In wake of TSA breach, a refresher on redacting PDFs

What works on printed documents doesn't doesn't translate to digital

News that the Transportation Security Administration (TSA) accidentally posted secret information detailing its airline screening practices may have had a familiar ring to feds. The information was exposed because of inadequate redaction procedures.

TSA’s operating manual had been posted on a procurement Web site in the spring in redacted form. But anyone who copied the document and pasted it into another format, such as Microsoft Word or Windows Notepad, could read the redacted sections. Some of those sections included the settings for X-ray machines and explosives detectors, as well as procedures for dealing with diplomats, CIA employees and law enforcement officers.

Information breaches due to improper redaction are not new. In 2005, the Multi-National Force-Iraq ran into a similar problem when a memo with redacted classified information about a shooting was posted on the Web. The classified information, however, wasn’t actually redacted so much as blacked out, and the information could be revealed by copying and pasting it into a different format.

The White House, Justice Department and United Nations also have encountered similar slip-ups.

In wake of those embarrassments, the National Security Agency issued guidance to federal agencies, titled “Redacting with Confidence: How to Safely Publish Sanitized Reports Converted From Word to PDF.”

In the guidance, NSA identified the three most common mistakes analysts make in redacting documents intended for the Web, all of them essentially the result of thinking that what works for a print copy works for a digital copy. The three most common mistakes:

  • Covering text, charts, tables, or diagrams with black rectangles, or highlighting text in black. most common mistake is covering text with black (or changing the background to black).
  • Covering up parts of an image with separate graphics such as black rectangles, or making images “unreadable” by reducing their size. As with text, this works only on printed copies.
  • Failing to remove metadata and documents properties, which is often as sensitive as the original document; its presence in downgraded or sanitized documents has historically led to compromise.

A few tips NSA offers on how to properly redact a document:

  • Save a copy of the original document; make changes to the copy and keep the original.
  • Delete, rather than black-out, sensitive text, diagrams, tables and images.
  • Turn off track changes, comments and other visible markups, which can contain potentially compromising hidden data.
  • Rename the document to show that manual redaction is complete.
  • Create a new Word document, and copy and paste the edited text.
  • Convert a Word document to PDF and review final output for missed redactions or formatting issues.

Metadata and recorded, but often not visible, changes to a document are potential dangers because they often go unnoticed by the user. Knowing how to find that data is the key to removing it.

About the Author

Kevin McCaney is a former editor of Defense Systems and GCN.

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