Impressioning is the act of creating a fully working key using nothing but a file, a key blank, and keen observation. While standing on the secure side of a locked door, and having no knowledge of the actual working key whatsoever, it is often possible to fabricate a functional key by means of an attack that is difficult to thwart or even to detect.

For years, impressioning was practiced only by the most covert of government operatives or the most elite of locksmiths. Now, this tactic is taught in high-end physical security penetration classes, such as those offered by The CORE Group.