Tamper resistant packaging for information protection in electronic circuitry

A tamper-resistant package for protecting information stored in electronic circuitry is described. An energy source provides energy (electrical current, optical energy, microwave energy or RF energy, for example) within a region occupied by the circuitry to be protected. The energy is applied to an energy distribution system comprising a path or paths for energy distribution. Sensing means are provided which respond to the distribution system for sensing an intrusion. The distribution system includes an arrangement for changing or altering the path or paths over which the energy travels or altering the topology of the path or paths. The sensing means is informed of the appearance of the distribution system and senses an intrusion by comparing the appearance of the path(s) with the predicted appearance. In one embodiment, electrical current is selectively applied to a subset of electrical conductors. The pattern of current flowing in the conductors is sensed and compared to an expected pattern which is determined by the subset of energized conductors. Intrusion is evidenced by a disparity between the pattern of energized paths sensed by the sensing means compared to the pattern predicted by the sensing means. Dynamically varying the identity of the energized paths makes it difficult for an attacker to bypass the tamper detection.

DESCRIPTION 
1. Technical Field 
The invention relates to the provision of physical security for information 
which is electronically stored. 
2. Related Application 
Copending application Ser. No. 927,629, filed Nov. 15, 1986, now U.S. Pat. 
No. 4,817,140 discloses a software protection mechanism which in part 
relies upon the security of information contained in hardware devices. The 
present invention an be employed to provide the physical security required 
by the method described in the copending application. Copending 
application Ser. No. 927,298, filed Nov. 5, 1986, now U.S. Pat. No. 
4,860,351, assigned to the assignee of this application, is directed at 
other features touching on physical security. 
3. Background Art 
The prior art describes a wide variety of devices to provide physical 
security to protect objects from unauthorized removal. Generally, such 
protected objects have macroscopic qualities. One such example is Wetz in 
U.S. Pat. No. 3,763,795. 
For the protection of programs or data contained in machine readable form, 
the computer industry has traditionally relied on the physical security of 
the computer installation itself, or that security in combination with 
legal protection afforded by copyright, contract, trade secret and patent 
laws. Encryption has been used to prevent unauthorized persons from using 
intercepted information. In the personal computer area, many different 
software copy protection schemes are used, but all are based upon one or 
more software traps of some kind built into the program, and none are 
effective against the determined pirate. 
U.S. Pat. No. 4,471,163 describes a lock identity code which is part of a 
program lock. To provide security for this information, the patent 
describes that a circuit board on which the program lock is mounted is 
enclosed by top and bottom protective plates. Battery power for the 
components is supplied through a conductor which is glued to the inner 
surfaces of the protective plates. By this technique, one attempting to 
gain access to the components on the printed circuit board would 
necessarily move at least one of the plates. Such movement results in 
breaking of the power lead to remove power from some of the components. 
If, as is suggested, the memory storing the information to be protected 
requires power, this interruption in the power lead would destroy the 
information that a pirate was seeking and therefore the information would 
remain protected. 
Jones Futurex, Inc., in a brochure describing their "Encryptor" product, 
describes physical security which is accomplished "by encasing sensitive 
components that hold the key words in a steel enclosure filled with 
epoxy." Some of the products such as the "Encryptor 304" include battery 
support which "not only protects keys in a power failure, but will trigger 
a self-destruct event if someone tampers with the board. The 304 has a 
steel/epoxy barrier that prevents direct reading of the keys by logic 
analyzer or indirect access through electronic signal analysis." 
The Jones Futurex brochure, although mentioning tamper protection, provides 
no teaching as to how that tamper protection is implemented. 
The tamper protection provided in U.S. Pat. No. 4,471,163 is of limited 
utility in that at most it can detect movement of that portion of the 
upper and lower cover plates which is directly glued to the power lead 49. 
A determined pirate with access to several such program locks could easily 
circumvent this protection, although it might mean destroying one or more 
circuit boards until he had discovered the extent of the protection. There 
is a need to provide for security of information rather than objects. The 
breach in security required to surreptitiously withdraw information may be 
minuscule as compared to the breach required to withdraw a physical 
object. Hence the security provisions for information may be different in 
kind from that required to protect objects. 
