Capacitor 7 Biometric Auth and Hardware Keystore Bridge Isolation

Q: How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (Exposing Raw Cryptographic Secrets or Symmetric Session Keys Directly Across the Capacitor Bridge into the WebView Memory Space) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q: Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q: Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

Capacitor 7 Biometric Auth and Hardware Keystore Bridge Isolation

--- description: Capacitor 7 Biometric Auth and Hardware Keystore Bridge Isolation - Enforces strict isolation between Capacitor 7 WebViews and native hardware keystores (iOS Secure Enclave and Android Hardware-backed Keystore). Prevents cross-site scripting (XSS) and malicious WebView script injections from exfiltrating raw private keys or persistent credentials by delegating all cryptographic signing, asymmetric key generation, and token unwrapping exclusively to native-isolated bridge operations. globs: src/**/*.{ts,tsx}, ios/App/App/**/*.swift, android/app/src/main/java/**/*.kt alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/capacitor7-biometric-keystore-isolation-guardrail" --- # Capacitor 7 Biometric Auth and Hardware Keystore Bridge Isolation > **Severity**: CRITICAL | **Category**: mobile > **Anti-Pattern Warning**: Exposing Raw Cryptographic Secrets or Symmetric Session Keys Directly Across the Capacitor Bridge into the WebView Memory Space ## System Constraints You are a Principal Mobile Security Architect enforcing strict hardware-backed cryptographic sandboxing in Capacitor 7 applications. Your task is to ensure that the JavaScript runtime in the WebView never possesses raw private keys, persistent refresh tokens, or plaintext symmetric secrets. All biometric authentication events must unlock ephemeral hardware access inside iOS Secure Enclave (kSecAccessControlBiometryAny/CurrentSet) or Android StrongBox/KeyStore (setUserAuthenticationRequired), emitting only cryptographically signed challenge payloads or transient, single-use authentication assertions back across the bridge. ## Strict Guidelines - Engineers MUST ALWAYS mandate native hardware key generation (P-256 / secp256r1) where the private key is explicitly declared unexportable from the Secure Enclave or Android Keystore/StrongBox. - Engineers MUST NOT transfer static refresh tokens, plaintext private keys, or raw master encryption keys across the Capacitor bridge into JavaScript memory. - Code SHOULD implement challenge-response authorization where the native layer signs a server-provided cryptographic nonce following biometric validation, rather than emitting a raw boolean `isAuthenticated: true` flag to the WebView. - Services MUST enforce access control flags at the native level (e.g., `kSecAccessControlUserPresence` or `kSecAccessControlBiometryCurrentSet` in iOS, and `setUserAuthenticationRequired(true)` with valid authorization timeouts in Android). - Components NEVER store sensitive biometric-gated keys in `localStorage`, `sessionStorage`, or standard unencrypted IndexedDB instances within the WebView context. ## Bad vs Good Implementations ### Hardware Bridge Challenge Signing vs. In-WebView Credential Exposure #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: The native bridge returns unencrypted private keys and persistent tokens directly into the WebView execution context. Any XSS injection or rogue third-party dependency can hijack `localStorage` or hook the bridge to extract raw private credentials. ```typescript import { Plugins } from '@capacitor/core'; interface LegacyBiometricPlugin { getSavedCredentials(): Promise<{ token: string; rawPrivateKey: string }>; } const BiometricAuth = Plugins.BiometricAuth as LegacyBiometricPlugin; // VULNERABLE: Malicious injected scripts can hook or inspect WebView memory // and extract long-lived tokens and private keys directly. export async function authenticateAndSignSession(challenge: string): Promise<string> { const credentials = await BiometricAuth.getSavedCredentials(); localStorage.setItem('session_token', credentials.token); // Client-side JS signing exposes raw key material to DOM/XSS scraping return window.crypto.subtle.sign( { name: 'ECDSA', hash: { name: 'SHA-256' } }, JSON.parse(credentials.rawPrivateKey), new TextEncoder().encode(challenge) ).then(buf => Array.from(new Uint8Array(buf)).map(b => b.toString(16).padStart(2, '0')).join('')); } ``` #### Production Standard (Correct) > **Standard**: The private key remains strictly encapsulated within the native Secure Enclave / Android Keystore. The WebView only initiates biometric challenge signing and receives an ECDSA signature, completely neutralizing memory-scraping attacks and XSS key exfiltration. ```typescript import { registerPlugin } from '@capacitor/core'; export interface SecureEnclaveSignOptions { alias: string; serverChallenge: string; promptMessage: string; } export interface SecureEnclavePlugin { signChallengeWithBiometrics(options: SecureEnclaveSignOptions): Promise<{ signature: string; keyId: string }>; } const SecureEnclave = registerPlugin<SecureEnclavePlugin>('SecureEnclave'); // SECURE: Challenge is passed to native enclave; raw private key never touches WebView memory. export async function authenticateAndSignSession(serverChallenge: string): Promise<{ signature: string; keyId: string }> { if (!serverChallenge || typeof serverChallenge !== 'string' || serverChallenge.length < 32) { throw new Error('Invalid challenge length: Nonce must be at least 256 bits.'); } // Hardware-backed signing occurs inside Secure Enclave / Android KeyStore. // The native layer prompts for biometrics with userPresence/biometryCurrentSet. return await SecureEnclave.signChallengeWithBiometrics({ alias: 'com.app.auth.identity', serverChallenge, promptMessage: 'Confirm identity to authorize transaction' }); } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Capacitor 7 Biometric Auth and Hardware Keystore Bridge Isolation

Capacitor 7 Biometric Auth and Hardware Keystore Bridge Isolation defines an industrial-grade behavioral standard for mobile engineering. Enforces strict isolation between Capacitor 7 WebViews and native hardware keystores (iOS Secure Enclave and Android Hardware-backed Keystore). Prevents cross-site scripting (XSS) and malicious WebView script injections from exfiltrating raw private keys or persistent credentials by delegating all cryptographic signing, asymmetric key generation, and token unwrapping exclusively to native-isolated bridge operations. 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 Cryptographic Secrets or Symmetric Session Keys Directly Across the Capacitor Bridge into the WebView Memory Space 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 Cryptographic Secrets or Symmetric Session Keys Directly Across the Capacitor Bridge into the WebView Memory Space	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 mandate native hardware key generation (P-256 / secp256r1) where the private key is explicitly declared unexportable from the Secure Enclave or Android Keystore/StrongBox.

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.