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securitycritical PriorityID: webauthn-fido2-prf-vault-derivation-standard

WebAuthn FIDO2 PRF Extension Zero-Knowledge Vault Derivation Guardrail

Mandates zero-knowledge symmetric key derivation using WebAuthn Level 3 FIDO2 Pseudo-Random Function (PRF) extensions without persisting key material to host storage. Derived keying material must undergo HKDF expansion with domain separation, instantiate non-extractable CryptoKey handles, and enforce immediate memory zeroization of intermediate buffers.

Prohibited Anti-Pattern: Persisting raw PRF outputs or derived master keys to client-side storage without HKDF domain separation or memory hygiene

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---
description: WebAuthn FIDO2 PRF Extension Zero-Knowledge Vault Derivation Guardrail - Mandates zero-knowledge symmetric key derivation using WebAuthn Level 3 FIDO2 Pseudo-Random Function (PRF) extensions without persisting key material to host storage. Derived keying material must undergo HKDF expansion with domain separation, instantiate non-extractable CryptoKey handles, and enforce immediate memory zeroization of intermediate buffers.
globs: src/**/*.{ts,tsx}, lib/**/*.{ts,tsx}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/webauthn-fido2-prf-vault-derivation-standard"
---

# WebAuthn FIDO2 PRF Extension Zero-Knowledge Vault Derivation Guardrail

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: Persisting raw PRF outputs or derived master keys to client-side storage without HKDF domain separation or memory hygiene

## System Constraints
You are a Principal Security Architect enforcing the WebAuthn FIDO2 PRF Extension Client Vault Derivation Standard. You must mandate zero-knowledge memory safety, guarantee proper HKDF domain separation, prevent disk or storage persistence of root secrets, and ensure non-extractable Web Crypto handles.

## Strict Guidelines
- Engineers MUST ALWAYS invoke WebAuthn credentials requests with the 'prf' extension using a cryptographically verified 32-byte salt and confirm 'authenticatorAttachment: platform' or 'cross-platform' capability.
- Engineers MUST ALWAYS process the raw authenticator PRF response through HKDF-SHA256 (RFC 5869) with unique application domain separation info parameters prior to generating AES-GCM or HMAC operational keys.
- Engineers MUST NOT persist raw PRF seed outputs, intermediate HKDF materials, or derived plaintext symmetric keys into localStorage, sessionStorage, IndexedDB, or filesystem caches.
- Code SHOULD import derived keys into the Web Crypto API with extractable set strictly to false to prevent programmatic key exfiltration via XSS vectors.
- Services MUST zeroize all transient ArrayBuffers, Uint8Arrays, and PRF output seeds immediately after importing into WebCrypto via Uint8Array.prototype.fill(0).
- Components NEVER store or transmit unencrypted vault secrets over the network or retain PRF outputs across client page reloads.

## Bad vs Good Implementations
### WebAuthn PRF Key Derivation and Secure Ephemeral Vault Handling

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: The insecure implementation persists raw PRF seed bytes to unencrypted localStorage (vulnerable to XSS and disk forensics), fails to apply HKDF domain separation, and imports the key as extractable: true, allowing malicious scripts to dump cryptographic keys.
```typescript
// INSECURE: Directly caching PRF output in localStorage, omitting HKDF, extractable key
export async function deriveAndStoreVaultKey(credentialId: Uint8Array, salt: Uint8Array) {
  const credential = await navigator.credentials.get({
    publicKey: {
      challenge: crypto.getRandomValues(new Uint8Array(32)),
      allowCredentials: [{ id: credentialId, type: 'public-key' }],
      extensions: {
        prf: {
          eval: { first: salt }
        }
      }
    }
  }) as PublicKeyCredential;

  const extensionResults = credential.getClientExtensionResults();
  const rawPrf = new Uint8Array(extensionResults.prf?.results?.first!);

  // DANGEROUS: Storing raw derived seed in browser persistent storage
  localStorage.setItem('cached_vault_key', btoa(String.fromCharCode(...rawPrf)));

  // DANGEROUS: Using PRF output directly as AES-GCM key with extractable: true
  return crypto.subtle.importKey(
    'raw',
    rawPrf,
    { name: 'AES-GCM' },
    true, // Exfiltratable key handle
    ['encrypt', 'decrypt']
  );
}
```

