Encoding MAC-Related Words

Words that are not directly used for MAC, yet imply the presence of a compartment or other MAC word, are encoded in the information labels: section using both compartment and marking bits. This situation typically occurs when there are multiple words, sometimes called codewords, associated with a compartment. In such a case, users are cleared for the compartment as a whole, not for the individual codewords. However, the presence of the codeword in an information label implies that the data is in the compartment. In such a case, the codeword must have a compartment bit associated to identify the compartment, but must additionally have one or more marking bits associated to distinguish the word as a codeword (as opposed to a MAC word) and to differentiate among the multiple codewords. An example of this case appears in Appendix B, Annotated Sample Encodings with the words alpha1, alpha2, and alpha3. All three words are associated with compartment bit 0 (and hence the compartment A), but additionally have marking bits associated. This particular pattern of marking bits determines which of the three codewords are present.

It is also possible to encode MAC-related words in the PRINTER BANNERS: section if desired. There is no such example in Appendix B, Annotated Sample Encodings.