The decoder stub is a small chunk of instructions that is prepended
to the encoded payload. When this new payload is executed on the target
system, the decoder stub executes first and is responsible for decoding the
original payload data. Once the original payload data is decoded, the decoder
stub passes execution to the original payload. Decoder stubs generally
perform a reversal of the encoding function, or in the case of an XOR
obfuscation encoding, simply perform the XOR again against the same key value.