CA2226386C

CA2226386C - Protection of software against use without permit

Info

Publication number: CA2226386C
Application number: CA002226386A
Authority: CA
Inventors: Sigurd Sigbjornsen; Magne Arild Haglund; Vladimir A. Oleshchuk
Original assignee: Sospita AS
Current assignee: Sospita AS
Priority date: 1995-07-13
Filing date: 1996-07-10
Publication date: 2006-12-05
Anticipated expiration: 2016-07-10
Also published as: CA2226386A1

Abstract

An arrangement to protect software, particularly freely distributed application software, against utilization without permission of the copyright holder. By encrypting the software employing a key (k1) which is different from that key (k2) which is employed in the decryption, better protection is obtained against unauthorized utilization when the decryption key is kept secret to the user. Further improved security is achieved by additionally executing scrambling-descrambling of the communication between the computer in which the software is utilized and the external unit in which the decryption key is stored. Also, the external unit is arranged such that it returns to the host computer, the result from its processing of data received from the host, the result then being utilized in the further execution of the respective program.

Description

WO 97!03398 PCT/N096/00171 PROTECTION OF SOFTWARE AGAINST USE WITHOUT PERMIT
Technical Field Present invention relates to the protection of software, in particular freely distributed application software, against use without permission of the copyright holder;
and concerns in particular a method of preventing unauthorized utilization of software in a computer; a method and device for preparing software for the utilization in a computer only with a corresponding authorization; and a method and device for allowing authorized utilization of software in a computer which has been prepared according to an ~o aspect of the invention.
Piracy copying of software, particularly software adapted to run on personal computers, is an extensive problem to software vendors which annually costs them large amounts of money. The conventional methods employed to protect software against unauthorized is utilization by demanding a password for the installation or operation of a specific computer program or program package, for example, have not provided sufficient security. Thus, an arrangement making it possible to use a certain computer program or a computer program package only if the permission of the copyright holder really exists would be of great importance.
Background Art Several attempts have already been made to establish arrangements in which simply typing the required password is not sufficient to gain access to a program.
For example, systems have been proposed which require that a special hardware unit is connected to the computer to make it possible to utilize a given program. This unit may take the form of a blind plug {also termed "dongle"), for example, which is connected directly to one of the input-output terminals of the computer, and containing fixed tables, identity number, or the like, stored in an internal memory from which information is read upon the request of a programmed additional routine included for this purpose in the application program 3o in question. The unit may also take the form of a card reader station, or the like, in which a card is inserted, the matter stored on the card in principle being similar to that of the blind plug above. Usually the checking procedure involves that the additional routine of the program makes a direct comparison of the program identity, for example, and a corresponding item of information present in the stored table.
An example of such an arrangement is described in published DE patent application no.
44 19 115 wherein the matter stored in a chipcard is read, and if the expected content

2 exists, this fact is regarded as being sufficient proof of identity for the use of the program. The checking may be done when a program is installed, or during the utilization thereof. Published DE patent application no. 42 39 865 discloses a similar system which in addition provides an arrangement by which the number of software s installations performed are noted, making it possible to limit the number thereof.
The additional routine which must be included in the software constitutes the main disadvantage of all such known devices. By simply removing such routines the software will operate normally, and the protection against unauthorized utilization would be lost.
yo Also, during the exchange of data between the processor of the computer and the memory of the unit or card, it is possible to observe the information, and as the course of this information exchanged is the same each time the program is used, it is possible also to reveal the matter stored in the external memory. Even if the contents of the memory is encrypted in one way or another, such kind of recurrence across the is communication interface makes it possible to simulate a corresponding hardware unit, for example, or "break the code" by means of relatively modest computing power.
In the arrangement described in published GB patent application no. 2 163 577 some of the flaws of the above type of hardware units are avoided by employing certain crypto 2o techniques, and by accommodating several storage means as well as a processor of its own in a tamper-proof housing. The processor in the housing makes use of a decryption key which is stored in the housing, and of instructions which also are stored in the housing, to decrypt and execute by itself an encrypted application program or program module transferred from the host computer to which the housing is connected.
2s Regarding the crypto technique itself, the arrangement according to GB
patent application no. 2 163 577 uses a so-called DES algorithm (DES - Data Encryption Standard, Bureau of Standards, U.S.A., 1977} for the encryption of the application software, and the corresponding inverse DES algorithm for decrypting the same, whereby one and the same key is used for both the encryption and the decryption.
so Hence, the DES standard is symmetrical, and the security resides only in the key itself.
Therefore, not to give away this security, the encryption also of the DES key itself is proposed in the GB patent application. For this purpose it is used a so-called RSA
algorithm (RSA - Rivest, Shamir, Adleman) having two different keys, that is, one for the encryption and another for the decryption, the deduction of one key from the other being 35 practically impossible. Hence, the RSA crypto system is an asymmetric, two-key system (also termed public-key / private-key crypto system), and in the case of the arrangement

3 according to GB patent application no. 2 163 577, one key only is used which, per se, may be known (the public key) to encrypt the DES key while another key which the user must not get to know (the private or secret key) is used to decrypt the DES
key. The latter key, i.e. the secret key, is stored in a memory in the tamper-proof housing and is s fetched by the processor in the housing when needed to decrypt encrypted DES
keys, each of which belonging to an encrypted application program, for the purpose of being able to execute such an application program.
In the arrangement according to GB patent application no. 2 163 577, however, it is also 1o possible to monitor the communication between the external unit and the host computer, and the course of communication is identical each time the same encrypted program module is to be executed. Since complete program modules are encrypted and such modules make up a relatively large part of the software, this kind of predictable recurrence across the communication intertace assists in the identification of respective ~s program modules which then easily can be separated from the rest of the software, to be processed, e.g. in off-line mode, for the purpose of decrypting the encrypted program module once and for all. Moreover, external decryption, storage and execution of complete application program modules would take an unacceptably long period of time, unless the circuits in the housing possess a sufficiently high data processing capacity 2o and the communication with the host computer from which the program modules originate, is very fast.
An object of the present invention is to provide a crypto arrangement giving suppliers and/or proprietors of the software an improved possibility of protecting their product 2s against unauthorized utilization, and which does not suffer from the drawbacks of prior art, in such a manner that the software can be copied and distributed without restrictions, but yet not be used unless the necessary permission is present.
A further object of the invention is to provide a crypto arrangement of a universal nature so which is able to accommodate not only individual software modules but entire program ' packages, wherein permission of use may be assigned at different levels, such as for selected parts of a program package.
These and other objects will appear more clearly from the description below of examples 3s of preferred embodiments of the present invention as seen in relation with the accompa-vying drawings.

