a virus code (x bytes long) is XORed with a x bytes long word (we call this T). This word T is built repeating multiple times a 8 byte secret key.
the infected file contains this XORed (obfuscated) virus code, the 8 bytes secret key, and a loader. We don't know the location of those 3 entities inside the file. The loader, when executed, uses the secret key to build T and extract the virus code.
If I have the original virus code (but not the loader nor the secret key), how do I determine if a file is infected?