It is conceivable that an attack on a device or installation to obtain 
information may be mounted in several stages: 
1) Removal of encapsulant or encapsulant and covers, 
2) Identification of location and function of security sensors, 
3) Bypassing of sensors to allow access to the next layer of protection, 
and so on. 
Use of such carefully directed techniques (which can be likened to brain 
surgery) could, given sufficient time and resources, defeat existing 
protection systems such as that described in Wetz in U.S. Pat. No. 
3,763,795. 
It is therefore an object of the invention to provide security against 
physical attack for information as opposed to security for physical 
objects or devices. It is a particular object of the invention to provide 
security for information which may be recorded or stored in an electronic 
device. It is another object of the invention to provide security which is 
exceedingly difficult to penetrate even if the attacker is aware of the 
architecture of the protection mechanisms. 
SUMMARY OF THE INVENTION 
Accordingly, the invention provides a tamper resistant or intrusion 
resistant package for protecting information stored in an electronic 
circuit. The package includes an enclosure or boundary space which 
substantially surrounds the electronic circuit to be protected. An energy 
source is provided to apply some form of energy (e.g. electrical current, 
optical energy, microwave energy or rf energy) to the enclosure. An energy 
distribution system, comprising a path or paths for energy distribution, 
is coupled to the energy source. Sensing means are provided, responsive to 
the distribution system for sensing an intrusion or an attempted 
intrusion. The distribution system includes an arrangement for changing or 
altering the path or paths over which the energy travels or altering the 
topology of the path or paths; in general the arrangement alters the 
appearance of the distribution system. The sensing means is informed of 
the appearance of the distribution system and senses an intrusion by 
comparing the appearance of the path(s) with the predicted appearance; any 
substantial difference is evidence of an intrusion. An impairing means may 
be provided which is responsive to the sensing means for impairing the 
electronic circuit in the presence of an intrusion. In order for the 
sensing system to be able to detect variations in the appearance of the 
distribution system it may require some a priori information relating to 
the distribution system appearance. For example the enclosure may be 
subjected to the energy for each possible appearance or topology and the 
response of the sensor recorded as the predicted appearance, or the 
transfer function of the enclosure for each possible appearance or 
topology, can be measured or computed and recorded. At selected times (or 
continuously) energy is applied to the enclosure and the actual response 
of the sensing means is compared to the expected response. In summary, the 
tamper detection system consists of a path for the transmission of energy 
(substantially surrounding the protected information containing circuit), 
means for changing the topology or appearance of that path, means for 
determining the actual appearance or topology of that path, and means for 
responding to correspondence or lack of correspondence of the path with 
its predicted appearance or topology, and the continuous performance of a 
cycle of change, test, and compare, using these components in order that 
efforts to tamper with the enclosure which involve means for misleading 
the system about the intact state of the paths must be able to track the 
possibly random and rapid changes in the topology or appearance of the 
paths or be detected by the failure to do so. 
In accordance with an aspect of the present invention, information which 
may be stored in the form of a CMOS RAM or other power requiring memory is 
protected from unauthorized access by surrounding it with a physical 
boundary space filled with or including one or more non-touching or 
insulated conductors following a complex path. The conductors in the 
boundary space are used to electrically sense any physical intrusion into 
this boundary space. The density of the conductors is made sufficiently 
high and the conductors are made sufficiently fragile so that any physical 
intrusion into the boundary space will cause either a short between two 
conductors or a break in one or more conductors. An opaque encapsulant is 
used to support the conductors, which encapsulant facilitates breaking the 
conductors in the event of an intrusion. If a short circuit or conductor 
break occurs, circuitry within the protected region senses the intrusion 
to immediately destroy the information being protected. For example, the 
conductors which have already been referred to in the boundary space might 
themselves carry the current necessary for maintaining the storage states 
of volatile memory elements. Accordingly, the breaking of conductor 
automatically obliterates the stored information. However, the conductors 
need not themselves carry the current necessary to maintain volatile 
memory, but can merely be employed to sense an intrusion, triggering a 
change in the supply of power to such volatile memory. The tamper 
resistant package may be enhanced so as to foil any attempt at probe 
guidance by use of imaging equipment using light, x-rays, sound, etc. 