Summary of the Invention In accordance with an aspect of the invention, there is provided a method of preventing unauthorized utilization of software in a computer, which includes the steps of: encrypting only a part of the software in accordance with a first algorithm (g1), wherein the first algorithm uses a public first key (k1 ); and decrypting the encrypted part of the software in accordance with a second algorithm (g2), the second algorithm (g2) together with a private second key (k2) to be employed in the decryption of the encrypted part of the software being stored in an external unit adapted to be connected to the computer, the external unit comprising at least a computer readable storage medium and a processor of its own, wherein the private second key (k2) is different than the public first key (k1) employed in the encryption.
In accordance with a further aspect of the invention, there is provided a method of preparing software for utilization in a computer only with a corresponding authorization, which includes the steps of: storing in an external unit a second algorithm (g2) and a private key (k2), encrypting only a part of the software in accordance with a first algorithm (g1) and a public first key (k1); and transferring the software encrypted to the external unit connected to the computer, the second algorithm (g2) together with the private second key (k2) being employed in decryption of the encrypted part of the software, the external unit comprising at least a computer readable storage medium and a processor of its own, wherein the private second key (k2) is different than the public first key (k1 ) employed in the encryption.
In accordance with a further aspect of the invention, there is provided a method of making authorized utilization possible in a computer of software prepared for utilization in the computer only with a corresponding authorization, which includes the steps of: encrypting in accordance with a first algorithm (g1) only a part of the software which by the utilization in the computer is decrypted in accordance with a second algorithm (g2), wherein a key which is employed for the encryption in accordance with the first algorithm (g1), is a public first key (k1) which is different from a private second key (k2) which is employed in decryption in accordance with the second algorithm (g2) of the part of the software which is encrypted in accordance with the first algorithm (g1) and the public first key (k1 ), the encryption of the part of the software in accordance with the first algorithm (g1) is executed on the source code for the software prior to compiling and linking to an executable program, or in similar processing steps S
producing an executable program, and at the same time adding an object code to the data library of the executable program associated with an external unit adapted to be connected to the computer, and the compiling andlor linking of the source code .to an executable program, or in similar processing steps producing an executable program, call 'sequences, or similar instructions, are . inserted which in the execution in the computer of the executable program causes a jump to a corresponding entry point to the object code, the object code being used to establish a communication channel between the computer and the external unit in which the decryption takes place in accordance with the second algorithm (g2); connecting the external unit to the computer, the external unit comprising a computer readable storage medium and a processor, the second algorithm (g2) and a the private second key k2 to be employed in the decryption of the encrypted part of the software being stored in the external unit;
and when the computer in the execution of the part of the software which is encrypted in accordance with the first algorithm (g1) encounters a call sequence, or a similar instruction, causing a jump to a corresponding entry point to the object code, the object code is utilized by the computer to establish a communication channel to the external unit through which communication channel the encrypted part of the software is transferred in a -first transfer session to the external unit to be decrypted by the processor of the external unit in accordance with the second algorithm (g2) and the private second key (k2) both of which being stored in the external unit, the second key (k2) being different from the public first key (k1 ) employed in the execution of the encryption of the part of the software in accordance with the first algorithm (g1), and the decrypted software part then being processed in the external unit and a result transferred in a second transfer session in the opposite direction through the communication channel for the further utilization in the computer.
In accordance with a further aspect of the invention, there is provided a device for the preparation of software to be utilized in a computer only with a corresponding authorization, which includes: crypto means effecting the encryption of only a part of the software in accordance with a first algorithm (g1 ) and a public first key (k1 ), and an external unit adapted to be connected to the computer, the external unit comprising a processor and a computer readable storage medium for storing a second algorithm (g2) and a private second key (k2), wherein the device further comprises generator means to provide the second algorithm (g2) and the private second key (k2) intended to be employed in decryption in accordance with the second algorithm (g2), the private second key (k2) being different from the public first key (k1 ) employed by the crypto means in the execution of the encryption of the part of the software in accordance with the first algorithm (g1).
In accordance with a further aspect of the invention, there is provided adevice for making authorized utilization of software in a computer possible, which includes: a device including crypto means for effecting the encryption of only a part of the software in accordance with a first algorithm (g1 ) and a public f rst key (k1 ), and an external unit adapted to be connected to the computer, the external unit comprising a processor and a computer readable storage medium for storing a second algorithm (g2) and a private second key (k2), and being disposed to execute decryption of the encrypted part of the software in accordance with the second algorithm (g2) and the private second key (k2); and generator means to provide the second algorithm (g2) and the private second key (k2) intended to be employed in the decryption in accordance with the second algorithm (g2), the private second key (k2) being different from the public first key (k1) employed by the crypto means in the execution of the encryption of the part of the software in accordance with the first algorithm (g1), the device comprising a computer adapted to serve as a host computer for the external unit, wherein the external unit comprises decryption means adapted to execute decryption in accordance with the second algorithm (g2) and the private second key (k2) produced by the generator means.
In accordance with a further aspect of the invention, there is provided amethod of preventing unauthorized utilization of software in a computer, which includes the steps of: encrypting only a part of the software in accordance with a first algorithm (g1) and a public first key (k1 ); .storing a second algorithm (g2) and a private second key from the public first key (k1 ) in an external unit having a computer readable storage medium and a processor, the external unit adapted to be connected to the computer;
and decrypting the encrypted part of the software using the second algorithm (g2) and the private second key (k2).
In accordance with a further aspect of the invention, there is provided adevice for the preparation of software to be utilized in a computer only with a corresponding authorization, which includes: determining means for determining a public private key to be used by the computer to decrypt only a part of the software in accordance with a decryption algorithm; an external unit connected to the computer, wherein the decryption algorithm and the private key are stored in the external unit and encrypting S means for encrypting only the part of the ~ software in accordance with an encryption algorithm and another private public key different from the public private key determined by the determining means.
In accordance with a further aspect of the invention, there is provided a method of preventing unauthorized utilization of software in a computer, which includes the steps of: encrypting at least a part of the software in accordance with a first algorithm (gi) and a first key (ki), the encryption being executed on the source code of the software prior to compiling and linking thereof, or during similar processing steps producing an executable program, and at the same time adding to the di#a library of the executable program an object code relating to the communication with a tamper-1 S proof external unit connectable to the computer, the external unit comprising at least a computer readable storage medium and a processor of its own, and decrypting the encrypted part of the software in accordance with a second algorithm (k2), the decryption being executed by employing a second key (k2) associated with the authorized utilization of the software and stored in the external unit, characterized in that the decryption of the encrypted part of the software takes place when the computer in the execution of the program encounters a call sequence, or a similar instruction, causing a jump to a corresponding entry point to the added object code, whereby a communication channel Is established between the computer and the external unit by utilization of the added object code, through which communication 2S channel the encrypted part of the software is transferred to the external unit to be decrypted by the units own processor, the decrypted software part then being processed in the external unit and the result thereof transferred in the opposite direction through the communication channel for further utilization in the computer.
In accordance with a further aspect of the invention, there is provided a method of preparing software, particularly software intended for free distribution, for utilization in a computer only with a corresponding authorization, which includes the steps of:
encrypting at least a part of the software in accordance with a first algorithm (g1) and a first key (k1), the encryption being executed on the source code of the software prior to compiling and linking thereof, or during similar processing steps 3S producing an executable program, and at the same time adding to the data library of the executable program an object code relating to the communication with a tamper-proof external unit connectable to the computer, the external unit comprising at least a computer readable storage medium and a processor of its own, and storing in the external unit a second algorithm (g2) and a second key (k2) associated with the authorized utilization of the software, the second key being intended for use in the decryption of the encrypted part of the software in accordance with the second algorithm, characterized in that in the compiling andlor linking of the source code to an executable program, or in similar processing steps producing an executable program, one or more call sequences, or similar instructions, are inserted therein, which in the execution in a computer of the program causes a jump to a corresponding entry point to the added object code, the object code being intended for being used to establish a communication channel between the computer and the external unit in which the decryption is to take place by the units own processor and the decrypted software part is to be processed and the result thereof transferred in the opposite direction through the communication channel for further utilization in the computer.
In accordance with a further aspect of the invention, there is provided a method of a device for the preparation of software, particularly software intended for free distribution, to be utilized in a computer only with a corresponding authorization, which includes: crypto means for effecting encryption of at least a part of the software in accordance with a first algorithm (gi) and a first key (k1), the crypto means being adapted to execute the encryption on the source code of the software befQe it is compiled and linked, or in any other way transformed into an executable program, the crypto means at the same time, effecting addition to the data library of the executable program, of an object code relating to the communication with a tamper-proof external unit connectable to the computer, the external unit at least comprising a processor of its own and a computer readable storage medium for storing a second algorithm (g2) and a second key (k2) associated with the authorized utilization of the software, and being arranged to execute decryption of the encrypted part of the software in accordance with the second algorithm, characterized in that the crypto means is adapted to Insert during the compiling andlor linking of the sourcecode to an executable program, or during a similar processing producing an executable program, one or more call sequences, or similar instructions, which in the execution of the program in the computer causes a jump to a corresponding entry point to the added object code, the object code being intended for being used to establish a communication channel between the computer and the external unit in which the decryption is to take place by the unit's own processor, and the decrypted software par# is to be processed and the result thereof transferred in the opposite direction through the communication channel for further utilization in the computer.

