#OK to post homework
#Joseph Koutsoutis, 03-31-2024, Assignment 18

read `C18.txt`:

#1

MakeCubic := proc(L,n):
 if nops(L) <> 20 then return(FAIL): fi:
 return(BinToIn([op(1..5,L)])+
        BinToIn([op(6..10,L)])*n+
        BinToIn([op(11..15,L)])*n^2+
        BinToIn([op(16..20,L)])*n^3):
end: 


#2

EncodeDES := proc(M,K,r) local i,M_enc:
 if nops(M) mod 2 <> 0 then return(FAIL): fi:
 if nops(K) <> 20*r then return(FAIL): fi:

 M_enc := M:
 for i from 1 to r do:
  M_enc := Feistel(M_enc, n -> MakeCubic(K[(20*(i-1)+1)..(20*(i))], n)):
 od:
 return(M_enc): 
end: 


#3

DecodeDES := proc(M,K,r) local i,M_dec:
 if nops(M) mod 2 <> 0 then return(FAIL): fi:
 if nops(K) <> 20*r then return(FAIL): fi:

 M_dec := M:
 for i from r to 1 by -1 do:
  M_dec := revFeistel(M_dec, n -> MakeCubic(K[(20*(i-1)+1)..(20*(i))], n)):
 od:
 return(M_dec): 
end: