A security mechanism employed in broadband passive optical network (BPON). Downstream transmissions are encrypted through the use of a byte-oriented churn key exchanged between the optical line terminal (OLT) at the network headend and the optical network unit (ONU).The ONU or optical network terminal (ONT) at the customer premises generates the key and sends it to the OLT, which uses it to encrypt downstream transmissions.The key is changed at least once a second, hence the term churning. See also BPON, byte, downstream, encryption, headend, OLT, ONT, ONU, and PON.