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securitycritical PriorityID: webauthn-prf-zero-knowledge-derivation

WebAuthn PRF Client-Side Zero-Knowledge Symmetric Key Derivation Standard

Enforces strict client-side symmetric key derivation from hardware authenticators using the WebAuthn prf extension. Guarantees raw PRF outputs and derived encryption keys never leave the client runtime or transit over the network. Requires HKDF key separation with explicit domain contexts before performing WebCrypto operations on sensitive payloads.

Prohibited Anti-Pattern: Transmitting raw PRF outputs, server-side derivation, or un-isolated key reuse across encryption contexts

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---
description: WebAuthn PRF Client-Side Zero-Knowledge Symmetric Key Derivation Standard - Enforces strict client-side symmetric key derivation from hardware authenticators using the WebAuthn prf extension. Guarantees raw PRF outputs and derived encryption keys never leave the client runtime or transit over the network. Requires HKDF key separation with explicit domain contexts before performing WebCrypto operations on sensitive payloads.
globs: src/**/*.{ts,tsx}, lib/**/*.{ts,tsx}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/webauthn-prf-zero-knowledge-derivation"
---

# WebAuthn PRF Client-Side Zero-Knowledge Symmetric Key Derivation Standard

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: Transmitting raw PRF outputs, server-side derivation, or un-isolated key reuse across encryption contexts

## System Constraints
You are a Principal Security Architect enforcing the WebAuthn PRF (Pseudo-Random Function) Passkey Zero-Knowledge Standard. You must ensure all sensitive payload encryption keys are derived client-side directly from hardware authenticators via the WebAuthn PRF extension. Never permit PRF outputs, master derivation secrets, or intermediate key material to be transmitted to the backend, stored unencrypted in persistent browser storage, or derived without HKDF cryptographical context isolation.

## Strict Guidelines
- Engineers MUST ALWAYS verify client support for WebAuthn PRF by inspecting window.PublicKeyCredential and PublicKeyCredential.getClientCapabilities() before issuing PRF assertion requests.
- Engineers MUST ALWAYS pass a high-entropy, constant application salt into the WebAuthn prf extension input (eval.first) during navigator.credentials.get().
- Engineers MUST ALWAYS feed the hardware-derived PRF output (extensionResults.prf.results.first) into HKDF (RFC 5869) via window.crypto.subtle to derive separate operational keys (e.g., encryption vs authentication) with explicit info context strings.
- Engineers MUST NOT transmit raw PRF output, intermediate derivation material, or derived symmetric keys over HTTP headers, payloads, or remote logging services.
- Engineers MUST NOT persist derived CryptoKey instances or raw keys in localStorage, sessionStorage, or unencrypted IndexedDB.
- Code MUST mark imported CryptoKey instances as non-extractable (extractable: false) when invoking crypto.subtle.importKey or crypto.subtle.deriveKey.
- Services MUST only receive and store ciphertext, initialization vectors (IVs), and authentication tags; the server MUST maintain zero-knowledge of the underlying plaintext and hardware master keys.

## Bad vs Good Implementations
### Hardware-backed symmetric key derivation via WebAuthn PRF and HKDF

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: The raw PRF output extracted from the hardware passkey is exposed to the network and backend service, destroying zero-knowledge guarantees and creating a server-side single point of failure.
```typescript
export async function unlockUserVault(credentialId: Uint8Array): Promise<string> {
  // Anti-pattern: Sending PRF seed/output to server for key derivation or vault decryption
  const assertion = (await navigator.credentials.get({
    publicKey: {
      challenge: crypto.getRandomValues(new Uint8Array(32)),
      allowCredentials: [{ id: credentialId, type: 'public-key' }],
      extensions: {
        prf: {
          eval: { first: new TextEncoder().encode('global-static-salt') }
        }
      }
    }
  })) as AuthenticationCredential;

  const prfResults = assertion.getClientExtensionResults().prf;
  const rawSecret = prfResults?.results?.first;

  // VULNERABILITY: Raw authenticator PRF bytes transmitted to server
  const res = await fetch('/api/vault/decrypt', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({
      credentialId: Array.from(credentialId),
      rawPrfOutput: Array.from(new Uint8Array(rawSecret!))
    })
  });
  const { plaintext } = await res.json();
  return plaintext;
}
```

