This invention relates to a security system including a group of electrical security circuit, components and to a method of potting the group. O/89/09708 discloses a security system, particularly for a motor vehicle, which comprises an electrical security key usable to make circuits which, either directly, or through relays controlling other circuits, render operable electrical devices necessary for the starting of the engine of the vehicle by a conventional ignition key and switch.

To confuse a person attempting to breach the security, conductors necessary to supply power to the devices are made discontinuous within a wiring loom of the vehicle and other conductors are tapped-in at sides of the discontinuities and extend to relay contacts operable by the use of the electrical security key. Further to confuse such person, further conductors are connected to other relay contacts but have no operating function. Electrical components of the security system, such as the relays, are mounted upon a printed circuit board from which the conductors extend.

The printed circuit board is mounted in a housing fixed to a suitable surface of the engine compartment of the vehicle. Although the printed circuit board and adjacent portions of the conductors in the form of electrical wires are potted in resin in the housing, it is desirable to improve the security of even such a secure system.

GB-A-1448674 discloses an encapsulated component assembly consisting of a television camera tube and a hollow cylindrical focus and deflection coil mounted within a protective metal cylindrical casing. Surrounding one end of the camera tube is a first helical coil the turns of which are in contact with the surface of the camera -tube. Adjacent turns of the helical coil do not touch, but are spaced slightly apart. A second, similar helical coil surrounds the other, larger diameter end of the camera tube and a third helical coil surrounds the focus and deflection coil. The turns of the third helical coil are in contact with the outer cylindrical surface of the focus and

1 SUBSTITUTE SHEET

deflection coil. The helical coils are each composed of a monofilament nylon thread. An encapsulant, in this case a silicone rubber, fills the otherwise unoccupied space within the casing, the ends of the helical coils respectively protruding from the encapsulant. Electrical leads to the focus and deflection coil extend through the encapsulant to allow connection to be made thereto.

A component may be removed from the encapsulated component assembly by pulling gently on the free end of a helical coil. By this means the structure of the encapsulant in the regions between the turns of the helical coil is sheared as the coil is unravelled. When the helical coil is completely removed the cylindrical space so formed around the component allows it to be easily removed without damage. It will be appreciated that the system is not designed to enhance the physical security of the assembly.

GB-A-2237937 discloses an electric switch for use in hazardous or potentially explosive atmospheres, comprising a solid state switching circuit embedded in a body of foamed plastics material, which fills an hermetically sealed plastics housing which is of rectangular cross-section internally and externally. Flameproof cable connectors are sealed to the housing for connection to incoming and outgoing electrical power cables and further cables are sealed to the housing and lead to a start/stop controller and a load, respectively. Again, the system is not designed to enhance security.

EP-A-0268142 discloses a tamper-resistant package for protection of information stored in electronic circuitry.

The. package includes apparatus for distributing electro¬ magnetic energy within a region occupied by the circuit to be protected. A sensing arrangement senses the distribution of the energy. In a preferred embodiment, electrical current is distributed through a coil or coils within which the circuit to be protected resides. The sensor includes an integrating circuit, and the output of the integrating circuit is presented to both a sample and

2 SUBSTITUTE SHEET

hold circuit and to a comparator. The comparator has high and low reference input terminals which are fed from a voltage divider to which is also applied an output from the sample and hold circuit. The high and low reference inputs to the comparator are, by reason of the sample and hold circuit, adaptive. The comparator produces an output if its input is outside the limits established by the high and low reference inputs; that output of the comparator is taken as evidence of tampering. An impairing device responds to the comparator output to obliterate, destroy or otherwise impair the information stored in the electronic circuit which is being protected.

In an example, the electronic circuitry is in the form of chips mounted on an electronic circuit card. Printed circuit wiring inter-connects the chips and connects these chips to an off-card connector. The stored information is accessible through the connector. It is intended that nothing should be capable of intruding within a protected space within which the card is contained. The protected space is in an oblong rectangular enclosure.