In accordance with one particular embodiment of the invention, the boundary 
space is defined by at least one substantially planar board or card which 
has a first side coated with a conductor and a second side on which exists 
a pattern of fine conductors. The sensing means includes an electrical 
circuit which is formed at least in part by the conductor coating and at 
least in part by at least some of the fine conductors. In this embodiment 
the fine conductors can represent both the sensing and impairing means if 
they themselves carry current to support volatile memory. In another 
embodiment the fine conductors represent only the sensing means, and the 
impairing means includes other conductors supplying current to support 
volatile memory. The first side of the board is coated with a conductor to 
inhibit probing the card structure with x-rays and to provide shielding to 
the sensor circuitry. 
It is by no means essential for the intrusion sensing to be electrical. 
Optical propagation paths or a microwave or other rf energy can be 
substituted for the electrical conductors. 
In any case (current conduction, optical propagation, microwave or rf 
energy) all paths could be in use and the disturbance of any path triggers 
an intrusion, alternatively some paths could be energized and others 
deenergized and the pattern of energized and deenergized paths determines 
the existence of an intrusion. The path selection could be variable from 
package to package and could be dynamically variable within a single 
package. Rather than disturbing predetermined voltage patterns through 
open or short circuits, sensing the disturbance of an optical propagation 
path can be used to sense an intrusion. Such disturbance could be caused 
by physically disturbing an optical propagation path. 
The use of dynamically variable patterns of active and passive paths allows 
the presence of a disturbance or intrusion to be detected by sensing the 
actual pattern of conduction and comparing it to the expected pattern. An 
active path may be deactivated by a disturbance or intrusion opening or 
breaking an active path. A passive path may become active by being shorted 
to, or having energy reflected therein from, an active path. Dynamically 
variable patterns preclude an intruder from merely bypassing the active 
paths. If this were effected, the intrusion would be detected as soon as 
the pattern was altered. In general, these embodiments of the invention 
rely on a query/response arrangement; a condition or pattern of 
active/passive paths is established, the response is sensed or detected, 
the detected condition or pattern is compared with the condition or 
pattern established in the first step, and the protected information 
maintained if the conditions or patterns correspond. In the absence of 
some intrusion, the security system can predict the response, i.e. active 
paths should be sensed as active and vice versa. Intrusion is sensed when 
the system response is different from the predicted response. 
Another example of a dynamically variable path may be derived from the 
foregoing version. In this version, the conductive paths are arranged on a 
mechanical arm. This arm is arranged to be moved by a motor so that the 
boundary of the protected space is substantially swept by the arm. The 
path is dynamically varied by randomly changing the motor speed and 
direction so that the position of the arm is not predictable by an 
attacker. The circuit driving the motor is arranged so that an effort to 
slow or stop the arm can be sensed. This can be achieved by optically 
encoding the motor armature position or by sensing the motor back voltage. 
These are both conventional techniques. The circuit driving the motor 
predicts the motor position on the basis of the random drive signal. The 
predicted path can be compared with the actual result. An attempt to sever 
the arm would be sensed by the conductive paths on the arm. Mechanical 
energy may thus be considered among the forms which are useful in this 
system. 
Accordingly, in another aspect, the invention provides a tamper resistant 
apparatus for detecting intrusion into a region, comprising: 
means for changing a condition of said region to any one of a plurality of 
conditions; and 
means for sensing predictable results for said changing conditions assumed 
by said region when there is no intrusion into said region, and detecting 
intrusion into said region when other than said predictable results are 
sensed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
For information which is electronically stored, e.g. in RAM, the problem 
posed to those attempting to protect the information is rendered difficult 
by the inability to predict by what means an attacker would use to attempt 
to gain access to the information. However, if we can confine the attacker 
to one or several points of access, then we can concentrate our resources 
in protecting the few exposed access paths. For information which is 
stored electronically, and which is to be so used, it or some derivative 
thereof must be capable of being communicated in the form in which it is 
stored to the outside world. Typically, this capability is implemented as 
a series of conductive pins or the like. It is the purpose of the present 
invention to ensure that the information which is electronically stored is 
available in no other fashion regardless of the ingenuity of an attacker. 
