  This paper concerns with the construction of codes over $GF(2)$ which reach
the max-flow for single source multicast acyclic networks with delay. The
coding is always a bitwise XOR of packets with equal lengths, and is based on
highly symmetrical and balanced designs. For certain setups and parameters, our
approach offers additional plausible security properties: an adversary needs to
eavesdrop at least max-flow links in order to decode at least one original
packet.
