Among the TEEs used in confidential computing, which correctly describes the difference between VM-based TEEs that protect a whole virtual machine (such as AMD SEV-SNP and Intel TDX) and enclave-based TEEs that protect part of an application (such as Intel SGX)?

Answer: B. VM-based TEEs run existing OSes and applications almost unchanged, but the whole guest OS becomes part of the trusted computing base (TCB)

An enclave runs only the part of an application that handles secrets in a protected region, so what must be trusted (the TCB: trusted computing base) shrinks to the CPU and the enclave code, but the application has to be split and rewritten. A VM-based TEE encrypts the memory of a whole virtual machine and protects it from the hypervisor and the cloud administrators, so existing software can be moved almost unchanged. In exchange, the guest OS and all the software on top of it enter the TCB, and any vulnerability there can leak secrets. A smaller TCB is easier to verify and offers a smaller attack surface, so the choice is a trade-off between ease of use and the size of what must be trusted.

• A. VM-based TEEs also have the CPU encrypt virtual machine memory with a per-VM key.
• B. Correct. Ease of migration is bought with a larger range of trust.
• C. It is the other way round. Enclaves do not trust the OS or hypervisor, while VM-based TEEs include the guest OS in what is trusted.