#### Production Standard (Correct)
> **Standard**: The secure implementation enforces HKDF-SHA256 domain separation, imports non-extractable CryptoKey objects inside WebCrypto, explicitly clears memory buffers in a finally block via Uint8Array.fill(0), and avoids any persistent client-side storage.
```typescript
// SECURE: Enforces HKDF-SHA256 domain separation, non-extractable keys, and zeroizes memory buffers
export async function deriveVaultEncryptionKey(
  credentialId: Uint8Array,
  applicationSalt: Uint8Array
): Promise<CryptoKey> {
  if (applicationSalt.byteLength !== 32) {
    throw new Error('Application PRF salt must be exactly 32 bytes.');
  }

  const credential = await navigator.credentials.get({
    publicKey: {
      challenge: crypto.getRandomValues(new Uint8Array(32)),
      allowCredentials: [{ id: credentialId, type: 'public-key' }],
      userVerification: 'required',
      extensions: {
        prf: {
          eval: { first: applicationSalt }
        }
      }
    }
  }) as PublicKeyCredential | null;

  if (!credential) {
    throw new Error('WebAuthn assertion failed or cancelled by user.');
  }

  const extensionResults = credential.getClientExtensionResults();
  const prfResult = extensionResults.prf?.results?.first;
  if (!prfResult) {
    throw new Error('Authenticator does not support PRF extension or evaluation failed.');
  }

  const prfBuffer = new Uint8Array(prfResult);

  try {
    // 1. Import PRF output as raw Initial Keying Material (IKM) into HKDF
    const ikmKey = await crypto.subtle.importKey(
      'raw',
      prfBuffer,
      { name: 'HKDF' },
      false,
      ['deriveKey']
    );

    const encoder = new TextEncoder();
    const hkdfInfo = encoder.encode('app:vault:v1:aes-256-gcm:encryption');
    const hkdfSalt = encoder.encode('fido2-prf-domain-salt-v1');

    // 2. Derive non-extractable operational AES-GCM-256 key handle
    return await crypto.subtle.deriveKey(
      {
        name: 'HKDF',
        hash: 'SHA-256',
        salt: hkdfSalt,
        info: hkdfInfo
      },
      ikmKey,
      { name: 'AES-GCM', length: 256 },
      false, // Invariant: Non-extractable key
      ['encrypt', 'decrypt']
    );
  } finally {
    // 3. Zeroize intermediate raw PRF output immediately from memory
    prfBuffer.fill(0);
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

Cryptographic Static Analysis MCP Server
Inspects cryptographic key lifecycles, WebCrypto parameters, and entropy management in AST pipelines.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

WebAuthn & FIDO2 Spec Verification CLI
Validates client extension dictionaries and WebCrypto API derivation pipelines against FIDO Alliance specifications.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: WebAuthn FIDO2 PRF Extension Zero-Knowledge Vault Derivation Guardrail

WebAuthn FIDO2 PRF Extension Zero-Knowledge Vault Derivation Guardrail defines an industrial-grade behavioral standard for security engineering. Mandates zero-knowledge symmetric key derivation using WebAuthn Level 3 FIDO2 Pseudo-Random Function (PRF) extensions without persisting key material to host storage. Derived keying material must undergo HKDF expansion with domain separation, instantiate non-extractable CryptoKey handles, and enforce immediate memory zeroization of intermediate buffers. 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: Persisting raw PRF outputs or derived master keys to client-side storage without HKDF domain separation or memory hygiene 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 ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftPersisting raw PRF outputs or derived master keys to client-side storage without HKDF domain separation or memory hygieneCritical
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 InconsistencyAI 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:

1

Engineers MUST ALWAYS invoke WebAuthn credentials requests with the 'prf' extension using a cryptographically verified 32-byte salt and confirm 'authenticatorAttachment

Engineers MUST ALWAYS invoke WebAuthn credentials requests with the 'prf' extension using a cryptographically verified 32-byte salt and confirm 'authenticatorAttachment: platform' or 'cross-platform' capability.

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

Engineers MUST ALWAYS process the raw authenticator PRF response through HKDF-SHA256 (RFC 5869) with unique application domain separation info parameters prior to generating AES-GCM or HMAC operational keys.

Engineers MUST ALWAYS process the raw authenticator PRF response through HKDF-SHA256 (RFC 5869) with unique application domain separation info parameters prior to generating AES-GCM or HMAC operational keys.

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

Engineers MUST NOT persist raw PRF seed outputs, intermediate HKDF materials, or derived plaintext symmetric keys into localStorage, sessionStorage, IndexedDB, or filesystem caches.

Engineers MUST NOT persist raw PRF seed outputs, intermediate HKDF materials, or derived plaintext symmetric keys into localStorage, sessionStorage, IndexedDB, or filesystem caches.

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
4

Code SHOULD import derived keys into the Web Crypto API with extractable set strictly to false to prevent programmatic key exfiltration via XSS vectors.

Code SHOULD import derived keys into the Web Crypto API with extractable set strictly to false to prevent programmatic key exfiltration via XSS vectors.

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
5

Services MUST zeroize all transient ArrayBuffers, Uint8Arrays, and PRF output seeds immediately after importing into WebCrypto via Uint8Array.prototype.fill(0).

Services MUST zeroize all transient ArrayBuffers, Uint8Arrays, and PRF output seeds immediately after importing into WebCrypto via Uint8Array.prototype.fill(0).

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
6

Components NEVER store or transmit unencrypted vault secrets over the network or retain PRF outputs across client page reloads.

Components NEVER store or transmit unencrypted vault secrets over the network or retain PRF outputs across client page reloads.

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is WebAuthn FIDO2 PRF Extension Zero-Knowledge Vault Derivation Guardrail critical for AI coding assistants?

Mandates zero-knowledge symmetric key derivation using WebAuthn Level 3 FIDO2 Pseudo-Random Function (PRF) extensions without persisting key material to host storage. Derived keying material must undergo HKDF expansion with domain separation, instantiate non-extractable CryptoKey handles, and enforce immediate memory zeroization of intermediate buffers. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (Persisting raw PRF outputs or derived master keys to client-side storage without HKDF domain separation or memory hygiene) 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.

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