That is to say that the attacker is unable to create a new set of pins 
through which unauthorized access can be made to the stored information. 
Protection of access to information via the package pins or terminals 
requires logical security which is not within the purview of the 
invention. FIG. 1 shows an enclosure 20, formed by top wall 21, and a 
bottom wall 22 within which a card 15 on which is mounted the electronic 
circuit or circuitry in which is stored the information to be protected. 
If the walls 21, 22 are planar, side walls may also be necessary to 
complete the enclosure 20; however the walls 21, 22 need not be planar and 
therefore side walls are not essential. The electronic circuitry is 
coupled to a connector 16, so that the information or a derivative thereof 
can be communicated to the outside world. It is thus the purpose of the 
invention to ensure that the information which is contained on the card 15 
is inaccessible by any other means. The electronic circuitry may take on a 
variety of levels of complexity from a simple device such as a few stages 
of shift registers and associated logic to more complex devices such as a 
microcomputer or even a mainframe computer. 
One of the goals of the invention is to ensure that the information stored 
on the card 15 cannot be accessed by removing the card from the enclosure 
20. That is, while it may be possible to physically remove the card from 
the enclosure 20, by the time the card is removed from the enclosure 20, 
it no longer contains, at least in usable form, the information which the 
attacker sought to obtain. 
In accordance with one embodiment of the invention, the enclosure 20 
includes a conductor or conductors which follow a complex path so as to 
largely fill or at least substantially surround the volume of the 
enclosure 20. The conductor or conductors contained within the volume 20 
are used as a sensor. By making the density of the conductors sufficiently 
large, and the conductors themselves sufficiently fragile, any material 
intrusion into the volume is likely to cause a short circuit between 
conductors or an open in one or more of the conductors. Because the 
conductors are preferably relatively fragile, to ensure the integrity of 
the device in the absence of an intrusion, we provide a support for the 
conductors. The supporting material is chosen so that it increases the 
likelihood of an open or short on intrusion while providing integrity in 
the absence of an intrusion. In addition, one of the goals of the 
invention is to thwart attempts at gaining access to the stored 
information by means of creating an electronic device with properties that 
mimic those of the sensor, so that this device may be substituted for some 
region of the sensor without being detected, so that the substituted 
region of the sensor may be removed, so that the underlying circuitry may 
be revealed and the information contained therein be removed or copied or 
altered. 
In an inexpensive embodiment, the volume 20 is enclosed within upper and 
lower cards 21, 22 which may be epoxy glass. The epoxy glass cards 21, 22 
have a pattern of fine conducting lines (formed by standard etching 
processes) on inside surfaces (on the surfaces closer to the card 15). The 
still largely unfilled space between the cards 21, 22 is then filled with 
a black silicone encapsulating material (or other equivalent material). 
The outward facing sides of the cards 21, 22 have a continuous copper (or 
other conductor) coating which serves to provide shielding for both the 
card to be protected as well as the lines on the interior surface of the 
protection cards 21, 22. This sensing arrangement is electrically 
interconnected to the card 15 which carries the circuitry containing the 
information to be protected and the circuitry which controls retention of 
that information. Such electrical interconnection can be provided by means 
of a plurality of metal pins distributed on the inside of the protection 
cards 21, 22 and projecting toward the card 15. These pins provide for 
mechanical support of the protection cards during manufacture as well as 
providing physical security by making it difficult to remove a card 
without breaking the conducting lines and thereby triggering some 
protective reaction. 
FIG. 2 shows a board 15 on which a plurality of electronic circuits 
151-153, etc. are supported. The cards 21 and 22 are arranged on either 
side of the card 15 to form an enclosure as seen in FIG. 1. The cards 21, 
22 then substantially surround the card 15. The outer surfaces of the 
cards 21, 22 have a conductive coating 210, 220 such as copper. On the 
inside surfaces the cards 21, 22 have a fine pattern of conductors 215, 
225 deposited thereon. Pins 216, 226 are selectively located to 
interconnect the pattern of conductors 215, 225 to circuitry on the card 
15. In this embodiment the pattern of conductors on the cards 21, 22 
follow a complex path and substantially surround the volume containing the 
circuits 151-153 to be protected. 
