Systems and methods for managing state
The disclosure relates to systems and methods for managing state using relatively small assistance from protected hardware. Obfuscated code segments may communicate with supporting protected hardware, store encrypted state values in main memory, and/or communicate via secure channels to secure platform hardware components. In various embodiments, consistent state may be achieved, at least in part, by computing secure tag information and storing the secure tag information in a secure and/or otherwise protected device register. Consistent with embodiments disclosed herein, the tag information may be used to derive keys used to encrypt and/or decrypt stored state information. Tag information may further be used in connection with verification operations prior to using the information to derive associated keys.
1. A method for managing program state information associated with a protected software function performed by a computing system comprising secure hardware, a processor, and a non-transitory computer-readable medium storing instructions that, when executed by the processor, cause the computing system to perform the method, the method comprising:
receiving software function inputs;
receiving encrypted initial program state information;
retrieving a previous secure tag and a current secure tag associated with the encrypted initial program state information from a protected memory register of the secure hardware;
decrypting the encrypted initial program state information using a fixed key to generate decrypted initial program state information and a third secure tag;
determining that the previous secure tag and the third secure tag match;
based on determining that the previous secure tag and the third secure tag match, executing the protected software function using, at least in part, the software function inputs and the decrypted initial program state information to generate updated program state information; and
generating an updated current secure tag based, at least in part, on a shared key, the software function inputs, and the current secure tag.
2. The method of claim 1 , wherein the method further comprises:
encrypting the updated program state information using the fixed key; and
storing the encrypted updated program state information in system memory maintained on the non-transitory computer-readable medium.
3. The method of claim 2 , wherein encrypting the updated program state information using the fixed key is performed using an authenticated encryption function.
4. The method of claim 1 , wherein the fixed key comprises a protected fixed key.
5. The method of claim 1 , wherein the shared key comprises a key shared by the protected software function and the secure hardware.
6. The method of claim 1 , wherein the method further comprises storing the updated current secure tag in the protected memory register.
7. The method of claim 1 , wherein the updated current secure tag comprises a message authentication code tag and wherein generating the updated current secure tag comprises cryptographically signing information including at least the protected software function inputs and the current secure tag using the shared key.
8. The method of claim 1 , wherein the software function inputs comprise at least one of an external program input, a dynamic device resource input, and a static device resource input.
9. The method of claim 1 , wherein the previous secure tag and the current secure tag comprise message authentication code tags.
10. The method of claim 9 , wherein the previous secure tag and the current secure tag comprise sequential tags included in the protected memory register.
11. The method of claim 9 , wherein the previous secure tag and the current secure tag comprise sequential tags included in a hash chain maintained in the protected memory register.
12. The method of claim 1 , wherein the encrypted initial program state information is retrieved from memory of the computing system.
13. The method of claim 1 , wherein the protected software function comprises an obfuscated software function.
14. The method of claim 1 , wherein the protected software function comprises a whitebox-protected software function.
15. The method of claim 1 , wherein the method further comprises:
generating a state encryption key using a key derivation function with the fixed key and the current secure tag as inputs to the key derivation function; and
encrypting the updated program state information using the state encryption key.
16. The method of claim 1 , wherein decrypting the encrypted initial program state information using the fixed key is performed using an authenticated decryption function.
17. The method of claim 1 , wherein the method further comprises validating the integrity of the previous secure tag and the current secure tag.