Abstract: A `Eurocylinder` configuration for an electronic lock and key system has a plug fitted into a main body housing which is readily mounted into a mortise of standardized dimensions. The plug has a keyway at one end and a deadbolt-engaging cam at the other end. To operate the lock, an electronic key is inserted into the keyway and, when given permission as a result of an opto-electronic communication exchange between a control processor in the key and a control processor in the lock, the key is rotated so as to operate a deadbolt. A spring biased translatable contact serves as a conductive interface between a flex circuit and a contact on the keyblade. The body also includes a solenoid device which is energized by way of the flex circuit. The solenoid device operates a plunger away from the plug to allow rotation of the plug by the key. The body and plug also include a segmented pin arrangement such that when the key and the plug are rotated from a key insertion position, the key cannot be removed.