i.o Brief Description of Drawings In the description below reference is made to appended drawings, on which:
Figure 1 illustrates a preferred hardware configuration according to the invention, Figure 2 is a simplified general software diagram. according to the invention, Figure 3 iliustrgtes~schematically how a common command~or execution file (.EXE file) ~ s , is generated without encryption, Figures 4 and 5, illustrates schematically how encryption on the level of source code can be carried out according to the invention, Figure fi ~iliustrates schematically the partitioning of software onto a magnetic storage disk and a random access memory (RAM), respectively, 2o Figure 7 illustrates schematically the utilization of protected software in a computer, Figure 8 shows an exempts of an encryption process of the type shovsm in Figures 4 and 5, Figure 9 illustrates, schemagcafly the utilizat'ron of protected software in a computer, Figure 10. illustrates schematically an application area including access checking or 25 authentication, and Figure 11 is a flowchart schematicaHy~itlustrating a course of processes including encryption-decryption and scrambling-descrambling according=to a preferred embodiment of the invention.
. so Description of Preferred. Embodiments First it is referred to Figure 1 which illustrates a preferred.hardware configuration of the ' invention and which shows a computer having the form of a workstation or personal computer (PC) serving'as a host cofnputer according to the invention. In the figure, an external unit according to the invention is shown to be in the form of a card reader or 3s processor, particularly for. Smart Cards, provided with a~ commercially available integrated microprocessor, e.g. of the CCA12103 type, the unit being included in the computer shown or disposed in a separate unit of equipment connected to the computer by a serial or parallel connection.
Figure 1 also illustrates that now the secured software may be distributed through s different types of data networks to which the computer may establish a connection, such . as wide area networks (WAN), local area networks (t~N), and, in particular, Internet.
Also, the software may in deed, as usual, be distributed on flexible disks and/or CC-ROMs (readable only, compact laser disks). In any case, the software may be copied and installed without restrictions.
Since the software is protected against unauthorized utilization, there is no need for any kind of copy protection of the software as the case otherwise often may be.
Here, the authorization is embedded in the Smart Card, and it is not available to anyone else but the supplier of the software who himself installs the necessary decryption algorithms and 1s the keys on the card. Hence, the permit to use a certain computer program is found on the card, not in the respective program, or another part of the software.
From fig. 2 showing a simplified general diagram, it appears that an arrangement according to the invention can be seen to comprise:
1. software protected against unauthorized utilization (the protection, however, not interfering with the copying thereof), 2. a Smart Card (or the like) holding the algorithm and the key for the decryption of the software in question, and 3. special software for the communication between the protected software (1 ) and the 2s Smart Card (2) (i.e. the added object code specified in the claims).
The protection is provided by the insertion, in different locations of the software, of program calls to the Smart Card, or to special software at the disposal to 'the card, thereby obtaining the information necessary to proceed correctly in the execution of the so protected program. For example, this information may be certain parameters which are used when the program is executed, and which is determined by those who wish to protect their software. Because they are necessary for the software to work properly, such program calls cannot be removed.
35 The interaction of the protected program with the Smart Card is controlled by the special software (object code) entered into the data library of the program when the original program is encrypted. This special software may also provide for scrambling of the communication between the computer and the Smart Card.
Figure 3 shows how a common command or execution file (.EXE file) is generated .
s without encryption; and Figures 4 and 5, each in their own manner, depict how encryption can be carried out on the source code level according to the invention.
Figure 6 illustrates that the software itself is placed on a magnetic storage disk, whereas the special program (object code or data library) is supplied to the random access ~o memory (RAM) of the computer.
Figures 7 and 9 illustrate such processes which take place when the protected software is utilized in a computer.
~s Figure 8 shows an example of an encryption process of the type shown in Figures 4 and 5. Here, it is assumed that the source code is present in a high level programming language, such as Pascal, C, Modula, or the like. It is the source code that is being encrypted and thus protected against so-called unauthorized use. In the source code a few parameters are selected which are encrypted by means of an encryption function g.
2o For example, an expression, x: = y f 10, present in a command in the unprotected source code can be represented as:
C: = decrypt (g(10) f t), x=y+(C-T), where: g(10) is an encrypted parameter, and 25 T is a variable, the random value of which, in this case, being fetched from the Smart Card.
To obtain a correct value of x the protected program must "arrive atu a value of C, which must be equal 10 f T. The decrypt program is located in the "spesial softwarep (the so added object code) which constitutes a part of the protected software (see Figure 5).
This special software also comprises scrambling and descrambling functions, which here are denoted f and f 1, as well programs for the communication with the Smart Card (see Figure 7). In the communication with the Smart Card, the functions f and f' employ keys which are fetched from the Smart Card, the Smart Card itself containing:
35 - a number generator to produce a random value of variable T located in the protected software, - a secret key for the decryption function g'f, - an algorithm for the decryption function g'l,,sand - one or more keys for functions f and f 1.
It should be noted that it is important that the encryption function g and decryption function g'' represent a public key crypto -system not being symmetric. This means that the encryption function g employs a public key which may be known, this key;
however, not being sufficient to arrive at the decryption function g'' (neither its algorithm, nor its key). Thus, the algorithm and key for the decryption function g'' is placed on the Smart ~o Card, from which they are never transmitted.
. ., _ ..
:,.
.,' ,.
~s Having this kind of encryption-decryption arrangement according to the invention, the following advantages and possibilities are realized:
- Great flexibility by the use of Smart Cards.
- The licensing of Smart Cards (i.e. users) can be provided by the importers or agents engaged by software producers. A Smart Card may then contain licenses, or per-missions, at several levels for various software packages which have the same authentication format and algorithms.
s5 - A first level of encryption employing an unsymmetric, dual key encryption arrangement (public key / private key crypto system), such as the RSA crypto system, whereby the public key is available only to the software producer, and the private key is a secret key which the manufacturer of the Smart Card enters into the read only memory (ROM) of the Smart Card according to specifications given by the software producer.
The private key may be different for each program package.
s - An unsymmetric, encrypted authentication key which is transferred to the Smart Card when the running of protected software begins and which is decrypted in the Smart Card by means of a private key no. 0 to initiate an authentication process in the Smart Card.
- Encryption at the level of source code, making the arrangement independent of the to operating system. By encrypting small parts, or fragments, only of files, such as of command files, it is difficult to identify those parts of the software being encrypted for the purpose of attacking such parts in one way or another. Also, the decryption algorithms and keys are easily entered onto the Smart Card.
- A second level of encryption whereby the communication between the host computer ~s and Smart Card is such that it becomes difficult to trace anything making sense from that communication by the logging thereof. The algorithms to be employed are located both in the protected software and the Smart Card, and both the encryption keys and the decryption keys are located in the Smart Card, i.e. hidden to the user.
The encryption algorithm and key may be different for various types of software.
Figure 10 serves to demonstrate that the application area of the arrangement according to the invention is extendable also to cover access checking, or authentication, for example, as further possibilities also may exist. In such an embodiment of the invention, the software to be protected may be provided with an authentication key encrypted by 2s employing a so-called public key and an identification number for the software package in question. Then, the external unit, such as the Smart Card, would contain decryption algorithms which preferably are mask programmed, and a private key no. 0 (in the ROM) to be used to decrypt the authentication key, as well as an access or authentication table which may be configured as the table shown below.

