Unconditionally secure codes using classical effects (non-quantum) can only be done by the so-called "one time pad". Assume that the message M you want to encode is a bit string of length n. Then the sender and the receiver need to share a secret code word C which is also a bit string of length n. The encoding is E=M+C where the bits are added modulo 2. Now E is sent from sender to receiver. The decoding is M=E+C. The method works because C+C=0 modulo 2.