In one embodiment of the invention power for CMOS RAM chips (such as 151, 
152, etc.) is carried by at least some of the conductors 215, 225 and 
coupled to the RAM chips by at least some of the pins 216, 226. Physical 
movement of the cards 21, 22 or the conductors thereon 215, 225 or the 
pins 216, 226 will interrupt the supply of power to the RAM chips 151, 152 
to thereby obliterate or at least alter the contents of that memory. 
In accordance with another embodiment of the invention, the printed circuit 
conductors 215, 225 and the associated pins 216, 226 do not supply power 
to the memory chips 151, 152, etc., rather the foregoing circuitry is used 
to control the delivery of power to the RAM chips. In accordance with this 
embodiment of the invention, the conductive pattern 215, 225 is divided 
into several isolated paths through which a sensing current may or may not 
be passed. Not all of these paths need be used at any one time and the 
pattern of paths which are actually used can be dynamically selected by 
circuitry which is built for this purpose and for example supported on the 
card 15. An embodiment of that circuitry is shown in block diagram form in 
FIG. 3. If the pattern of sense current delivery circuits actually 
carrying current fails to match the patterns set by the power control 
circuitry (selector and distributor of FIG. 3) then that circuitry powers 
down and crowbars the RAM chips destroying the contents. 
As shown in FIG. 3, a battery 301 and/or other conventional power supply 
302 are provided to supply power to a CMOS RAM 151 and other circuitry 
152. A conventional power sensing element 303 is used to turn on power 
through power gate 252 to components which are not part of the protected 
memory or protection circuitry in the event that external power is 
supplied. This would be the case when the system was turned on for use. 
The CMOS RAM 151 is provided with a power gate 251 which under certain 
circumstances can allow power to flow to the associated CMOS RAM 151. The 
circumstances under which the power gate 251 will allow power to be 
conducted to the associated circuitry is now described. 
A distributor 304 is arranged to selectively energize one or a group of 
sense lines 304-1 through 304-n. Different ones of these sense lines are 
formed by different patterns of conductors 215, 225 and the associated 
pins 216, 226. While all of the sense lines could be energized by the 
distributor 304, its presence allows less than all of the sense lines to 
be energized. The sense lines which are energized are changed as a 
function of time. The distributor 304 is controlled by a selecting element 
305 to determine which of the plural sense lines are actually energized at 
any time. The select element 305 can be implemented in a variety of forms. 
It can be dynamic in that the selection of the sense lines which will be 
energized can change as a function of time, or any other parameter that 
the select element 305 can respond to. In its dynamic form the select 
element 305 generates a series of drive signals, changing as a function of 
time. Each drive signal is a composite to control each of the sense lines 
304-1 through 304-n. A compare element 306 receives two sets of inputs; a 
first type of input to the compare element 306 is derived from the output 
of the select element 305 and identifies the sense lines of the 
distributor 304 which are energized. Each of the sense lines 304-1 through 
304-n includes a sensing resistor such as R1, . . . Rn, so that when 
current flows through a particular sense line, a corresponding voltage is 
developed across the associated resistor. For sense lines which do not 
conduct current of course no voltage should be developed across the 
resistor. The compare element 306 includes an input from each of the 
sensing resistors. The compare element 306 then compares the identity of 
those sense lines which should be conducting current (information derived 
from the select element 305) with those sense lines which are actually 
conducting current (from information provided by the voltages sensed 
across the resistors). Only if the two patterns match identically does the 
compare element 306 allow the power gate 251 to conduct power to the RAM 
151. In this way, the compare element 306 can determine that every sense 
line that should be conducting current is conducting current (thus 
assuring that none of the patterns has developed an open circuit because 
of an intrusion for example) and that none of the sense lines which should 
not be conducting current are conducting current (as they might be if an 
attempted intrusion has resulted in a short circuit between one of the 
sense lines which has been commanded to conduct current with one of the 
sense lines which has been commanded not to conduct current) or in the 
case that an attacker has intentionally bridged equipotential lines to 
make room in which to work. 
