Hardware key signing: Secure Enclave / StrongBox / TPM integration

[joelteply]

Summary

Phase 2 signing using platform hardware security modules for non-exportable ES256 keys.

Platforms

• macOS/iOS: SecKeyCreateSignature with Secure Enclave key (Touch ID protected)
• Android: KeyStore with setIsStrongBoxBacked(true)
• Windows: TPM 2.0 via NCryptSignHash

Design

This is NOT WebAuthn — it's raw Secure Enclave access. WebAuthn signs clientDataJSON (challenge-response), not arbitrary data. The platform's native crypto API signs the JCS-canonical attestation payload directly.

The credentialId field optionally links to a WebAuthn credential for cross-referencing.

Depends on

• JCS canonicalization (RFC 8785 JCS canonicalization + ES256 signing implementation #1)
• Key registry (Key registry: hosted service with revocation and rotation #2)