How you inject codes?

[Hero]

Hi all
I write a small program that inject codes in other programs.Normally I have no
problem with small codes(I will make a routine and write my codes in it in asm
then inject it with coping routine codes in my target address).
But what can I do if my injecting code is too big?
For example,I want to inject an encryption algorithm into my target program,
and I can't find its asm implementation...
Is there any simpler way for this,except writing all of it in asm myself???

sincerely yours