A lock includes a stationary key mass having a first key mass bore and a keyway, and a movable spring mass including a first spring mass bore. A first spring mass pin is mounted in the first spring mass bore, and a first key mass pin is mounted in the first key mass bore for reciprocation between a locked position immobilizing the spring mass and an unlocked position unconstraining motion in at least one direction. The unlocked position is a function of the presence of a matching key in the keyway. Multiple key mass and spring mass pins can be used, some or all of which can have stepped configurations, and corresponding single-bladed or multi-bladed keys, or multiple keys, can be used. The keys can be straight or curved in any of various planes, or they can be straight but flexible to match curved keyways. Curves can be unidirectional or compound. Relative motion between the key mass and spring mass can be in a plane or along straight line or circular.