Redaction

For example, if the black pen or tape is not wide enough, careful examination of the resulting photocopy may still reveal partial information about the text, such as the difference between short and tall letters.

The exact length of the removed text also remains recognizable, which may help in guessing plausible wordings for shorter redacted sections.

Where computer-generated proportional fonts were used, even more information can leak out of the redacted section in the form of the exact position of nearby visible characters.

This process, internally complex, can be carried out very easily by a user with the aid of "redaction" functions in software for editing PDF or other files.

US government documents released under the Freedom of Information Act are marked with exemption codes that denote the reason why the content has been withheld.

The US National Security Agency (NSA) published a guidance document which provides instructions for redacting PDF files.

In some contexts (notably the US NSA, DoD, and related organizations), "sanitization" typically refers to countering the data remanence problem.

The published version of the report was in PDF format, and had been incorrectly redacted by covering sensitive parts with opaque blocks in software.

The Challenge of Multilevel Security gives an example of a sanitization failure caused by unexpected behavior in Microsoft Word's change tracking feature.

In both of these cases, the redacted material still exists in the document underneath the visible appearance and is subject to searching and even simple copy and paste extraction.

A 1953 US government document on Project MKUltra that has been redacted prior to release.
A heavily redacted page from a 2004 lawsuit filed by the ACLU — American Civil Liberties Union v. Ashcroft
A page of a classified document that has been sanitized for public release. This is page 13 of a U.S. National Security Agency report [1] Archived 2004-03-13 at the Wayback Machine on the USS Liberty incident , which was declassified and released to the public in July 2003. Classified information has been blocked out so that only the unclassified information is visible. Notations with leader lines at top and bottom cite statutory authority for not declassifying certain sections. Click on the image to enlarge.