Provided in the enclosure and for sensing the presence of an intrusion is a closely spaced coil of insulated conductor. The coil is sufficiently tightly wound so that the presence of the coil itself physically prevents access to the card. Any attacker then is forced to displace one or more turns of the conductor so as to obtain access, but the apparatus is arranged to inhibit or prevent displacing the conductor or at least sense any attempt to displace the conductor. In a preferred embodiment the conductor is formed of insulated nichrome wire which is relatively thin, for. example about 0.089mm. (35/10 , OOOths of an inch) . Thus, the conductor is relatively fragile so many attempts to displace it will result in actually breaking the conductor, which is a basis for detection. It is conceivable that an attacker would attempt physically to remove portions of the conductor and fool the protection system into thinking the conductor was present by bypassing or simulating the effect of the portion of the conductor

3 SUBSTITUTE SHEET

which had been removed. Use of the relatively t.iin nichrome wire renders this effort difficult to accomplish since the nichrome wire is difficult to attach to.

The resistance of the conductor can vary by one of three physical mechanisms; the resistance will vary if the conductor becomes open-circuited, or if adjacent or closely adjacent turns are shorted to one another, or if the conductor is damaged with the effect of reducing the cross- section. An intrusion is implied on detection of such a circumstance.

To enhance further the protection provided, after the conductor is wrapped, the entire package, e.g. the card and the coil along with the sensing circuit, is potted preferably in an opaque, cross-linked filled epoxy. Filler materials such as alumina or silica may be employed, to make machining (a process that might be used by an attacker) difficult. The off-card connector projects from the epoxy block at a location wherein an edge of the card is at a boundary of the block. Randomness in the placement of the conductor is also desirable. This minimizes the intelligence a pirate can achieve by "sacrificing" a unit for measurement purposes and then employing that information in attacking a second unit.

By using a slightly loose wrapping tension combined with the small size of the selected wire and the opacity of the epoxy, there is randomness in the position of individual turns. This requires the pirate to work slowly if trying to work mechanically into the epoxy but yet stop before damaging one of the wires. The sensing arrangement is arranged to detect the breaking or merely damaging of a wire (causing the opening of the circuit or an increase in resistance) or shorting different points on the wire (since the resulting change in resistance will be detected) .

There is wide latitude in selecting the manner in which the strands are wound. Close winding can be employed; close winding is a conventional technique which produces a smooth even wrap, that is, the turns are each regular and essentially parallel to each other. On the

4

SUBSTITUTE SHEET

other hand, a scramble winding technique could be employed to produce an irregular winding.

Further advantage is gained by wrapping or coiling the conductor about more than one axis, that is a first layer of the conductor is wrapped about one axis, a second layer wrapped about a different axis, etc. For example, at least two axes are used which are mutually perpendicular. In wrapping the conductor about the card, a completely enclosed volume is produced so that there are no "holes" through which small probes or other intrusions could be inserted. Thus, beyond the extremities of the card, a single winding layer will abut to "close the ends".

In spite of being very complex, particularly in having an electrical security circuit to protect the electronic circuitry, the system has a number of security weaknesses. In particular the projection of the off-card connector from the epoxy block and the positioning of an edge of the card at an edge of the block immediately gives the attacker a good idea of the position and orientation of the card and the protective coil(s) in the block.

According to one aspect of the present invention, there is provided a security system including a group of electrical security circuit components, a body of potting material containing said components and a housing fittingly receiving said body, characterized in that said housing has circular-cylindrical, co-axial, internal and external surfaces.

Owing to this aspect of the invention, a thief cannot ascertain the orientation of the group of components within that housing by simply looking at the housing.

It is also advantageous if the group of components is located relatively randomly in the housing prior to potting.

According to a second aspect of the present invention, there is provided a method of potting a group of electrical security circuit components, comprising locating said group of components in a housing, introducing potting material in a flowable condition into said housing and around said group of components, and then causing or allowing the potting

SUBSTITUTE SHEET

material to solidify, characterized in that said locating is performed relatively randomly.

