A Trusted Execution Environment (TEE) is a hardware-enforced isolated execution context whose confidentiality and integrity are protected by the CPU itself, rather than by software policy. The defining property of a TEE is that its guarantees hold against an adversary with full control of the software stack outside it — the hypervisor, the host operating system, the BIOS firmware, and even a user with physical access to the machine — because the enforcement is implemented in silicon and cannot be overridden by software. Code and data inside a TEE are encrypted in DRAM using a key held within the CPU’s memory controller or on-die security processor, CPU register state is isolated from the host at context switch boundaries, and memory integrity protection prevents the host from replaying, remapping, or aliasing TEE memory pages. The threat model TEEs are designed against is therefore the infrastructure provider: a cloud operator, a data centre staff member, or a co-tenant who controls the hypervisor layer — the party that traditional virtualisation, namespaces, and access control cannot protect against because they depend on a trusted host kernel.
TEEs are not a single technology but a family of implementations that share the same conceptual model at different granularities and from different CPU vendors. At the process level, Intel SGX (Software Guard Extensions) creates small isolated regions called enclaves within a process’s address space — the enclave code and data are hardware-encrypted and inaccessible to the OS or hypervisor, but the enclave must be written to run without system calls (or via a shim layer), making it suitable for small, cryptographic, or credential-holding components rather than general workloads. At the virtual machine level, Intel TDX (Trust Domain Extensions) and AMD SEV-SNP (Secure Encrypted Virtualization — Secure Nested Paging) lift the TEE boundary to encompass an entire VM: the guest kernel, all userspace processes, and the full memory of the VM are protected, requiring no application changes and making it practical to run existing workloads in a confidential environment by simply changing the hypervisor configuration. Arm CCA (Confidential Compute Architecture) and its Realm construct provide an analogous VM-level TEE on AArch64 platforms. At the SoC level, Arm TrustZone divides the CPU into a secure world and a normal world at the hardware level, used in embedded and mobile devices (Android’s Trusted Execution Environment for key storage, biometric processing, and DRM) rather than in server confidential computing.
The two properties that make a TEE actionable beyond mere isolation are attestation and sealing. Attestation is the mechanism by which a TEE proves its identity and configuration to a remote party: the CPU produces a signed evidence document — an SGX Quote, a TDX Quote, or a SEV-SNP attestation report — containing the hash of the code and initial data loaded into the TEE, the platform’s firmware version, and security-relevant configuration, all signed by a hardware-rooted key whose certificate chain is published by the CPU vendor (Intel’s DCAP infrastructure, AMD’s Key Distribution Service). A remote verifier can check this evidence against the vendor certificate chain and against expected reference values to confirm that the TEE is genuine hardware running unmodified, specific software — before sending it any secrets. Sealing is the complementary local operation: the TEE asks the CPU to encrypt a secret (a key, a credential, a policy) bound to the TEE’s identity so that only the same TEE configuration on the same hardware can decrypt it, providing persistence across TEE restarts without exposing the secret to the host. Together, attestation and sealing are what transform hardware isolation from a passive containment property into an active trust anchor: Trustee (for CoCo) uses attestation to gate secret delivery; Keylime uses TPM-anchored attestation (TPM being a related but distinct hardware root of trust) for conventional host integrity; LUKS disk encryption can be sealed to platform measurements via TPM PCRs, which is the non-TEE equivalent of TEE sealing. The CCC (Confidential Computing Consortium), hosted by the Linux Foundation, is the industry body that coordinates TEE standards, open-source TEE software stacks, and the definition of confidential computing as a discipline — its members include Intel, AMD, Arm, IBM, Google, Microsoft, and Red Hat.