codec writing

[MiKiOnE]

Hi all.
I'm have binary protocol with fixed sizes of headers (2 types)
the protocol have 2 types of packages enrypted by two diferent algorythms
seciefed by first byte of header
The first bbyte also define the size of Lenght secction in header
[Head|size|package spec][body]
My question is what base use to defragment the protocol and decrypt it after
defragmentation?