Software protection system using a single-key cryptosystem, a hardware-based authorization system and a secure coprocessor

Claims

Having thus described our invention, what we claim as new, and desire to secure by Letters Patent is:

1. A method of restricting use of software to an authorized computer comprising the steps of:

providing said software in a form in which at least a portion thereof is encrypted,

providing an encrypted decryption key for decrypting said encrypted software portion,

providing a physically secure coprocessor coupled to said computer, which coprocessor is capable of decrypting said software if it retains said decryption key,

coupling a transfer token source to said physically secure coprocessor, which transfer token source stores a token whose presence is required to effect a step of retaining said encrypted decryption key,

transferring said encrypted decryption key to said physically secure coprocessor along with said token from said transfer token source,

wherein said transferring step includes the step of altering said transfer token source as said token is transferred to said physically secure coprocessor so that said transfer token source is incapable of again transferring said token.

2. The method of claim 1 which includes the further step of decrypting said encrypted portion of said software in said coprocessor with said decryption key.

3. The method of claim 1 which includes the further step of reproducing said software including said encrypted portion.

4. The method of claim 1 which includes the further step of executing said software on said computer and decrypting said encrypted portion, during said execution, by said coprocessor with said decryption key.

5. The method of claim 1 in which said step of providing said software includes providing a magnetic medium on which said software is recorded.

6. The method of claim 1 in which said step of providing said software includes connecting a communication link to said computer and transmitting said software over said communication link.

7. The method of claim 1 in which said step of providing said physically secure coprocessor and coupling said physically secure coprocessor to said computer includes directly coupling said physically secure coprocessor to an internal bus of said computer.

8. The method of claim 1 in which said step of providing said physically secure coprocessor and coupling said physically secure coprocessor to said computer includes removably coupling said physically secure coprocessor to an input port of said computer.

9. The method of claim 1 in which said source of said transfer token is physically secure.

10. The method of claim 1 which includes the further steps of:

decrypting, in said coprocessor, said encrypted decryption key using a vendor decryption key stored in said coprocessor,

using said decryption key to decrypt said encrypted software portion at those times as said computer attempts to execute said software.

11. The method of claim 1 in which said step of altering said source of said transfer token includes destroying said token as stored in said transfer token source.

12. The method of claim 1 in which said step of transferring said transfer token includes the step of verifying, by said coprocessor, that said transfer token is authentic.

13. The method of claim 12 which includes the step of rejecting said transfer token and said encrypted decryption key in the event that said transfer token is not authentic.

14. The method of claim 12 in which said step of transferring said transfer token includes:

(a) generating a random number and coupling said random number to said transfer token source,

(b) generating, in response to said random number a selected portion of said stored token and coupling said portion to said coprocessor.

15. The method of claim 14 which includes the further steps of:

(c) transferring an encrypted token to said coprocessor,

(d) decrypting said encrypted token to produce a clear text token,

(e) applying said random number to said clear text token to generate in response a clear text token portion,

(f) comparing said selected token portion, generated in said step (b) with said clear text token portion, generated in step (e), and

(g) considering said token source authentic if a result of said step (f) identifies a first relation and considering said token source not authentic if said step (f) identifies any other relation.

16. The method of claim 15 which includes the further step of:

(h) decrypting said software prior to execution, and in which a key used for said decryption is identical to a key employed in said step (d).

17. The method of claim 1 in which said physically and logically secure coprocessor provides for establishing a dual privilege computation environment, a first lower privilege for executing that portion of said software which had been provided in encrypted form, and a higher level privilege for manipulating said decryption key.

18. A method of restricting execution of software to authorized processors and preventing execution of said software by unauthorized processors comprising the steps of:

(a) distributing said software in a form in which at least a significant portion is encrypted,

(b) providing a coprocessor in association with a potentially authorized processor, which coprocessor has a memory space secured against external access for storing decrypted software and operating instructions,

(c) coupling said software to said processor,

(d) coupling a distinct right to execute to said coprocessor and storing said distinct right to execute in said secure memory of said coprocessor,

(e) in response to presence of said distinct right to execute, decrypting and storing said significant portion of said software in said coprocessor, and

(f) executing said stored software portion in said coprocessor.

19. The method of claim 18 in which, along with said step (f), said processor executes encrypted portions of said software.

20. The method of claim 18 in which said distinct right to execute comprises a decryption key and said step (d) includes:

(d1) providing a token cartridge storing a clear text token,

(d2) coupling said token cartridge to said coprocessor,

(d3) generating a token query in said coprocessor and coupling said token query to said token cartridge,

(d4) generating, in response to said token query a token response and coupling said token response to said coprocessor,

(d5) verifying authenticity of said token cartridge, in said coprocessor by testing said token response,

(d6) if and only if said token cartridge is determined to be authentic, transferring a decryption key, representing said distinct right to execute, into said secure storage of said coprocessor.

21. The method of claim 20 in which said step (e) includes using said decryption key to decrypt said software portion.

22. The method of claim 20 in which said step (d5) includes:

(d5a) coupling encrypted token data to said coprocessor,

(d5b) decrypting said encrypted token data in said coprocessor to produce clear text token data,

(d5c) combining said clear text token data with said token query to produce a computed response,

(d5d) comparing said computed response with said token response, and

(d5e) verifying authenticity of said cartridge if said responses are identical, otherwise failing to verify authenticity of said cartridge.