While the apparatus of FIG. 3, as just described, includes a selector 
element 305 which dynamically alters the distribution or pattern of 
energized and deenergized paths, it is within the scope of the invention 
to provide a tamper resistant package in which the selectors 305 are not 
dynamic; as a function of time they are static. Rather, in a simpler 
embodiment different ones of the packages have different selectors 305. 
Accordingly, in a first package, a first selector 305 establishes a first 
pattern of energized and de-energized paths, in a second package a second 
selector 305 establishes a different pattern of energized and de-energized 
paths. Thus in any package the pattern of energized and de-energized paths 
is constant, but an attacker cannot employ information learned in 
attempting to breach one package, to assist him in attempting to breach 
another different package. This is true even though there may be thousands 
of packages having an identical pattern of energized and de-energized 
paths. 
In an even still simpler embodiment of the invention, the selector 305 is 
static, and all packages have the same pattern of energized and 
de-energized paths. 
An alternative system of distribution and sensing is shown in FIGS. 4 and 
5(a)-5(c). FIG. 4 shows a circuit in which a pair of switch banks 420 and 
440 control the position in the detection system which is occupied by the 
sense lines 462, 464, 466 and 468 which compose the conducting paths 
within the tamper detector 460. The "positions" of the switches 422, 424, 
426, 428, 442, 444, 446, 448 within the switch banks are controlled by the 
selection generator 480 via the drive signals from selection generator 480 
to the switches 422, 424, 426, 428, 442, 444, 446 and 448. It is to be 
understood that in any real embodiment, the switches will be electronic 
devices such as CMOS analog gates and that the selection generator will be 
designed so that all selections controlling the switch "positions" will 
configure useful arrangement of the sense lines. This may be accomplished 
either by recording all useful arrangement or by generating them 
algorithmically. In either case the methods needed in such a generator are 
obvious to those skilled in the art of digital design. Any useful 
arrangement of the sense lines will cause a complete circuit to be formed 
so that current from the current source 410 will flow through an internal 
jumper 412, a load resistance 414, and a current detector 416. It should 
be understood that a tamper detector 460 consisting of four lines is shown 
only for the sake of simplifying explanation of the system function. More 
lines could be used in any real embodiment. It should also be understood 
that the length of these lines is sufficient to fill the requirement that 
the tamper detector substantially fill the space which surrounds both the 
information containing circuitry and circuitry which configures and reads 
the tamper detector. It should also be understood that the circuit 
components (current source 410, jumper 412, load 414, and current detector 
416) may be replaced by a wide variety of matched sets of components 
including any usable form of signal source and detector, and that current 
is used in this example only for the purpose of simplification of the 
description. 
FIGS. 5A, 5B and 5C show the events which may occur in an attack on this 
tamper detector which is mounted with the object of removing some large 
fraction of the detector by substituting a circuit with the same 
electronic properties as the removed section. In FIG. 5A the tamper 
detector is configured identically to FIG. 4. The attacker has exposed 
widely separated points A, B, C and D, by carefully removing the material 
in which the lines are encased. By means of conventional electronic 
techniques it is possible to determine that the sub-circuit containing the 
four points may be replaced by an external resistance 55 of appropriate 
value. The attacker may substitute this resistance as shown in FIG. 5B in 
a time period too short to be detected by the current detector 416. The 
current detector shown consists of a one transistor invertor of 
conventional design with an "L" section low pass filter at its input so 
that the transient absence of current at the time the switch banks change 
state will not cause the protected information to be erased. FIG. 5C shows 
the configuration of the system after switch banks 420 and 440 have 
changed state (as may happen at any time). In the new configuration the 
external resistance no longer effectively substitutes for the area of the 
detector which has been removed hence the intrusion is detected by the 
lack of current flow in the current detector, and is indicated by the 
change in state of the status output line. 
The net effect of the ability of this system to detect attack by 
substitution is to limit the attacker to the tedious, accident sensitive, 
exacting attack of line by line substitution. This attack is thwarted by 
the correct construction of the tamper detector or by use of this system 
in conjunction with a more conventional measurement system which could 
detect the cumulative changes in the electronic properties of the 
individual lines as substitutions are made. 
