Oligomorphism vs polymorphism vs metamorphism in malware

Question

What is the difference between these three? As I understand it, polymorphic malware code actually is always the same, and it's just that it appears to be different (through encryption, data appending/pre-pending), while in metamorphic malware code the code itself gets different each time it propagates (through adding useless instructions, function reordering, loops etc.)? Am I right about these two?

And how about oligomorphic code? It is like a simpler version of polymorphic code but I don't exactly get it in which way it is simpler, so could someone please explain it a bit more?

Answer 1

Polymorphic code: The (same) code takes many forms (like encryptions)
Oligomorphic code: The (same) code takes one of a few predefined forms (and thus can be possibly matched against signatures that can cover all cases)
Metamorphic code: The code mutates, so the code itself is different in each execution (but the functionality the same)