Polymorphic Code - Example

Example

This example is not a really polymorphic code but will serve as an introduction to the world of encryption via the 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.

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 e 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.