The current detector shown consists of a one transistor invertor of 
conventional design with an "L" section low pass filter at its input so 
that the transient absence of current at the time the switch banks change 
state will not be misinterpreted as an attack and cause the protected 
information to be erased. In this circuit, the steady state potential 
which appears across the capacitor 59 after it has charged, is set by the 
voltage divider formed by the resistors 57 and 414 and the potential 
across both the current source 410 and the emitter-base junction of the 
transistor 61. The values of components used in the circuit are chosen so 
that this potential is more than enough to cause the current flow through 
the emitter-base junction of the transistor 61 to cause the transistor 61 
to conduct strongly. In addition to dividing the voltage across the 
current sensor, the resistor 57 is chosen so that the time constant for 
the discharge of the RC circuit consisting of the emitter-base junction of 
transistor 61 and capacitor 59 and resistor 57 is sufficiently long that 
the transistor 61 will continue to conduct strongly even during the short 
periods when the current sensor is disconnected from the remainder of the 
circuit as the switch banks 420 and 440 reconfigure the circuit. The 
direct consequence of the transistor 61 being in a strongly conducting 
state is that the potential at the status output terminal 63 connected to 
the collector of the transistor 61 is close to the ground potential to 
which the transistor 61 emitter is connected. If this circuit is connected 
to a conventional TTL logic circuit, the state of the status output would 
be interpreted by that circuit as a logic level "zero". A circuit using 
this status output should interpret this level as indicating an untampered 
state. In the event that the flow of current through the circuit is 
interrupted for more than the switching period, as would be the case in 
FIG. 5C, the discharging capacitor 59 will soon be unable to supply the 
current needed to keep the transistor 61 in a conductor state. When this 
happens, the status output 63 will no longer be close to ground potential. 
This change in state will be interpreted by a conventional TTL circuit as 
a logic level "one" which should be taken as an indication that tampering 
has been detected. The circuit described can easily be improved upon by 
one skilled in the art to include other detection capabilities such as 
detection of over-voltage conditions or detection of circuit resistance 
changes. This circuit is only intended to illustrate the principles of 
operation of such a system. 
Attempts at breaching the card security by inserting probes in from the 
side of the card is made difficult by the difficulty of routing a probe 
accurately through the encapsulating material 30 and by the length and 
narrowness of that path. Alternatively, the path could be covered by 
sensors. Positioning assistance with guidance equipment such as x-ray or 
sound is not possible because of the sensitivity of the RAM to x-rays and 
the difficulty of acoustic imaging through the silicone. 
A random vertical distribution of conducting lines resulting from a 
textured surface could render mechanical milling or plasma etching 
difficult. That is, distributing the lines so that different portions of a 
single path lie in different horizontal sections makes the use of plasma 
etching or mechanical milling unavailing to the intruder. 
Multiple designs with electrically identical function but different 
conductor position would serve to thwart repeated attempts by making data 
obtained by disassembling cards less likely to be useful when attacking 
other cards. 
In an optical analog to the embodiment of FIGS. 2/3, the electrical supply 
is replaced by an optical source, the sense lines 215/216 are replaced by 
free optical paths, the distribution function is replaced by optical 
scanning, and the resistors R.sub.1 -R.sub.n are replaced by photo 
sensitive devices. Intrusion is detected by lack of optical energy at a 
particular photo sensitive device because an intruding object has broken 
an optical path or by the presence of optical energy at another device 
produced by reflection off an intruding object. 
It should also be apparent that the opaque encapsulant must be omitted or 
channeled to provide a free space propagation path for optical energy. In 
the case of free or guided optical propagation a single source (such as a 
laser diode) can supply or drive multiple optical paths, each with a 
dedicated photo responsive diode or the like. Energized/deenergized paths 
may be selected by blocking access from the source to selected paths or by 
redirecting the optical source. Alternatively, multiple optical sources 
can be selectively energized/deenergized. 
From the foregoing, it should be apparent that many changes can be made to 
the various embodiments specifically described herein without departing 
from the spirit and scope of the invention which is to be determined from 
the attached claims.