The example said it itself "This is not really a polymorphic code [sic]". It was unreferenced and didn't describe anything resembling polymorphism in computer viruses, probably OR. No value was provided by keeping it.
The first known polymorphic virus was written by Mark Washburn. The virus, called [[1260 (computer virus)|1260]], was written in 1990. 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]].
== 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.
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.
