You have seen the screenshots: a card number with only the last four digits showing, or a contract with black bars over the salary. Both are ways of showing part of a document while hiding part of it.

The two techniques behind those pictures are masking and redaction, and they are not the same thing. Mixing them up is how hidden data ends up in files that were supposed to be clean.

A simple analogy

Think of a photo of a street scene. Masking is like blurring a face. The shape is still there, you can tell a person was standing on that corner, but you cannot name them.

Redaction is like cutting the face out and taping paper over the hole. The shape is gone too. A blur can sometimes be reversed with clever software; a cut cannot, because the information is no longer in the picture. That difference between "hidden" and "removed" is most of this subject.

What masking and redaction do

Masking replaces real values with altered ones that look similar.…