PID AcL PK
(Program LD. No.) (Access Level) (Private Key;l PID (1 ) AcL (1 } PK (1 ) PID (2) AcL (2) PK (1 ) PID (3) AcL (3) PK (3) PID(n) AcL (n) PK (n) io In the table, P1D denotes the identity number of the software, such that different programs are assigned dissimilar identity numbers which also may contain the version number of the respective software products, or the like, for example. AcL
denotes the access level or status, such as:
- two different levels, namely access permitted and access not permitted, - a limited number of times the respective program can be run, - a time limit for the use of a program, e.g. a permission expiration date, - access to a shortened variant only of the program, e.g. a so-called demo-variant.
The entries in the access level column, AcL, of the table are amendable t>y the importer or agent of the software product, for example.
In the private key column, PK, the software producer specifies the secret keys to be employed in the decryption of the encrypted fragments dependent on the identity number, PID, of the software. The secret keys are mask programmed in the Smart Card and are not available to anyone else.
Figure 11 is a flow chart schematically illustrating a principally complete course of processes according to a preferred embodiment of the invention, the steps being:
- encryption of the source code (g1, k1), - scrambling of the encrypted data (g3, k3), so - transfer of the scrambled encrypted data to the external unit and descrambling thereof (g4, k4), - decryption of the transferred and descrambled data (g2, k2), - processing of the decrypted data, and scrambling of the result (g5, k5), - transfer of the scrambled result to the host computer and descrambling thereof (g6, k6), and - output of the decrypted result for further utilization.
s In the examples shown of embodiments of the invention, a Smart Card constitutes the preferred implementation of the external units indicated in the claims below.
This is ' quite simply because the Smart Card technology is considered as being the most "tamper-proof" protection of the algorithms, keys, a.s.o., which necessarily have to be stored in the external unit, or in a separate article, according to the annexed claims. The io small number of manufacturers of such caPds in the world, and i.a. the large values being at disposal by the use of such cards, in particular cards having the form of payment transaction cards, contribute strongly to the fact that the knowledge required to falsify such Smart Cards, will not be readily available to persons having dishonest intentions. However, this does not impede a possible development of new techniques ~s which may be just as well, or better, suited for the purpose of the present invention.
Therefore, the patent claims are meant also to cover such future external units and separate articles indicated in the claims which may provide at least the same degree of security as the Smart Card now preferred.

Claims

What is claimed are:

1. A method of preventing unauthorized utilization of software in a computer, the method comprising the steps of:
encrypting only a part of the software in accordance with a first algorithm (g1), wherein said first algorithm uses a public first key (k1); and decrypting said encrypted part of the software in accordance with a second algorithm (g2), said second algorithm (g2) together with a private second key (k2) to be employed in said decryption of said encrypted part of the software being stored in an external unit adapted to be connected to the computer, said external unit comprising at least a computer readable storage medium and a processor of its own, wherein said private second key (k2) is different than said public first key (k1) employed in said encryption.

