ISO 26430-3:2008 presents a specification for a generic extra-theater message (ETM) format for use with unidirectional communications channels used in security communications for digital cinema systems.

The ETM specification is a generic extensible markup language (XML) security wrapper that includes specific fields which can be extended to carry different kinds of information to meet various application-level requirements. The ETM uses W3C XML to represent the information payload. It provides security using the XML encryption and signature primitives.