Owing to this aspect of the invention, a thief cannot ascertain the orientation of the group of components by looking at the housing.

It is particularly advantageous if the housing has a circular-cylindrical internal surface, because then the group of components is more readily located randomly in the housing than if the housing were of, for example, rectangular cross-section, when the components could tend to orientate themselves more often with reference to the corners of the housing. It is further advantageous if the housing has a circular-cylindrical external surface co-axial with the internal surface. The group of components may be mounted upon a printed circuit board and with it constitute a unit contained within the body of potting material. It is advantageous if the printed circuit board and terminals whereby electrical wires are connected to the components are spaced inwardly from all of the boundaries of the potting body, because then a thief cannot ascertain the orientation thereof by inspecting the boundaries of the potting body.

According to a third aspect of the present invention, there is provided a security system including a group of electrical security circuit components, a body of potting material containing said components, and electrical wires having portions thereof in said body and extending at substantially all six sides of said group of components, characterized in that said wires are electrically connected to said components.

It is particularly desirable that the wire portions should extend at all six sides of the group of components.

Owing to this aspect of the invention, particularly the use of the electrical wires not only for the purpose of connection to the components but also for the purpose of protecting those components against intrusion, it is possible very simply to ensure that a thief attempting to remove, for example by drilling-out, part of the potting

SUBSTITUTE SHEET.

material in order to obtain access to at least one of the components is liable to cut at least one of the electrical wires and thereby cause a malfunction.

In order that the invention may be clearly understood and readily carried into effect, reference will now be made, by way of example, to the accompanying drawing, which shows a fragmentary, diagrammatic, perspective view of a control device of a security system with parts cut away for ease of understanding. The security system may be of the character disclosed in WO/89/09708, the control device being mounted in the engine compartment of the vehicle.

Referring to the drawing, the device comprises a printed circuit board 1 with various electrical components 2 mounted thereon and having electrical supply and signal conductors in the form of wires 3 extending therefrom. Portions of the wires 3 extend at all six sides of the unit consisting of the printed circuit board 1 and the electrical components 2. In practice, there are many more wires 3 than are shown in the drawing, so that all six sides of the unit 1,2 will be fairly well covered with wire portions. It will be noted from the drawing that portions of the wires 3 are deliberately brought forward (as seen in the drawing) from the printed circuit board 1 so as to loop over the components 2. Alternatively to the arrangement of the wire portions as shown, they may instead be wrapped as a bundle a number of times around the unit 1,2 before extending along a housing 5 away from the unit. The board 1, the components 2 and the relevant portions of the wires 3 are potted in a body 4 of resin within the housing 5. The housing 5 has circular-cylindrical, co-axial, internal and external surfaces 6 and 7 each of constant radius and, of course, fittingly receives the body 4. The board 1, the components 2 and the terminals whereby the wires 3 are connected to the components 2 are spaced inwardly from all of the boundaries of the body 4.

In assembling the device shown, the unit 1,2 is introduced into and randomly located in the housing 5. Such

SUBSTITUTE SHEET

random location is particularly, but not necessarily solely, related to the orientation of the unit about the axis of the housing 5. Then the potting resin is introduced into the housing 5 and around the unit 1,2 and the relevant portions of the wires 3 and thereafter sets.

Because the components 2 are surrounded on all six sides by portions of the wires 3, any attempt to obtain access to the components 2 by removing part of the potting resin is likely to lead to severing of at least one of the wires 3. Moreover, the random orientation of the unit 1,2 about the axis of the housing 5 makes it impossible for a thief to be sure where individual components 2 are located. Such random orientation is promoted by the circular- cylindrical surface 6 which does not tend to retain the unit 1,2 in any particular orientation. A further difficulty for the thief is that the circular-cylindrical co-axial, internal and external surfaces 6 and 7 give no hint to the thief as to the orientation of the unit 1,2 therein.

⁸ SUBSTITUTE SHEET