2. A method according to claim 1, wherein said encryption of said part of the software in accordance with said first algorithm (g1) is executed on a source code of the software prior to compiling and linking, or during similar processing steps producing an executable program, and at the same time adding to a data library of said executable program an object code associated with said external unit.

3. A method according to claim 2, wherein said decryption of said encrypted part of the software which is executed in accordance with said second algorithm (g2), takes place when the computer in said execution of the software encounters a call sequence, or a similar instruction, causing a jump to a corresponding entry point to said object code, whereby a communication channel is established between the computer and said external unit by utilization of said object code, through which communication channel said encrypted part of the software is transferred in a first transfer session to said external unit to be decrypted by said external unit's own processor, said decrypted software part then being processed in said external unit and a result transferred in a second transfer session the opposite direction through said communication channel for the further utilization in the computer.

4. A method according to claim 3, wherein the method further comprises:
a step wherein said part of the software which is encrypted in accordance with said first algorithm (g1), prior to said first transfer session, is caused to be scrambled according to a third algorithm (g3) and a third key (k3), said third key (k3) being based on a number which, for the respective transfer session, is selected randomly and preferably produced by said external unit, and a step wherein said part of the software which is encrypted in accordance with said first algorithm (g1), and which is scrambled according to said third algorithm (g3) and transferred to said external unit in said first transfer session, is caused to be descrambled in said external unit in accordance with a fourth algorithm (g4) and a fourth key (k4), said fourth key (k4) being based on said randomly selected number for said third key (k3), and said fourth algorithm (g4) being the inverse algorithm of said third algorithm (g3).

5. A method according to claim 4, wherein the method further comprises:
a step wherein said result produced in said external unit on the basis of said part of the software which is decrypted in accordance with said second algorithm (g2), prior to said second transfer session, is caused to be scrambled according to a fifth algorithm (g5) and a fifth key (k5), said fifth key (k5) being based on a number which, for the respective transfer session, is randomly selected and preferably provided by said external unit, and a step wherein said result produced in said external unit on the basis of said part of the software which is decrypted in accordance with said second algorithm (g2), and which is scrambled according to said fifth algorithm (g5) and transferred to the computer in said second transfer session, is caused to be descrambled in the computer in accordance with a sixth algorithm (g6) and a sixth key (k6), said sixth key (k6) being based on said randomly selected number for said fifth key (k5), and said sixth algorithm (g6) being the inverse algorithm of said fifth algorithm (g5).

6. A method according to claim 5, wherein said randomly selected number for said third and fourth keys (k3, k4) and said randomly selected number for said fifth and sixth keys (k5, k6) are the same number.

7. A method of preparing software for utilization in a computer only with a corresponding authorization, the method comprising the steps of;
encrypting in accordance with a first algorithm (g1) only a part of the software;
transferring the software encrypted using said first algorithm (g1) to an external unit connected to the computer;
decrypting within said external unit the software encrypted using said first algorithm (g1) in accordance with a second algorithm (g2), wherein a key which is employed for said encryption in accordance with said first algorithm (g1), is a public first key (k1) witch is different from a private second key (k2) which is employed in an execution of said decryption in accordance with said second algorithm (g2) of that part of the software which is encrypted in accordance with said first algorithm (g1) and said public first key (k1).

8. A method according to claim 7, wherein said encryption of said part of the software in accordance with said first algorithm (g1) is executed on a source code of said software prior to compiling and linking to an executable program, or in similar processing steps producing an executable program, and at the same time adding an object code to a data library of said executable program associated with said external unit.

9. A method according to claim 8, wherein said compiling and/or linking of the source code of an executable program, or in similar processing steps producing an executable program, call sequences, or similar instructions, are inserted which in the execution in the computer of said executable program causes a jump to a corresponding entry point to said object code, said object code being used to establish a communication channel between the computer and said external unit in which said decryption takes place in accordance with said second algorithm (g2).

10. A method according to claim 9, further comprising a step wherein a third key (k3) is inserted in said part of the software which is encrypted in accordance with said first algorithm (g1), said third key (k3) being intended to be employed in a third algorithm (g3) for the scrambling of said part of the software prior to its transfer to said external unit.

11. A method of making authorized utilization possible in a computer of software prepared for utilization in the computer only with a corresponding authorization, the method comprising:
encrypting in accordance with a first algorithm (g1) only a part of the software which by the utilization in the computer is decrypted in accordance with a second algorithm (g2), wherein a key which is employed for said encryption in accordance with said first algorithm (g1), is a public first key (k1) which is different from a private second key (k2) which is employed in decryption in accordance with said second algorithm (g2) of said part of the software which is encrypted in accordance with said first algorithm (g1) and said public first key (k1), said encryption of said part of the software in accordance with said first algorithm (g1) is executed on the source code for the software prior to compiling and linking to an executable program, or in similar processing steps producing an executable program, and at the same time adding an object code to the data library of said executable program associated with an external unit adapted to be connected to the computer, and said compiling and/or linking of said source code to an executable program, or in similar processing steps producing an executable program, call sequences, or similar instructions, are inserted which in the execution in the computer of said executable program causes a jump to a corresponding entry point to said object code, said object code being used to establish a communication channel between the computer and said external unit in which said decryption takes place in accordance with said second algorithm (g2);
connecting said external unit to the computer, said external unit comprising a computer readable storage medium and a processor, said second algorithm (g2) and a said private second key (k2) to be employed in said decryption of said encrypted part of the software being stored in said external unit; and when the computer in the execution of said part of the software which is encrypted in accordance with said first algorithm (g1) encounters a call sequence, or a similar instruction, causing a jump to a corresponding entry point to said object code, said object code is utilized by the computer to establish a communication channel to said external unit through which communication channel said encrypted part of the software is transferred in a first transfer session to said external unit to be decrypted by said processor of said external unit in accordance with said second algorithm (g2) and said private second key (k2) both of which being stored in said-external unit, said second key (k2) being different from said public first key (k1) employed in the execution of said encryption of said part of the software in accordance with said first algorithm (g1), and said decrypted software part then being processed in said external unit and a result transferred in a second transfer session in the opposite direction through said communication channel for the further utilization in the computer.

