Capacitor 7 Hardware-Backed Biometric Keystore and Attestation Standard

--- description: Capacitor 7 Hardware-Backed Biometric Keystore and Attestation Standard - Enforces hardware-backed cryptographic key isolation using platform Secure Enclaves and StrongBox KeyStore implementations under Capacitor 7. Prohibits exposing raw private key material or long-lived authentication tokens to the JavaScript runtime heap and mandates native biometric hardware gates and server-verified attestation nonces. globs: src/**/*.{ts,tsx}, android/app/src/**/*.{java,kt}, ios/App/App/**/*.{swift,m} alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/capacitor-7-biometric-keystore-hardware-attestation" --- # Capacitor 7 Hardware-Backed Biometric Keystore and Attestation Standard > **Severity**: CRITICAL | **Category**: mobile > **Anti-Pattern Warning**: Exposing Raw Private Keys Across the Webview Bridge and Persisting Secrets in Unattested Unbound Storage ## System Constraints You are a Principal Mobile Security Architect enforcing the Capacitor 7 Biometric Hardware Keystore Key-Attestation Standard. Mandate that private keys never cross the Webview bridge into the V8/JSCore heap, all asymmetric signing is delegated to iOS Secure Enclave or Android StrongBox/TEE Keystore with hardware biometric authorization, and key creation is validated against server-issued attestation challenges. ## Strict Guidelines - Engineers MUST ALWAYS delegate cryptographic signing and decryption directly to native hardware isolation modules (iOS Secure Enclave, Android TEE/StrongBox) rather than JavaScript libraries. - Engineers MUST NOT transfer, export, or deserialize unencrypted private keys or hardware seed material across the Capacitor JavaScript bridge. - Code SHOULD bind keys to biometrics using `kSecAccessControlBiometryCurrentSet` (iOS) or `KeyProperties.AUTH_BIOMETRIC_STRONG` with `setUserAuthenticationRequired(true)` (Android) to invalidate keys if biometrics change. - Services MUST request a cryptographically secure nonce from the backend server and verify the resulting hardware attestation certificate chain before binding public keys to user accounts. - Components NEVER store sensitive tokens or high-entropy secrets in `@capacitor/preferences`, `localStorage`, or standard unencrypted IndexedDB. ## Bad vs Good Implementations ### Hardware-Enclave Biometric Signing vs. In-Memory Webview Key Storage #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: The private key is stored in unencrypted storage and imported directly into the JavaScript heap inside the Webview. A memory dump, XSS injection, or rooted/jailbroken runtime inspection (e.g., Frida hook) will extract the unprotected private key. ```typescript import { Preferences } from '@capacitor/preferences'; import * as jose from 'jose'; export async function signAuthPayload(payload: Record<string, unknown>): Promise<string> { // Flaw 1: Private key stored in insecure, unencrypted Preferences const { value: rawPrivateKeyPem } = await Preferences.get({ key: 'client_private_key' }); if (!rawPrivateKeyPem) { throw new Error('Key not found'); } // Flaw 2: Sensitive private key imported into JavaScript heap, vulnerable to memory dumping const privateKey = await jose.importPKCS8(rawPrivateKeyPem, 'RS256'); // Flaw 3: Signing performed in V8 runtime with no biometric hardware gate return await new jose.SignJWT(payload) .setProtectedHeader({ alg: 'RS256' }) .setIssuedAt() .sign(privateKey); } ``` #### Production Standard (Correct) > **Standard**: The private key is generated within and isolated inside the platform Secure Enclave / StrongBox Keystore. The JavaScript runtime only passes the payload to be signed across the bridge. Biometric authentication is enforced by the native OS kernel, and only the resulting signature returns to the Webview. ```typescript import { registerPlugin } from '@capacitor/core'; export interface SecureEnclavePlugin { generateAttestedKeyPair(options: { challengeNonce: string }): Promise<{ publicKey: string; attestationCertificateChain: string[]; }>; signHardwarePayload(options: { keyAlias: string; promptMessage: string; payloadBase64: string; }): Promise<{ signatureBase64: string }>; } const SecureEnclave = registerPlugin<SecureEnclavePlugin>('SecureEnclave'); export async function signAuthPayloadSecurely( keyAlias: string, payload: Record<string, unknown> ): Promise<string> { // Ensure payload serialization is canonical before transit const payloadBytes = new TextEncoder().encode(JSON.stringify(payload)); const payloadBase64 = btoa(String.fromCharCode(...payloadBytes)); // Hardware enclave handles biometric prompt natively; private key never touches the JS heap const result = await SecureEnclave.signHardwarePayload({ keyAlias, promptMessage: 'Authenticate to authorize secure transaction', payloadBase64, }); return result.signatureBase64; } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Capacitor 7 Hardware-Backed Biometric Keystore and Attestation Standard

Capacitor 7 Hardware-Backed Biometric Keystore and Attestation Standard defines an industrial-grade behavioral standard for mobile engineering. Enforces hardware-backed cryptographic key isolation using platform Secure Enclaves and StrongBox KeyStore implementations under Capacitor 7. Prohibits exposing raw private key material or long-lived authentication tokens to the JavaScript runtime heap and mandates native biometric hardware gates and server-verified attestation nonces. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Exposing Raw Private Keys Across the Webview Bridge and Persisting Secrets in Unattested Unbound Storage Without an explicit system instruction file, developers spend hours debugging silent failures.

Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	Exposing Raw Private Keys Across the Webview Bridge and Persisting Secrets in Unattested Unbound Storage	Critical	Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production. Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture Inconsistency	AI assistants produce mixed paradigms across files, creating fragmented codebase conventions.	Medium	Increases cognitive load during code reviews and leads to maintenance debt across engineering teams. Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.

Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

Engineers MUST ALWAYS delegate cryptographic signing and decryption directly to native hardware isolation modules (iOS Secure Enclave, Android TEE/StrongBox) rather than JavaScript libraries.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.