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securitycritical PriorityID: webauthn-prf-zero-knowledge-vault-standard

WebAuthn PRF Hardware-Bound Zero-Knowledge Vault Decryption Guardrail

Enforces deterministic derivation of symmetric vault encryption keys via the WebAuthn PRF (Pseudo-Random Function) extension while maintaining strict zero-knowledge architecture. Mandates that raw PRF outputs are immediately imported into non-extractable WebCrypto keys via HKDF and never serialized, persisted, or transmitted to any server.

Prohibited Anti-Pattern: Directly using raw PRF output as a cipher key, persisting derived seeds, or leaking PRF buffers over network boundaries

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---
description: WebAuthn PRF Hardware-Bound Zero-Knowledge Vault Decryption Guardrail - Enforces deterministic derivation of symmetric vault encryption keys via the WebAuthn PRF (Pseudo-Random Function) extension while maintaining strict zero-knowledge architecture. Mandates that raw PRF outputs are immediately imported into non-extractable WebCrypto keys via HKDF and never serialized, persisted, or transmitted to any server.
globs: src/**/crypto/**/*.{ts,tsx}, src/**/vault/**/*.{ts,tsx}, src/**/auth/**/*.{ts,tsx}, src/**/storage/**/*.{ts,tsx}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/webauthn-prf-zero-knowledge-vault-standard"
---

# WebAuthn PRF Hardware-Bound Zero-Knowledge Vault Decryption Guardrail

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: Directly using raw PRF output as a cipher key, persisting derived seeds, or leaking PRF buffers over network boundaries

## System Constraints
You are a Principal Security Architect enforcing the WebAuthn PRF Extension Zero-Knowledge Client Vault Decryption Standard. Hardware-bound symmetric key derivation must strictly isolate cryptographic seeds within client authenticators (HSM/Secure Enclave), process intermediate PRF outputs as ephemeral buffers, and derive non-extractable WebCrypto keys via RFC 5869 HKDF. You must reject any pattern that persists unencrypted PRF outputs, exports derived vault keys, or sends credential derivation material to remote services.

## Strict Guidelines
- Engineers MUST ALWAYS verify authenticator PRF support via credential creation extension outputs ('prf.enabled === true') before attempting vault registration.
- Engineers MUST NEVER use raw PRF evaluation outputs directly as AES encryption/decryption keys; all outputs MUST be derived using HKDF-SHA-256 with distinct domain-separation info strings.
- Engineers MUST ALWAYS derive vault symmetric keys as non-extractable ('extractable: false') WebCrypto CryptoKey instances.
- Code MUST explicitly wipe/zeroize in-memory raw PRF ArrayBuffers and TypedArrays immediately after key derivation using TypedArray.prototype.fill(0).
- Services MUST NEVER receive, log, or persist PRF outputs, master derivation seeds, or derived vault symmetric keys; vault payloads MUST be encrypted and decrypted entirely client-side.
- Components NEVER store PRF outputs or derived keys in localStorage, sessionStorage, IndexedDB, or serializable state management stores (e.g., Redux, Zustand).

## Bad vs Good Implementations
### Vulnerable: Storing Raw PRF Output in LocalStorage and Sending to API

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Directly reading raw PRF results, saving them unencrypted to localStorage, and transmitting the derivation material across the network completely breaks zero-knowledge guarantees and exposes the client vault key to persistent XSS and server compromise.
```typescript
export async function unlockVault(credentialId: string, salt: Uint8Array): Promise<void> {
  const assertion = (await navigator.credentials.get({
    publicKey: {
      challenge: crypto.getRandomValues(new Uint8Array(32)),
      allowCredentials: [{ id: Uint8Array.from(atob(credentialId), c => c.charCodeAt(0)), type: 'public-key' }],
      extensions: {
        prf: { eval: { first: salt } }
      }
    }
  })) as PublicKeyCredential;

  const results = assertion.getClientExtensionResults();
  // CRITICAL ANTI-PATTERN: Direct PRF usage, persistence, and network exfiltration
  const rawPrf = new Uint8Array(results.prf!.results!.first);
  localStorage.setItem('raw_vault_key', btoa(String.fromCharCode(...rawPrf)));
  
  await fetch('/api/vault/unlock', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ credentialId, prfToken: btoa(String.fromCharCode(...rawPrf)) })
  });
}
```

