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Sanitization (classified information)
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Sanitization is the process of removing sensitive information from a document or other message (or sometimes encrypting it), so that the document may be distributed to a broader audience. When the intent is secrecy protection, such as in dealing with classified information, sanitization attempts to reduce the document's classification level, possibly yielding an unclassified document. When the intent is privacy protection, it is often called data anonymization. Originally, the term sanitization was applied to printed documents; it has since been extended to apply to computer media and the problem of data remanence as well.
Redaction in its sanitization sense (as distinguished from its other editing sense) is the blacking out or deletion of text in a document, or the result of such an effort. It is intended to allow the selective disclosure of information in a document while keeping other parts of the document secret. Typically the result is a document that is suitable for publication or for dissemination to others than the intended audience of the original document. For example, when a document is subpoenaed in a court case, information not specifically relevant to the case at hand is often redacted.
Government secrecy
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In the context of government documents, redaction (also called sanitization) generally refers more specifically to the process of removing sensitive or classified information from a document prior to its publication, during declassification.
Secure document redaction techniques
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A 1953 US government document 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
The traditional technique of redacting confidential material from a paper document before its public release involves overwriting portions of text with a wide black pen, followed by photocopying the result—the obscured text may be recoverable from the original. Alternatively opaque "cover up tape" or "redaction tape", opaque, removable adhesive tape in various widths, may be applied before photocopying.
This is a simple process with only minor security risks. 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.
The UK National Archives published a document, Redaction Toolkit, Guidelines for the Editing of Exempt Information from Documents Prior to Release, "to provide guidance on the editing of exempt material from information held by public bodies."
Secure redacting is a far more complicated problem with computer files. Word processing formats may save a revision history of the edited text that still contains the redacted text. In some file formats, unused portions of memory are saved that may still contain fragments of previous versions of the text. Where text is redacted, in Portable Document Format (PDF) or word processor formats, by overlaying graphical elements (usually black rectangles) over text, the original text remains in the file and can be uncovered by simply deleting the overlaying graphics. Effective redaction of electronic documents requires the removal of all relevant text or image data from the document file. This either requires a very detailed understanding of the internal operation of the document processing software and file formats used, which most computer users lack, or software tools designed for sanitizing electronic documents (see external links below).
Redaction usually requires a marking of the redacted area with the reason that the content is being restricted. US government documents being released under the Freedom of Information Act are marked with exemption codes that denote the reason why the content has been
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