feat(patterns): add TEE-based privacy pattern

patterns/pattern-tee-based-privacy.md

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+---
+title: "Pattern: TEE-Based Privacy"
+status: draft
+maturity: pilot
+layer: offchain
+privacy_goal: Protect computation and data confidentiality via hardware-isolated execution
+assumptions: Hardware vendor trust, attestation infrastructure, physical security of deployment environment
+last_reviewed: 2026-01-14
+works-best-when:
+  - Confidential computation needed with lower latency than ZK proofs
+  - Parties accept hardware trust assumptions over cryptographic-only solutions
+  - Institutional-grade infrastructure with known operators and vendors
+avoid-when:
+  - Threat model includes nation-state physical access or supply-chain compromise
+  - Full trustlessness required (prefer ZK or MPC alternatives)
+  - Long-term secrets that outlive hardware security lifecycle
+dependencies: [Intel SGX, AMD SEV-SNP, AWS Nitro Enclaves, Azure Confidential Computing]
+---
+
+## Intent
+
+Use Trusted Execution Environments (TEEs) to isolate sensitive computation from the host operating system, hypervisor, and operator. TEEs provide hardware-enforced confidentiality and integrity for code and data during execution, enabling privacy-preserving operations without exposing plaintext to infrastructure providers.
+
+This is a foundational pattern describing TEE trust models and failure modes. Specific applications (key management, settlement, matching) build on this foundation.
+
+## Ingredients
+
+- **Hardware Platforms**:
+  - **Intel SGX**: Process-level enclaves with memory encryption (SGX1, SGX2, SGX3)
+  - **AMD SEV-SNP**: VM-level isolation with memory encryption and integrity
+  - **AWS Nitro Enclaves**: Isolated VMs with no persistent storage, no network access
+  - **Azure Confidential Computing**: SGX and SEV-SNP offerings with attestation services
+  - **ARM TrustZone**: Mobile/embedded TEE (less common in institutional settings)
+
+- **Attestation Infrastructure**:
+  - Remote attestation services (Intel IAS/DCAP, AMD KDS, AWS Nitro attestation)
+  - Attestation verification logic (on-chain or off-chain)
+  - Measurement registries for approved code/configurations
+
+- **Operational Requirements**:
+  - Secure provisioning and sealing key management
+  - Firmware/TCB update policies
+  - Physical security of hosting environment
+  - Incident response runbooks
+
+## Trust Model
+
+### Who Must Be Trusted
+
+| Entity | Trust Requirement | Mitigation |
+|--------|-------------------|------------|
+| **Hardware Vendor** | Correct implementation, no backdoors | Vendor reputation, third-party audits, multi-vendor strategy |
+| **Firmware/Microcode** | No vulnerabilities, timely patches | TCB recovery, update policies, attestation checks |
+| **Cloud Provider** | Physical security, correct hypervisor | Contractual obligations, attestation, multi-cloud |
+| **Operator** | Correct deployment, no tampering | Remote attestation, sealed secrets, audit logs |
+| **Code Author** | Correct enclave logic | Open source, audits, formal verification |
+
+### What TEEs Protect
+
+- **Confidentiality**: Data and code encrypted in memory; inaccessible to host OS/hypervisor
+- **Integrity**: Tamper detection via hardware measurements and attestation
+- **Isolation**: Execution separated from other processes and privileged software
+
+### What TEEs Do NOT Protect
+
+- **Availability**: Host can deny service, power off, or refuse to schedule enclave
+- **Side Channels**: Timing, cache, power, and speculative execution leaks possible
+- **Physical Attacks**: Sophisticated attackers with hardware access may extract secrets
+- **Supply Chain**: Compromised hardware from manufacturing may have backdoors
+
+## Protocol (concise)
+
+1. **Provision**: Deploy TEE workload with measured code image; generate or import secrets.
+2. **Attest**: TEE produces attestation report; verifier checks measurement against expected values.
+3. **Establish Trust**: Clients verify attestation before sending sensitive data.
+4. **Execute**: TEE processes confidential data; results encrypted before leaving enclave.
+5. **Audit**: Log attestation events and operations for compliance; seal state for persistence.
