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 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 at least one protected input from the secure hardware and 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 at least one protected input using a program input decryption key to generate decrypted program input and a nonce value;
comparing the nonce value and the previous secure tag to determine that the nonce value and the previous secure tag match;
generating a state decryption key based on a protected fixed key and the previous secure tag;
decrypting the encrypted initial program state information using the state decryption key to generate decrypted initial program state information;
executing the protected software function using, at least in part, the decrypted program input and the decrypted initial program state information to generate updated program state information and program output information;
generating a state encryption key based on the protected fixed key and the current secure tag;
encrypting the updated program state information using the state encryption key to generate encrypted updated program state information; and
storing the encrypted updated program state information in system memory maintained on the non-transitory computer-readable medium.
2 . The method of claim 1 , wherein the method further comprises validating the previous secure tag and the current secure tag.
3 . The method of claim 2 , wherein validating the previous secure tag and the current secure tag comprises generating a hash value based, at least in part, on the previous secure tag and comparing the generated hash value with the current secure tag.
4 . The method of claim 3 , wherein validating the previous secure tag and the current secure tag further comprises determining, based on the comparison, that the generated hash value and the current secure tag are the same value.
5 . The method of claim 3 , wherein generating the hash value is further based on a shared key.
6 . The method of claim 1 , wherein the program input decryption key is generated using, at least in part, a shared key.
7 . The method of claim 6 , wherein the shared key comprises a key shared between the secure hardware and the protected software function.
8 . The method of claim 1 , wherein the method further comprises receiving one or more software function inputs and wherein executing the protected software function to generate the updated program state information and the program output information further comprises executing the protected software function further using the one or more software function inputs.
9 . The method of claim 8 , 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.
10 . The method of claim 1 , wherein the previous secure tag and the current secure tag comprise message authentication code tags.
11 . The method of claim 10 , wherein the previous secure tag and the current secure tag comprise sequential tags included in the protected memory register.
12 . The method of claim 11 , wherein the previous secure tag and the current secure tag comprise sequential tags included in a hash chain maintained in the protected memory register.
13 . The method of claim 1 , wherein receiving the encrypted initial program state information comprises retrieving the encrypted initial program state information from the system memory.
14 . The method of claim 1 , wherein the protected software function comprises an obfuscated software function.
15 . The method of claim 1 , wherein the protected software function comprises a whitebox protected software function.
16 . The method of claim 1 , wherein generating the state decryption key comprises generating the state decryption key using a key derivation function with the protected fixed key and the previous secure tag as inputs to the key derivation function.
17 . The method of claim 1 , wherein generating the state encryption key comprises generating the state encryption key using a key derivation function with the protected fixed key and the current secure tag as inputs to the key derivation function.
18 . The method of claim 1 , wherein decrypting the encrypted initial program state information using the state decryption key is performed using an authenticated decryption function.
19 . The method of claim 1 , wherein encrypting the updated program state information using the state encryption key is performed using an authenticated encryption function.