Secure Code Execution from External Memory via Authenticated Encryption

The patent applies preliminary action by encrypting the code in external memory before the SoC is powered on or exits low power mode. The encryption is performed in advance during a secure provisioning phase, so that when the code is later read and executed, it is already protected. This prevents adversaries from reading or manipulating the code when the device is off, as the code remains encrypted until the final decryption step during execution.

The patent introduces encryption as an intermediary layer between the code and the external memory. Instead of storing plaintext code that can be directly read and replaced, the code is encrypted using a block cipher with authenticated encryption mode. This intermediary encryption layer prevents adversaries from reading or replacing the code, as any modification would be detected during authentication. The encryption acts as a mediator that protects the code while allowing it to be stored in external memory.

The patent converts the potential harm of using external memory (which is vulnerable to physical attacks) into a benefit by applying authenticated encryption. The encryption scheme includes authentication tags that detect any tampering or fault injection attempts. Even if adversaries attempt side channel attacks or fault injection, the authentication mechanism will detect the tampering and prevent execution of compromised code. This transforms the vulnerable external memory into a secure storage medium by using the encryption as a protective measure against the very attacks that external memory is susceptible to.