From the Globe and Mail:

The Department of National Defence has produced a guide instructing the military how to justify its arguments for censoring records requested by the public and journalists.

The Guide on Protection of Operational Security Information was distributed in January across the Forces and National Defence, a department in a constant struggle with journalists over how much should be made public about the war Canada is fighting in Afghanistan.

The guide lays out myriad ways that the federal law allowing censorship of government documents, the Access to Information Act, can be used to withhold information.

Examples include the excuse that records "would offend other nations or citizens of foreign countries" or identify "deficiencies in Canada's ability to defend against a threat." ...

The guide explains how even innocuous information can be withheld on the grounds its release would help piece together a larger picture of military operations, producing what it calls a "mosaic effect" that causes harm to Canada.

The Canadian Forces has often cited "operational security" when making its case for the censorship of records to the Access to Information directorate at National Defence, an office that functions as a chief information gatekeeper for public document requests.

But there's no definition of "operational security" in the Access to Information Act. That means the military must find other justifications in the access law when it argues its case before National Defence's access to information co-ordinator, who controls what is released to the public.