12. A method according to claim 11, wherein a third key (k3) is inserted in said part of the software which is encrypted in accordance with said first algorithm (g1), said third key (k3) being intended to be employed in a third algorithm (g3) for the scrambling of said part of the software prior to its transfer to said external unit, the method further comprising:
prior to said first transfer session to said external unit, said part of the software which is encrypted in accordance with said first algorithm (g1) is caused to be scrambled according to said third algorithm (g3) and said third key (k3), said third key (k3) being based on a number randomly selected for the respective transfer session and provided by said external unit, and said part of the software which is encrypted in accordance with said first algorithm (g1), and which is scrambled according to said third algorithm (g3) and transferred to said external unit in said first transfer session, is caused to be descrambled in said external unit in accordance with a fourth algorithm (g4) and a fourth key (k4), said fourth key (k4) being based on said randomly selected number for said third key (k3), and said fourth algorithm (g4) being the inverse algorithm of said third algorithm (g3).

13. A method according to claim 12, wherein the method further comprises:
a step wherein said result produced in said external unit on the basis of said part of the software which is decrypted in accordance with said second algorithm (g2), prior to said second transfer session, is caused to be scrambled according to a fifth algorithm (g5) and a fifth key (k5), said fifth key (k5) being based on a randomly selected number for the respective transfer session and preferably provided by said external unit, and a step wherein said result produced in said external unit an the basis of said part of the software which is decrypted in accordance with said second algorithm (g2), and which is scrambled according to said fifth algorithm (g5) and transferred in said second transfer session to the computer is caused to be descrambled in the computer in accordance with a sixth algorithm (g6) and a sixth key (k6), said sixth key (k6) being based on said randomly selected number for said fifth key (k5), and said sixth algorithm (g6) being the inverse algorithm of said fifth algorithm (g5).

14. A method according to claim 1, wherein said first algorithm (g1), said second algorithm (g2), said public first key (k1) and said private second key (k2) are determined in accordance with an RSA crypto system.

15. A method according to claim 1, wherein said part of the software to be encrypted in accordance with said first algorithm (g1) is selected by being required for the use of the software by the computer, and comprising one or more instructions in a command or execution file, such as a .COM or .EXE file.

16. A device for the preparation of software to be utilized in a computer only with a corresponding authorization, the device comprising:
crypto means effecting the encryption of only a part of the software in accordance with a first algorithm (g1) and a public first key (k1), and an external unit adapted to be connected to the computer, said external unit comprising a processor and a computer readable storage medium for storing a second algorithm (g2) and a private second key (k2), wherein the device further comprises generator means to provide said second algorithm (g2) and said private second key (k2) intended to be employed in decryption in accordance with said second algorithm (g2), said private second key (k2) being different from said public first key (k1) employed by said crypto means in the execution of said encryption of said part of the software in accordance with said first algorithm (g1).

17. A device according to claim 16, wherein said crypto means is adapted to execute said encryption of said part of the software in accordance with said first algorithm (g1) on a source code of the software before it is compiled and linked, or in any other way is transformed to an executable program, and that said crypto means at the same time effects the addition to a data library of said executable program, of an object code associated with said external unit.

18. A device according to claim 17, wherein said crypto means is adapted to insert during said compiling and/or linking of said source code to an executable program, or during a similar processing producing an executable program, call sequences, or similar instructions, which in the execution of said executable program in the computer provide for a jump to a corresponding entry point to said object code which the computer then utilizes to establish a communication channel to said connected external unit.

19. A device according to claim 18, wherein said crypto means is adapted to enter into said part of the software which is encrypted in accordance with said first algorithm (g1), a third key (k3) intended to be employed in a third algorithm (g3) for scrambling of said part of the software prior to its transfer to said external unit.

20. A device for making authorized utilization of software in a computer possible comprising:
a device including crypto means for effecting the encryption of only a part of the software in accordance with a first algorithm (g1) and a public first key (k1), and an external unit adapted to be connected to the computer, said external unit comprising a processor and a computer readable storage medium for storing a second algorithm (g2) and a private second key (k2), and being disposed to execute decryption of said encrypted part of the software in accordance with said second algorithm (g2) and said private second key (k2), and generator means to provide said second algorithm (g2) and said private second key (k2) intended to be employed in said decryption in accordance with said second algorithm (g2), said private second key (k2) being different from said public first key (k1) employed by said crypto means in the execution of said encryption of said part of the software in accordance with said first algorithm (g1), said device comprising a computer adapted to serve as a host computer for said external unit, wherein said external unit comprises decryption means adapted to execute decryption in accordance with said second algorithm (g2) and said private second key (k2) produced by said generator means.

21. A device according to claim 20, wherein said decryption means is adapted to execute said decryption in accordance with said second algorithm (g2) when said host computer in the execution of said part of the software which is encrypted in accordance with said first algorithm (g1) encounters a call sequence, or the like, which provides for a jump to a corresponding entry point to said object code, whereby said host computer then uses said object code to establish a communication channel to said external unit through which said host computer effects that said encrypted part of the software is transferred in a first transfer session to said external unit and subjected to said decryption, and that said external unit's own processor is adapted to process said decrypted part of the software and effect that a result is transferred the opposite direction in a second transfer session through said communication channel for the further utilization of said result in the computer.

22. A device according to claim 21, wherein said crypto means is adapted to enter into said part of the software which is encrypted in accordance with said first algorithm (g1), a third key (k3) intended to be employed in a third algorithm (g3) for scrambling of said part of the software prior to its transfer to said external unit, wherein said external unit comprises descrambling means to descramble in accordance with a fourth algorithm (g4) and a fourth key (k4) said part of the software being encrypted in accordance with said first algorithm (g1), and which, prior to said first transfer session, is scrambled by said host computer according to said third algorithm (g3) and transferred to said external unit in said first transfer session, said third key (k3) used by said host computer in said scrambling being based on a number randomly selected for the respective transfer session and preferably provided by a number generator in said external unit, and said fourth key (k4) used by said descramble means in said external unit based on said number randomly selected, said fourth algorithm (g4) being the inverse algorithm of said third algorithm (g3).

23. A device according to claim 22, wherein said external unit further comprises:
scrambler means to scramble, prior to said second transfer session, in accordance with a fifth algorithm (g5) and a fifth key (k5), said result produced in said external unit on the basis of said part of the software which said external unit has decrypted in accordance with said second algorithm (g2), said fifth key (k5) used by said scrambler means to scramble being based on a number which is randomly selected for the respective transfer session and produced by a number generator in said external unit, said host computer being capable of descrambling in accordance with a sixth algorithm (g6) and a sixth key (k6), said result produced in said external unit for further utilization in said host computer, said result prior to said second transfer session being scrambled according to said fifth algorithm (g5) by means of said scrambler means in said external unit and transferred to the computer in said second transfer session, said sixth key (k6) being based on the same randomly selected number as that for said fifth key (k5), and said sixth algorithm (g6) being the inverse algorithm of said fifth algorithm (g5).