23. The method of claim 22 in which said encrypted token data resides in common on a medium supporting said software.

24. The method of claim 22 in which said encrypted token data is stored in said cartridge.

25. The method of claim 22 in which said encrypted token data is coupled over a communication link to said coprocessor.

26. The method of claim 20 in which said step (d4) includes the step of erasing said clear text token data as said token response is generated.

27. The method of claim 20 in which said step (d3) includes the step of coupling said token query to said token cartridge in a bit serial form of N bit length, where N is an integer, and in which said step (d4) includes the steps of:

(d4a) dividing said clear text token data into first and second portions,

(d4b) for each bit of said query, selecting a bit from either said first or second portions and returning said selected bit as a portion of said token response, and replacing both the selected bit and a corresponding unselected bit of said first and second portions,

whereby after a last bit of the token response is generated said clear text token data in said token cartridge is at least reduced in bit length by twice the bit length of said query.

28. The method of claim 27 in which said bit length of said query is equal to the bit length of said clear text token data portions whereby at conclusion of generation of said token response all said clear text token data has been replaced.

29. A method of securing protected software against unauthorized use without perturbing existing channels of software distribution and, at the same time not interfering in users creation of unlimited backup copies of protected software, said method comprising the steps of:

(a) providing to a user a physically secure coprocessor and coupling said coprocessor to a user host computer to support bidirectional communication therebetween to create a composite computing system including said host computer and said coprocessor,

(b) providing logical security to said coprocessor by:

(1) providing a first privilege level including first level secure memory and first level operating instructions, secured against access or variation by said user, for executing protected software,

(2) providing a second privilege level including second level secure memory and second level operating instructions, secured against access or variation by said user or any author of protected software, for controlling authorization for execution of said protected software by said first privilege level,

(c) distributing protected software in a form in at which at least a portion is inexecutable by said host computer but which is executable by said coprocessor but only with authorization by said second privilege level,

(d) distributing a further tangible element distinct from said protected software representing a right to execute said protected software,

(e) providing said composite computing system access to said protected software and to said further tangible element,

(f) verifying authenticity of said further tangible element by said coprocessor at said second privilege level,

(g) altering said second level secure memory in a distinctive fashion to reflect a determination by said second privilege level of authenticity of said tangible element, and

(h) executing said protected software so long as said alteration of said second privilege level secure memory is detected and denying said request if said alteration is not present.

30. A method as recited in claim 29 in which said software as distributed in said step (c) has at least a portion encrypted and said second privilege level subsequent to performance of said step (f) has access to a decryption key and in which said step (h) comprises performing the following steps on each subsequent request by said user to execute said protected software:

(1) responding, at said second privilege level to check for said alteration of said second level secure memory, if said alteration is present honoring said request by;

(a) initiating decryption of said protected software and storage of said decrypted software in said first privilege level secure memory,

(b) authorizing execution of said decrypted software by said first privilege level and initiating operation of said first privilege level,

and, if said alteration is not present, refusing said request.

31. A method as recited in claim 29 wherein said protected software as distributed in said step (c) includes a portion which is encrypted under a software key which itself is distributed encrypted under a different key, and in which said second privilege level has access to said different key so that said encrypted software key can be decrypted by said second privilege level, and in which said step (g) includes:

(1) decrypting said software key with said second key,

(2) altering said second privilege level secure memory by writing said software key therein.

32. A method as recited in claim 31 in which said composite computing system is provided information with which to verify said tangible element and in which said step (d) includes:

(1) providing said tangible element with physical security,

(2) providing said tangible element with electronic storage having a destructive read function, and

(3) storing a verifiable entity in said electronic storage.

33. A method as recited in claim 32 in which said information with which to verify said tangible element comprises said verifiable entity encrypted under said software key and in which said step (f) includes:

(1) generating a random number and storing said random number for later use,

(2) transmitting said random number to said tangible element,

(3) transmitting from said tangible element a reply comprising a transformation of said verifiable entity determined by said random number,

(4) using said software key to decrypt said encrypted verifiable entity,

(5) simulating at said coprocessor said transformation using said random number and said decrypted verifiable entity to generate an expected reply, and

(6) comparing said reply and expected reply and determining authenticity only if a result of said comparison is an equality,

whereby said destructive read property of said tangible element restricts it to a use once device and review of said random number and said reply is inadequate to specify said verifiable entity so that operation of said tangible device cannot be simulated notwithstanding access to said random number and said reply.

34. A method as recited in claim 29 in which said composite computing system is provided information with which to verify said tangible element and in which said step (d) includes:

(1) providing said tangible element with physical security,

(2) providing said tangible element with electronic storage having a destructive read function, and

(3) storing a verifiable entity in said electronic storage.

35. A method as recited in claim 34 wherein:

said protected software as distributed in said step (c) includes a portion which is encrypted under a software key which itself is distributed encrypted under a different key,

in which said second privilege level has access to said different key so that said encrypted software key can be decrypted by said second privilege level,

in which said information with which to verify said tangible element comprises said verifiable entity encrypted under said software key, and

in which said step (f) includes:

(1) generating a random number and storing said random number for later use,

(2) transmitting said random number to said tangible element,

(3) transmitting from said tangible element a reply comprising a transformation of said verifiable entity determined by said random number,

(4) using said software key to decrypt said encrypted verifiable entity,

(5) simulating at said coprocessor said transformation using said random number and said decrypted verifiable entity to generate an expected reply, and

(6) comparing said reply and expected reply and determining authenticity only if a result of said comparison is an equality,

whereby said destructive read property of said tangible element restricts it to a use once device and review of said random number and said reply is inadequate to specify said verifiable entity so that operation of said tangible device cannot be simulated notwithstanding access to said random number and said reply.