virus software targets this small unencrypted portion of code.

Malicious programmers have sought to protect their polymorphic code from this virus-scanning strategy by rewriting the unencrypted decryption engine 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.

The first known polymorphic virus was written by Mark Washburn. The virus, called 1260, was written in 1990. A more well-known polymorphic virus was invented in 1992 by the Bulgarian cracker Dark Avenger (a pseudonym) as a means of avoiding pattern recognition from antivirus-software.

An algorithm that uses, for example, the variables A and B but not the variable C could stay intact even if you added lots of code that changed the contents of the variable C.

The original algorithm:

Start:
GOTO Decryption_Code
Encrypted:
    ...
    lots of encrypted code
    ...
Decryption_Code:
    *A = Encrypted
Loop:
    B = *A
    B = B XOR CryptoKey
    *A = B
    A = A + 1
    GOTO Loop IF NOT A = Decryption_Code
    GOTO Encrypted
 CryptoKey:
    some_random_number

The same algorithm, but with lots of unnecessary C-altering code:

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 code inside "Encrypted" ("lots of encrypted code") could then search the code between Decryption_Code and CryptoKey and remove all the code that alters the variable C. Before the next time the encryption engine is used, it could input new unnecessary codes that alters C, or even exchange the code in the algorithm for new code that does the same thing. Usually the coder uses a zero key for the first generation of the virus, making it easier for him because with this key the code is not encrypted. He then implements an incremental key algorithm or a random one.

Another polymorphism technique is to autoinject NOP (No Operation) or other opcodes that don't alter the algorithm.

• Timeline of notable computer viruses and worms
• Metamorphic code
• Self-modifying code
• alphanumeric code
• shellcode
• software cracking
• security cracking

• Diomidis Spinellis. Reliable identification of bounded-length viruses is NP-complete. IEEE Transactions on Information Theory, 49(1):280–284, January 2003. doi:10.1109/TIT.2002.806137