24. A device according to claim 16, wherein said external unit comprises adapter means and a separate article, being made tamper-proof, in or on which electronic circuits are disposed which at least comprise said external unit processor and computer readable memory and which is arranged such that a communication channel can be established between the circuits of said separate article and said host computer when said separate article is inserted into a slot in said adapter means, or in any other way is connected to said adapter means, said second algorithm (g2) and said private second key (k2) preferably being stored in such a way in said memory that they cannot be read by other means than said external unit processor of said separate article and never become available outside said separate article.

25. A device according to claim 24, wherein said separate article takes the form of a small substrate plate, such as a plastic card, having the size of a usual credit card, or Smart Card, said adapter means then comprising a card reader station for said substrate plate which optionally may be incorporated with said host computer or a peripheral unit connected thereto.

26. A device according to claim24, wherein said separate article takes the form of an electronic circuit card adapted to be placed in a card position or slot provided in said host computer for extension cards, the circuit card, in the case of a portable host computer, being configured in accordance with one of the standard designs used for such insertable cards, such as the PCMCIA
standard.

27. A device according to claim 24, wherein said separate article takes the form of an electronic blind plug, or the like, said adapter means then comprising an input-output terminal of said host computer.

28. A method of preventing unauthorized utilization of software in a computer, comprising the steps of:
encrypting only a part of the software in accordance with a first algorithm (g1) and a public first key (k1);
storing a second algorithm (g2) and a private second key from said public first key (k1) in an external unit having a computer readable storage medium and a processor, said external unit adapted to be connected to the computer; and decrypting said encrypted part of the software using said second algorithm (g2) and said private second key (k2).

29. A device for the preparation of software to be utilized in a computer only with a corresponding authorization, comprising:
determining means for determining a public private key to be used by the computer to decrypt only a part of the software in accordance with a decryption algorithm;
an external unit connected to the computer, wherein said decryption algorithm and said private key are stored in said external unit and encrypting means for encrypting only said part of the software in accordance with an encryption algorithm and another private public key different from said public private key determined by said determining means.

30. A method of preventing unauthorized utilization of software in a computer, the method comprising the steps of:
encrypting at least a part of said software in accordance with a first algorithm (g1) and a first key (k1), the encryption being executed on the source code of the software prior to compiling and linking thereof, or during similar processing steps producing an executable program, and at the same time adding to the data library of the executable program an object code relating to the communication with a tamper-proof external unit connectable to said computer, the external unit comprising at least a computer readable storage medium and a processor of its own, and decrypting the encrypted part of the software in accordance with a second algorithm (k2), the decryption being executed by employing a second key (k2) associated with the authorized utilization of said software and stored in said external unit, characterized in that said decryption of the encrypted part of the software takes place when the computer in the execution of said program encounters a call sequence, or a similar instruction, causing a jump to a corresponding entry point to said added object code, whereby a communication channel Is established between the computer and the external unit by utilization of the added object code, through which communication channel the encrypted part of the software is transferred to the external unit to be decrypted by the unit's own processor, the decrypted software part then being processed in the external unit and the result thereof transferred in the opposite direction through the communication channel for further utilization in the computer.

31. A method according to claim 30, further comprising the steps of:
scrambling in the computer said encrypted part of the software, in accordance with a third algorithm (g3) and a third key (k3), and transferring said scrambled and encrypted part of the software to the external unit, the third key (k3) being based on a number randomly selected for each transfer, and descrambling in the external unit said scrambled and encrypted part of the software in accordance with a fourth algorithm (g4) and a fourth key (k4), the fourth key (k4) being based on said randomly selected number for said third key (k3), and the fourth algorithm (g4) being the inverse algorithm of said third algorithm (g3).

32. A method according to claim 31, further comprising the steps of:
scrambling in the external unit said result produced therein, in accordance with a fifth algorithm (g5) and a fifth key (k5), and transferring said scrambled result to the computer, the fifth key (k5) being based on a number randomly selected for each transfer , and descrambling in the computer said scrambled result in accordance with a sixth algorithm (g6) and a sixth key (k6), the sixth key (k6) being based on said randomly selected number for said fifth key (k5), and the sixth algorithm (g6) being the inverse algorithm of said fifth algorithm (g5).

33. A method according to claim 31 or 32, wherein the same random number is selected for the third and fourth keys (k3, k4), and the same random number is selected for the fifth and sixth keys (k5, k6), respectively.

34. A method of preparing software, particularly software intended for free distribution, for utilization in a computer only with a corresponding authorization, the method comprising the steps of:
encrypting at least a part of said software in accordance with a first algorithm (g1) and a first key (k1), the encryption being executed on the source code of the software prior to compiling and linking thereof, or during similar processing steps producing an executable program, and at the same time adding to the data library of the executable program an object code relating to the communication with a tamper-proof external unit connectable to said computer, the external unit comprising at least a computer readable storage medium and a processor of its own, and storing in said external unit a second algorithm (g2) and a second key (k2) associated with the authorized utilization of said software, the second key being intended for use in the decryption of said encrypted part of the software in accordance with the second algorithm, characterized in that in said compiling and/or linking of the source code to an executable program, or in similar processing steps producing an executable program, one or more call sequences, or similar instructions, are inserted therein, which in the execution in a computer of the program causes a jump to a corresponding entry point to said added object code, the object code being intended for being used to establish a communication channel between the computer and the external unit in which said decryption is to take place by the unit's own processor and the decrypted software part is to be processed and the result thereof transferred in the opposite direction through the communication channel for further utilization in the computer.

35. A method according to claim 34, further comprising the step of generating a third key (k3) intended for being used with a third algorithm (g3) for scrambling said encrypted part of the software prior to its transfer via said communication channel to the external unit.

36. A method of making authorized utilization possible in a computer, of software, particularly freely distributed soft-ware, prepared according to claim 34, the method comprising the steps of:
connecting said external unit to the computer, when the computer in the execution of the program encounters a call sequence, or similar instruction, making a jump to a corresponding entry point to said added object code, and utilizing in the computer the object code to establish said communication channel to the external unit, through which channel the encrypted part of the software is transferred to the external unit to be decrypted by the unit's own processor, the decrypted software part then being processed in the external unit and the result thereof transferred in the opposite direction through the communication channel for further utilization In the computer.