+6. **Rotate/Upgrade**: Update code or firmware; re-attest; migrate sealed state if needed.
+
+## Failure Modes
+
+| Failure Mode | Description | Impact | Mitigation |
+|--------------|-------------|--------|------------|
+| **Supply Chain Compromise** | Backdoored hardware from manufacturing | Complete confidentiality loss | Multi-vendor, hardware audits, attestation checks |
+| **Firmware Vulnerability** | Exploitable bugs in microcode/firmware | Enclave escape, secret extraction | TCB recovery, rapid patching, version policies |
+| **Side-Channel Attack** | Cache timing, Spectre/Meltdown variants | Partial secret leakage | Constant-time code, partitioning, updates |
+| **Rollback Attack** | Replay of old sealed state | Policy bypass, double-spend | Monotonic counters, external anchoring |
+| **Denial of Service** | Host refuses to run enclave | Availability loss | Redundancy, fallback procedures, SLAs |
+| **Key Exfiltration** | Bug in enclave code leaks secrets | Complete compromise | Audits, formal verification, minimal TCB |
+
+## Guarantees
+
+- **Confidentiality**: Protected from host/operator under stated trust assumptions
+- **Integrity**: Attestation proves code and configuration match expectations
+- **Auditability**: Attestation logs provide verifiable execution history
+- **Portability**: Secrets can be sealed and migrated between attested enclaves
+
+## Trade-offs
+
+- **Hardware Dependency**: Locked to specific vendors; supply constraints possible
+- **Trust Surface**: Larger than pure cryptographic solutions; hardware/firmware vulnerabilities affect all deployments
+- **Performance**: Fast execution but limited memory (SGX); context switches have overhead
+- **Lifecycle**: Hardware security erodes over time as attacks improve; plan for migration
+- **Regulatory Uncertainty**: Some regulators may not accept "black box" execution for audit purposes
+
+## When TEEs Are Appropriate
+
+| Use Case | TEE Fit | Notes |
+|----------|---------|-------|
+| Hot key management | Good | Faster than MPC; acceptable trust for operational keys |
+| Private matching engine | Good | Real-time performance; ZK too slow for orderbooks |
+| Bridge/oracle relayer | Acceptable | Defense in depth with other controls |
+| Long-term custody | Poor | Prefer MPC or cold storage for years-long secrets |
+| Regulatory-critical audit | Uncertain | Depends on regulator acceptance |
+
+## Upgrade Paths
+
+TEEs should be viewed as a transitional technology when pure cryptographic solutions are impractical:
+
+1. **TEE → MPC**: Migrate key management to threshold schemes as MPC performance improves
+2. **TEE → ZK**: Replace confidential computation with ZK proofs when circuits become practical
+3. **TEE + ZK Hybrid**: Use TEE for execution, ZK for verifiable output (see pattern-tee-zk-settlement)
+4. **Multi-TEE**: Distribute across vendors/platforms to reduce single-vendor risk
+
+## Example
+
+**Confidential Order Matching**
+
+1. Exchange deploys matching engine inside AWS Nitro Enclave.
+2. Traders verify attestation showing approved matching code.
+3. Orders submitted encrypted; decrypted only inside enclave.
+4. Matching occurs privately; only fills are published.
+5. Attestation logs prove matching logic was not tampered with.
+6. Auditors can verify code measurements without seeing order flow.
+
+## See also
+
+- [TEE Key Manager](pattern-tee-key-manager.md) - Specific application for custody
+- [Hybrid TEE + ZK Settlement](pattern-tee-zk-settlement.md) - TEE execution with ZK verification
+- [MPC Custody](pattern-mpc-custody.md) - Cryptographic alternative to TEE key management
+- [Verifiable Attestation](pattern-verifiable-attestation.md) - On-chain attestation verification
+
+## See also (external)
+
+- Intel SGX: https://www.intel.com/content/www/us/en/architecture-and-technology/software-guard-extensions.html
+- AMD SEV: https://www.amd.com/en/developer/sev.html
+- AWS Nitro Enclaves: https://aws.amazon.com/ec2/nitro/nitro-enclaves/
+- Azure Confidential Computing: https://azure.microsoft.com/en-us/solutions/confidential-compute/
+- TEE security research: https://tee.dev/