Impressioning: Fitting a Key to a Lock Without Disassembly

Definition

The Lock Industry Standards and Training (LIST) Council's Professional Locksmith Dictionary defines the impression technique as "a means of fitting a key directly to a locked cylinder by manipulating a blank in the keyway and cutting the blank where the tumblers have made marks." An impression, in the same dictionary, is either the mark a tumbler leaves on a key cut, or the act of fitting a key that way. An impressioning tool is a holder for a blank, or a tool that holds a material that takes marks easily.

The definition stops at that concept. How marks are made, read, or cut is outside the scope of this page.

Impressioning, Decoding, and Picking

Three neighboring terms are easy to confuse:

• Picking aims to operate the lock without its key. See lockpicking.
• Decoding, in ANSI/BHMA A156.30, is "obtaining the dimensions of a key or cylinder without disassembling the cylinder." Decoding produces information; impressioning produces a key. See decoding.
• Impressioning fits a blank to the lock so that the blank becomes a working key.

A156.30 tests resistance to decoding separately from resistance to picking. Impressioning sits next to both: it uses the lock's own feedback to shape a key, and standards treat it as its own covert-entry concern.

Legitimate Use and Locksport

Locksmiths use impressioning when no key is available and the lock should be preserved for continued use, or when a key must be originated from the lock itself. TOOOL Netherlands's Blackbag reports on the Dutch Open impressioning competition at LockCon: "The challenge in impressioning is to make a working key by reading the feedback from the lock and adjusting the key accordingly." In 2024, forty people entered. The same club's ethics page requires permission: members are "to only open locks that you have permission for to open from the owner."

How Standards Account for It

Lab standards treat impressioning as a timed covert-entry method.

• UL 437 (Key Locks). A 2007 DEF CON presentation by Marc Weber Tobias and Matt Fiddler summarizes UL 437 as including timed covert-entry tests for picking and impressioning (listed at ten minutes each in that summary), alongside forced-entry tests.
• ANSI/BHMA A156.30 (High Security Cylinders). The 2014 edition defines decoding separately and sets pick-resistance requirements that include detainers "that have false notches, mushroom shapes or other constructions that impede manual picking." It cautions that its tests "are performed under laboratory conditions."
• EN 1303:2026 (Europe). The 2026 edition of the European cylinder standard adds "requirements and evaluation methods for manipulation," defined as an unauthorized attempt to open a cylinder by non-destructive techniques.

A rating describes a defined test. It is not a guarantee.

Defenses at the Concept Level

High-security design raises the cost of impressioning without claiming to eliminate it.

• Restricted keyways and blanks. The LIST dictionary defines a high-security cylinder as one offering greater resistance to two or more of picking, impressioning, key duplication, drilling, or other forced entry. Controlling which blanks exist is part of that picture. See key control and restricted keyways.
• Security pins and sidebars. Spool, mushroom, and step pins (LIST) and sidebar mechanisms add feedback that is harder to read as a simple mark. See security pins and sidebar locks.

Ethics and Law

Authorized impressioning for a lockout or key origination, with the owner's permission, is a legitimate locksmith service. Unauthorized use is a crime. California Penal Code § 466 makes possessing listed instruments "with the intent to feloniously break or enter" a misdemeanor, and covers knowingly making or altering a key to open a lock without being asked by someone with the right to open it. Laws vary. This article gives no legal advice.

Related Topics

• Decoding
• Lockpicking
• Key control
• Restricted keyways
• Security pins
• Locksport
• High-security locks
• Forensic locksmithing

Sources