37. A method according to claim 36, the method further comprising the steps of:
scrambling in the computer said encrypted part of the software in accordance with a third algorithm (g3) and a third key (k3), and transferring said scrambled and encrypted part of the software to the external unit, the third key (k3) being based on a number randomly selected for each transfer, and descrambling in the external unit said scrambled and encrypted part of the software in accordance with a fourth algorithm (g4) and a fourth key (k4), the fourth key (k4) being based on said randomly selected number for said third key (k3), and the fourth algorithm (g4) being the inverse algorithm of said third algorithm (g3).

38. A method according to claim 36, the method further comprising the steps of:
scrambling in the external unit said result produced therein, in accordance with a fifth algorithm (g5) and a fifth key (k5), and transferring said scrambled result to the computer, the fifth key (k5) being based on a number randomly selected for each transfer, and descrambling in the computer said scrambled result in accordance with a sixth algorithm (g6) and a sixth key (k6), the sixth key (k6) being based on said randomly selected number for said fifth key (k5), and the sixth algorithm (g6) being the inverse algorithm of said fifth algorithm (g5).

39. A method according to any one of claims 30 to 38, wherein said first and second algorithms (g1, g2) and said first and.second keys (k1, k2) are determined in accordance with a coventional or symmetric crypto arrangement, or an unsymmetric, dual key crypto arrangement (public-key / private-key crypto system), such as the RSA crypto system.

40. A method according to any one of claims 30 to 39, wherein said part of the software to be encrypted in accordance with said first algorithm (g1) is selected by being fully decisive for the use of the software by a computer.

41. A device for the preparation of software, particularly software intended for free distribution, to be utilized in a computer only with a corresponding authorization, the device comprising crypto means for effecting encryption of at least a part of said software in accordance with a first algorithm (g1) and a first key (k1), said crypto means being adapted to execute said encryption on the source code of the software before it is compiled and linked, or in any other way transformed into an executable program, said crypto means at the same time effecting addition to the data library of the executable program, of an object code relating to the communication with a tamper-proof external unit connectable to the computer the external unit at least comprising a processor of its own and a computer readable storage medium for storing a second algorithm (g2) and a second key (k2) associated with the authorized utilization of said software, and being arranged to execute decryption of the encrypted part of the software in accordance with said second algorithm, characterized in that said crypto means is adapted to insert during said compiling and/or linking of the source code to an executable program, or during a similar processing producing an executable program, one or more call sequences, or similar instructions, which in the execution of the program in said computer causes a jump to a corresponding entry point to said added object code, the object code being intended for being used to establish a communication channel between the computer and the external unit in which said decryption is to take place by the unit's own processor, and the decrypted software part is to be processed and the result thereof transferred in the opposite direction through the communication channel for further utilization in the computer.

42. A device according to claim 41, wherein said crypto means is adapted to generate a third key (k3) intended for use with a third algorithm (g3) for scrambling said encrypted part of the software prior to its transfer to said external unit.

43. A device for making authorized utilization of software possible, particularly freely distributed software, prepared by means of a device according to claim 41, the device comprising a computer adapted to serve as a host computer for a tamper-proof external unit which at least comprises a processor of its own and a computer readable storage medium, and being connectable to the host computer for the communication therewith, characterized in that when the host computer in its execution of the program encounters a call sequence, or the like, which causes a jump to a corresponding entry point to said added object code, the host computer is adapted to utilize the object code to establish said communication channel to the external unit for transfer of the encrypted part of the software through the channel to the external unit for decryption by the unit's own processor, the external unit being adapted to process the decrypted part of the software and to provide for transfer of the result thereof in the opposite direction through the communication channel for further utilization in the computer.

44. A device according to claim 43, wherein:
the host computer is adapted to scramble, prior to Its transfer to the external unit, said encrypted part of the software, the scrambling being executed by the host computer in accordance with a third algorithm (g3) and a third key (k3), the third key (k3) being based on a number randomly selected for each transfer, and the external unit is adapted to descramble said scrambled and encrypted part of the software when transferred to the external unit, the descrambling being executed by the external unit in accordance with a fourth algorithm (g4) and a fourth key (k4), the fourth key (k4) being based on said randomly selected number for said third key (k3), and the fourth algorithm (g4) being the inverse algorithm of said third algorithm (g3).

45. A device according to claim 43 or 44, wherein:
the external unit is adapted to scramble, prior to its transfer to the computer, said result produced in therein, the scrambling being executed by the external unit in accordance with a fifth algorithm (g5) and a fifth key (k5), the fifth key (k5) being based on a number randomly selected for each transfer, and the host computer is adapted to descramble the scrambled result produced in the external unit when transferred to the computer, the descrambling being executed by the host computer in accordance with a sixth algorithm (g6) and a sixth key (k6), the sixth key (k6) being based on said randomly selected number for said fifth key (k5), and the sixth algorithm (g6) being the inverse algorithm of said fifth algorithm (g5).

46. A device according to any one of claims 41 to 45, wherein said external unit comprises adapter means and a separate, tamper-proof article in or on which electronic circuits are disposed which at least comprise the external unit processor and computer readable memory and which is arranged such that a communication channel can be established between the circuits of the separate article and said host computer when the article is inserted into a slot in the adapter means, or in any other way is connected to the adapter means, said second key (k2) being stored in such a way in said memory that it cannot be read by other means than the processor of the separate article and never become available outside the separate article.

47. A device according to claim 46, wherein said separate article takes the form of a small substrate plate, such as a plastic card, having the size of a usual credit card, or Smart Card, the adapter means then comprising a card reader station for said substrate plate which optionally may be incorporated with the host computer or a peripheral unit connected thereto.

48. A device according to claim 46, wherein said separate article takes the form of an electronic circuit card adapted to be placed in a card position or slot provided for extension cards in the host computer, the circuit card, in the case of a portable host computer, being configured in accordance with one of the standard designs used for such insertable cards, such as the PCMCIA
standard.

49. A device according to claim 46, wherein said separate article takes the form of an electronic blind plug, or the like, said adapter means then comprising an input-output terminal of the host computer.

50. A method according to claim 31, 37 or 44, wherein the third key (k3) is based on a number randomly selected for each transfer and is produced by the external unit.

51. A method according to claim 32 or 38, wherein the fifth key (k5) is based on a number randomly selected for each transfer and is provided by the external unit.

52. A method according to claim 45, wherein the fifth key (k5) is based on a number randomly selected for each transfer and is provided by a number generator in the external unit.

53. A method according to claim 39, wherein the first key is a public key, and the second key is a private key of the crypto arrangement.

54. A method according to claim 40, wherein said part of the software to be encrypted in accordance with said first algorithm (g1) is selected by being fully decisive for the use of the software by a computer, comprising one or more instructions in a command or execution file, such as a .COM or .EXE file.