#### Production Standard (Correct)
> **Standard**: Raw PRF hardware output is immediately ingested into WebCrypto as a non-extractable HKDF key. The operational AES-256-GCM key is derived locally with domain separation info and marked non-extractable, ensuring zero-knowledge client decryption without leaking keys off-device.
```typescript
const VAULT_SALT = new TextEncoder().encode('app.production.vault.prf.v1');
const HKDF_INFO_ENCRYPTION = new TextEncoder().encode('aes-gcm-256-payload-encryption');

export async function deriveVaultKeyFromHardware(credentialId: Uint8Array): Promise<CryptoKey> {
  const assertion = (await navigator.credentials.get({
    publicKey: {
      challenge: crypto.getRandomValues(new Uint8Array(32)),
      allowCredentials: [{ id: credentialId, type: 'public-key' }],
      userVerification: 'required',
      extensions: {
        prf: {
          eval: { first: VAULT_SALT }
        }
      }
    }
  })) as AuthenticationCredential | null;

  if (!assertion) {
    throw new Error('Authenticator assertion failed');
  }

  const extResults = assertion.getClientExtensionResults();
  const rawPrfOutput = extResults.prf?.results?.first;
  if (!rawPrfOutput) {
    throw new Error('WebAuthn PRF extension not supported or denied by authenticator');
  }

  // Import raw PRF bytes as non-extractable HKDF base key
  const prfBaseKey = await window.crypto.subtle.importKey(
    'raw',
    rawPrfOutput,
    'HKDF',
    false,
    ['deriveKey']
  );

  // Derive non-extractable AES-GCM-256 symmetric key for local decryption
  return window.crypto.subtle.deriveKey(
    {
      name: 'HKDF',
      hash: 'SHA-256',
      salt: new Uint8Array(32), // Deterministic zero-salt; salt entropy already bound to PRF evaluation
      info: HKDF_INFO_ENCRYPTION
    },
    prfBaseKey,
    { name: 'AES-GCM', length: 256 },
    false,
    ['encrypt', 'decrypt']
  );
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

WebAuthn Protocol Inspector MCP
Inspects and validates W3C WebAuthn Level 3 PRF extension inputs, authenticator extension outputs, and CBOR structures.
Recommended Agent Skills (askills.dev)

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

WebCrypto & HKDF Integrity Audit
Scans client-side cryptography code for key leakage, missing HKDF domain context separation, and extractable CryptoKey violations.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: WebAuthn PRF Client-Side Zero-Knowledge Symmetric Key Derivation Standard

WebAuthn PRF Client-Side Zero-Knowledge Symmetric Key Derivation Standard defines an industrial-grade behavioral standard for security engineering. Enforces strict client-side symmetric key derivation from hardware authenticators using the WebAuthn prf extension. Guarantees raw PRF outputs and derived encryption keys never leave the client runtime or transit over the network. Requires HKDF key separation with explicit domain contexts before performing WebCrypto operations on sensitive payloads. 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: Transmitting raw PRF outputs, server-side derivation, or un-isolated key reuse across encryption contexts 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 DriftTransmitting raw PRF outputs, server-side derivation, or un-isolated key reuse across encryption contextsCritical
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 client support for WebAuthn PRF by inspecting window.PublicKeyCredential and PublicKeyCredential.getClientCapabilities() before issuing PRF assertion requests.

Engineers MUST ALWAYS verify client support for WebAuthn PRF by inspecting window.PublicKeyCredential and PublicKeyCredential.getClientCapabilities() before issuing PRF assertion requests.

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 pass a high-entropy, constant application salt into the WebAuthn prf extension input (eval.first) during navigator.credentials.get().

Engineers MUST ALWAYS pass a high-entropy, constant application salt into the WebAuthn prf extension input (eval.first) during navigator.credentials.get().

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 feed the hardware-derived PRF output (extensionResults.prf.results.first) into HKDF (RFC 5869) via window.crypto.subtle to derive separate operational keys (e.g., encryption vs authentication) with explicit info context strings.

Engineers MUST ALWAYS feed the hardware-derived PRF output (extensionResults.prf.results.first) into HKDF (RFC 5869) via window.crypto.subtle to derive separate operational keys (e.g., encryption vs authentication) with explicit info context 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.
4

Engineers MUST NOT transmit raw PRF output, intermediate derivation material, or derived symmetric keys over HTTP headers, payloads, or remote logging services.

Engineers MUST NOT transmit raw PRF output, intermediate derivation material, or derived symmetric keys over HTTP headers, payloads, or remote logging services.

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

Engineers MUST NOT persist derived CryptoKey instances or raw keys in localStorage, sessionStorage, or unencrypted IndexedDB.

Engineers MUST NOT persist derived CryptoKey instances or raw keys in localStorage, sessionStorage, or unencrypted IndexedDB.

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

Code MUST mark imported CryptoKey instances as non-extractable (extractable

Code MUST mark imported CryptoKey instances as non-extractable (extractable: false) when invoking crypto.subtle.importKey or crypto.subtle.deriveKey.

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.
7

Services MUST only receive and store ciphertext, initialization vectors (IVs), and authentication tags

Services MUST only receive and store ciphertext, initialization vectors (IVs), and authentication tags; the server MUST maintain zero-knowledge of the underlying plaintext and hardware master 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.
Technical Questions & Answers

Frequently Asked Technical Questions

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

Q:Why is WebAuthn PRF Client-Side Zero-Knowledge Symmetric Key Derivation Standard critical for AI coding assistants?

Enforces strict client-side symmetric key derivation from hardware authenticators using the WebAuthn prf extension. Guarantees raw PRF outputs and derived encryption keys never leave the client runtime or transit over the network. Requires HKDF key separation with explicit domain contexts before performing WebCrypto operations on sensitive payloads. 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 (Transmitting raw PRF outputs, server-side derivation, or un-isolated key reuse across encryption contexts) 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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