1. Field of the Invention
This invention relates to a data carrier having a semiconductor chip in which secret data are stored. The invention relates in particular to a smart card.
2. Description of Related Art
Data carriers containing chips are used in a great number of different applications, for example for performing monetary transactions, paying for goods or services, or as an identification means for access or admission controls. In all said applications the data carrier chip normally processes secret data which must be protected from access by unauthorized third parties. Said protection is ensured by, among other things, giving the inner structures of the chip very small dimensions so that it is very difficult to access said structures with the aim of spying out data processed in said structures. In order to impede access further, one can embed the chip in a very firmly adhering compound whose forcible removal destroys the semiconductor plate or at least the secret data stored therein. It is also possible to provide the semiconductor plate during its production with a protective layer which cannot be removed without destroying the semiconductor plate.
With corresponding technical equipment, which is extremely expensive but nevertheless fundamentally available, an attacker could possibly succeed in exposing and examining the inner structure of the chip. Exposure could be effected for example by special etching methods or a suitable grinding process. The thus exposed structures of the chip, such as conductive paths, could be contacted with microprobes or examined by other methods to determine the signal patterns in said structures. Subsequently, one could attempt to determine from the detected signals secret data of the data carrier, such as secret keys, in order to use them for purposes of manipulation. One could likewise attempt to selectively influence the signal patterns in the exposed structures via the microprobes.
The invention is based on the problem of protecting secret data present in the chip of a data carrier from unauthorized access.
The inventive solution does not aim, like the prior art, at preventing exposure of the internal structures of the chip and the mounting of microprobes. Instead measures are taken to make it difficult for a potential attacker to infer secret information from any signal patterns intercepted. Said measures consist according to the invention in manipulating security-relevant operations so that the secret data used in performing said security-relevant operations cannot be determined without including further secret information. For this purpose the security-relevant operations are disguised or falsified with the aid of suitable functions before execution. In order to impede or even prevent in particular a statistical evaluation in case of multiple execution of the security-relevant operations, a random component enters into the disguising function. As a result, an attacker cannot determine the secret data from any data streams intercepted.
The security-relevant operation will be represented in the following by function h mapping input data x on output data y, i.e. y=h(x). To prevent secret input data x from being spied out the invention provides, in one example, for a disguised function hR1 to be determined, so that the following holds:
h(x)=hR1(x{circle around (x)}R1)
as shown in
y{circle around (x)}R2=hR1R2(x{circle around (x)}R1),
as shown in
The security-relevant operation is now performed by means of disguised function hR1R2 whose input data are not authentic secret data x but disguised secret data x{circle around (x)}R1 generated by combining authentic secret data x with random number R1. Without knowledge of random number R1 one cannot determine authentic secret data x from disguised secret data x{circle around (x)}R1. As a result of applying disguised function hR1R2 to disguised secret data x{circle around (x)}R1 one obtains disguised output data y{circle around (x)}R2. From disguised output data y{circle around (x)}R2 one can determine output data y by suitable combination. Before each new execution of the security-relevant function one can preset new random numbers R1 and R2 from which new disguised function hR1R2 is determined in each case. Alternatively, a plurality of disguised functions hR1R2 can be permanently stored, one of which is selected randomly before execution of the security-relevant operation. It is especially advantageous to use two functions hR1R2 and hR1′R2′, random numbers R1′ and R2′ being the inverse values of random numbers R1 and R2 with respect to the type of combination selected for disguising. In a further variant, random numbers R1 and R2 can also be identical. In particular, random numbers R1 and R2 can be selected statistically independently so that there is no correlation between input and output data which can be used for an attack.
If further operations are executed before or after security-relevant operation h in question here, random numbers R1 and R2 can also be used for disguising the data processed with the further operations.
The inventive solution can be used especially advantageously for security-relevant operations containing nonlinear functions. With nonlinear functions one cannot apply known protective measures based on disguising the secret data before execution of the functions. Known protective measures presuppose that the functions are linear with respect to the disguising operations so that disguising can be undone after execution of the functions. In the inventive solution, however, not only the secret data are falsified or disguised but also the security-relevant operations processing the secret data. The disguising of the secret data and the security-relevant operations is coordinated such that the authentic secret data can be derived from the disguised secret data after execution of the security-relevant operations. Coordination between disguising of the secret data and the security-relevant operations can be realized especially simply if the security-relevant operations are realized in the form of tables, so-called lookup tables. In the stated tables each input value x has output value y associated therewith. The functions realized by the tables are executed by looking up output values y belonging to particular input values x.
The invention will be explained below with reference to the embodiments shown in the figures, in which:
a, 3b, 3c and 3d show representations of lookup tables.
The invention makes it very difficult or even impossible for an attacker to gain access to in particular secret data of the chip even if he has managed to remove the protective layer of chip 5 without destroying the circuit and to contact exposed structures 6 of chip 5 with microprobes or intercept them in some other way. The invention is of course also effective if an attacker gains access to the signal patterns of chip 5 in another way.
a, 3b, 3c and 3d show simple examples of lookup tables in which the input and output data each have a length of 2 bits. All table values are represented as binary data. The first line states input data x, and the second line output data y associated therewith in the particular column.
a shows a lookup table for undisguised function h.
b shows an intermediate step in determining the disguised lookup table of
One usually arranges the individual columns of a lookup table according to ascending input data x. A table determined by accordingly sorting the table in
If the table according to
d shows the result of applying said further XOR operation. In said operation the elements of the second line of the table according to
With the table shown in
Use of the table shown in
Disguising the security-relevant operations of the input values can be effected not only by XORing but also by other suitable types of combination, for example modular addition. Furthermore, the invention is not limited to the application of nonlinear functions represented by means of lookup tables. One can also use any nonlinear and even linear functions for which a suitable disguised function can be determined.
Number | Date | Country | Kind |
---|---|---|---|
198 41 676 | Sep 1998 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/EP99/06580 | 9/7/1999 | WO | 00 | 4/26/2001 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO00/16272 | 3/23/2000 | WO | A |
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4549075 | Saada et al. | Oct 1985 | A |
5153581 | Hazard | Oct 1992 | A |
20010053220 | Kocher et al. | Dec 2001 | A1 |