#### Production Standard (Correct)
> **Standard**: Adheres to zero-knowledge standards by importing PRF outputs directly into an ephemeral non-extractable HKDF key, deriving a non-extractable AES-GCM CryptoKey using domain-separated info parameters, wiping intermediate memory with Uint8Array.fill(0), and strictly preventing server transmission.
```typescript
export async function deriveVaultKey(credentialId: BufferSource, salt: Uint8Array): Promise<CryptoKey> {
  const assertion = (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 extResults = assertion.getClientExtensionResults();
  const prfResult = extResults.prf?.results?.first;
  if (!prfResult) {
    throw new Error('Authenticator did not return PRF evaluation result');
  }

  const rawPrfBytes = new Uint8Array(prfResult);
  let hkdfKey: CryptoKey | null = null;
  try {
    // Import ephemeral raw PRF bytes into HKDF key space (non-extractable)
    hkdfKey = await window.crypto.subtle.importKey(
      'raw',
      rawPrfBytes,
      { name: 'HKDF' },
      false,
      ['deriveKey']
    );

    // Derive dedicated AES-GCM 256-bit vault encryption key
    return await window.crypto.subtle.deriveKey(
      {
        name: 'HKDF',
        hash: 'SHA-256',
        salt: salt,
        info: new TextEncoder().encode('zk-client-vault:aes-gcm-256:v1')
      },
      hkdfKey,
      { name: 'AES-GCM', length: 256 },
      false,
      ['encrypt', 'decrypt']
    );
  } finally {
    // Secure cleanup: Zeroize ephemeral memory buffers containing raw entropy
    rawPrfBytes.fill(0);
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

Fetch Security Auditor
Inspects external API schema dependencies and verifies client cryptographic boundaries.
Recommended Agent Skills (askills.dev)

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

WebCrypto Invariant Checker
Validates proper non-extractable flags, buffer sanitization, and WebAuthn PRF extension parameters.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: WebAuthn PRF Hardware-Bound Zero-Knowledge Vault Decryption Guardrail

WebAuthn PRF Hardware-Bound Zero-Knowledge Vault Decryption Guardrail defines an industrial-grade behavioral standard for security engineering. Enforces deterministic derivation of symmetric vault encryption keys via the WebAuthn PRF (Pseudo-Random Function) extension while maintaining strict zero-knowledge architecture. Mandates that raw PRF outputs are immediately imported into non-extractable WebCrypto keys via HKDF and never serialized, persisted, or transmitted to any server. 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: Directly using raw PRF output as a cipher key, persisting derived seeds, or leaking PRF buffers over network boundaries 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 DriftDirectly using raw PRF output as a cipher key, persisting derived seeds, or leaking PRF buffers over network boundariesCritical
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 verify authenticator PRF support via credential creation extension outputs ('prf.enabled === true') before attempting vault registration.

Engineers MUST ALWAYS verify authenticator PRF support via credential creation extension outputs ('prf.enabled === true') before attempting vault registration.

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 NEVER use raw PRF evaluation outputs directly as AES encryption/decryption keys

Engineers MUST NEVER use raw PRF evaluation outputs directly as AES encryption/decryption keys; all outputs MUST be derived using HKDF-SHA-256 with distinct domain-separation info strings.

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 ALWAYS derive vault symmetric keys as non-extractable ('extractable

Engineers MUST ALWAYS derive vault symmetric keys as non-extractable ('extractable: false') WebCrypto CryptoKey instances.

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 MUST explicitly wipe/zeroize in-memory raw PRF ArrayBuffers and TypedArrays immediately after key derivation using TypedArray.prototype.fill(0).

Code MUST explicitly wipe/zeroize in-memory raw PRF ArrayBuffers and TypedArrays immediately after key derivation using TypedArray.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.
5

Services MUST NEVER receive, log, or persist PRF outputs, master derivation seeds, or derived vault symmetric keys

Services MUST NEVER receive, log, or persist PRF outputs, master derivation seeds, or derived vault symmetric keys; vault payloads MUST be encrypted and decrypted entirely client-side.

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 PRF outputs or derived keys in localStorage, sessionStorage, IndexedDB, or serializable state management stores (e.g., Redux, Zustand).

Components NEVER store PRF outputs or derived keys in localStorage, sessionStorage, IndexedDB, or serializable state management stores (e.g., Redux, Zustand).

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 PRF Hardware-Bound Zero-Knowledge Vault Decryption Guardrail critical for AI coding assistants?

Enforces deterministic derivation of symmetric vault encryption keys via the WebAuthn PRF (Pseudo-Random Function) extension while maintaining strict zero-knowledge architecture. Mandates that raw PRF outputs are immediately imported into non-extractable WebCrypto keys via HKDF and never serialized, persisted, or transmitted to any server. 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 (Directly using raw PRF output as a cipher key, persisting derived seeds, or leaking PRF buffers over network boundaries) 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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