@Joe Riel 

cl:=proc(l, shift)
local n,el,mn:
n:=L_T_I(l)+shift:
mn:=modp(n,26);
el:=I_T_L(mn);
return(el);
end proc; 



with lti being

L_T_I:=proc(L)
convert(L, 'bytes')[1]-96;
end proc;


and itl being

I_T_L:=proc(n)
convert([n+96],'bytes');
end proc;


I have tried it with the word shift not being the same in both procedures. This is for a project I don't want to do it I just have to, I'm a total beginner I don't know how to answer your question, oops!