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securitycritical PriorityID: webauthn-prf-zk-encryption-standard

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

Enforces strict client-side evaluation of WebAuthn Pseudo-Random Function (PRF) extension inputs to derive deterministic symmetric encryption keys directly within client memory. Mandates that raw PRF outputs and derived key material are never exposed to the authentication backend, ensuring hardware-bound zero-knowledge encryption guarantees.

Prohibited Anti-Pattern: Transmitting raw WebAuthn PRF extension results to backend authentication services or deriving encryption keys server-side.

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---
description: WebAuthn PRF Client-Side Zero-Knowledge Symmetric Key Derivation Standard - Enforces strict client-side evaluation of WebAuthn Pseudo-Random Function (PRF) extension inputs to derive deterministic symmetric encryption keys directly within client memory. Mandates that raw PRF outputs and derived key material are never exposed to the authentication backend, ensuring hardware-bound zero-knowledge encryption guarantees.
globs: src/**/*.{ts,tsx}, lib/**/*.{ts,tsx}, packages/**/*.{ts,tsx}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/webauthn-prf-zk-encryption-standard"
---

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

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: Transmitting raw WebAuthn PRF extension results to backend authentication services or deriving encryption keys server-side.

## System Constraints
You are a Principal Security Architect enforcing the WebAuthn PRF Client-Side Zero-Knowledge Encryption Standard. You must guarantee that passkey PRF extension results never leave the browser environment, key derivation strictly uses the Web Crypto API with HKDF-SHA-256, and backend interactions receive only authenticated ciphertext and standard WebAuthn assertion signatures.

## Strict Guidelines
- Engineers MUST ALWAYS strip PRF client extension outputs from the PublicKeyCredential assertion object prior to transmitting the authentication response to the backend.
- Engineers MUST NOT transmit raw PRF byte outputs (`results.first` or `results.second`) or intermediate key material over the network or log them in monitoring buffers.
- Code MUST derive deterministic symmetric keys using the client-side Web Crypto API (`SubtleCrypto.importKey` with `HKDF`, followed by `SubtleCrypto.deriveKey` for `AES-GCM-256`).
- Services MUST enforce that the server remains completely blind to symmetric encryption keys, storing only application ciphertext, IVs, and public cryptographic salts.
- Components NEVER store derived master keys or raw PRF outputs in unencrypted browser storage (localStorage, sessionStorage, or unencrypted IndexedDB).

## Bad vs Good Implementations
### Client-Side PRF Evaluation and Symmetric Key Derivation

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Sending the PRF extension result to the backend completely invalidates zero-knowledge isolation and gives the server permanent access to the user's hardware-bound encryption key.
```typescript
async function authenticateAndDecrypt(credentialId: BufferSource, salt: Uint8Array) {
  const assertion = (await navigator.credentials.get({
    publicKey: {
      challenge: new Uint8Array(32),
      allowCredentials: [{ id: credentialId, type: 'public-key' }],
      extensions: {
        prf: { eval: { first: salt } }
      }
    }
  })) as PublicKeyCredential;

  const extensionResults = assertion.getClientExtensionResults();
  const rawPrfKey = extensionResults.prf?.results?.first;

  // CRITICAL VULNERABILITY: Leaking raw hardware PRF secret directly to backend
  await fetch('/api/auth/complete', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({
      assertion,
      leakedKey: Array.from(new Uint8Array(rawPrfKey as ArrayBuffer))
    })
  });
}
```

#### Production Standard (Correct)
> **Standard**: Keeps the PRF-derived key strictly within client-side Web Crypto primitives, marks keys as non-extractable, and only sends clean WebAuthn assertion data to the server.
```typescript
async function authenticateAndDeriveKey(
  credentialId: BufferSource,
  salt: Uint8Array,
  info: 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 extensionResults = assertion.getClientExtensionResults();
  const prfResult = extensionResults.prf?.results?.first;

  if (!prfResult) {
    throw new Error('Authenticator did not return a valid PRF result.');
  }

  // In-memory zero-knowledge key derivation using HKDF-SHA-256
  const initialKeyMaterial = await crypto.subtle.importKey(
    'raw',
    prfResult,
    { name: 'HKDF' },
    false,
    ['deriveKey']
  );

  const derivedKey = await crypto.subtle.deriveKey(
    {
      name: 'HKDF',
      hash: 'SHA-256',
      salt: salt,
      info: info
    },
    initialKeyMaterial,
    { name: 'AES-GCM', length: 256 },
    false,
    ['encrypt', 'decrypt']
  );

  // Strip PRF outputs: Transmit ONLY standard assertion data to server
  await fetch('/api/auth/verify', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({
      id: assertion.id,
      rawId: Array.from(new Uint8Array(assertion.rawId)),
      response: {
        authenticatorData: Array.from(new Uint8Array(assertion.response.authenticatorData)),
        clientDataJSON: Array.from(new Uint8Array(assertion.response.clientDataJSON)),
        signature: Array.from(new Uint8Array(assertion.response.signature)),
        userHandle: assertion.response.userHandle
          ? Array.from(new Uint8Array(assertion.response.userHandle))
          : null
      }
    })
  });

  return derivedKey;
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

WebAuthn & Cryptography MCP Inspector
Validates WebAuthn payloads and verifies that cryptographic secrets are not serialized across client-server boundaries.
Recommended Agent Skills (askills.dev)

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

Web Crypto Static Analysis Audit
Scans client bundles for insecure key exports, memory persistence, or network leakages of WebAuthn PRF extension results.
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 evaluation of WebAuthn Pseudo-Random Function (PRF) extension inputs to derive deterministic symmetric encryption keys directly within client memory. Mandates that raw PRF outputs and derived key material are never exposed to the authentication backend, ensuring hardware-bound zero-knowledge encryption guarantees. 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 WebAuthn PRF extension results to backend authentication services or deriving encryption keys server-side. 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 WebAuthn PRF extension results to backend authentication services or deriving encryption keys server-side.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 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 strip PRF client extension outputs from the PublicKeyCredential assertion object prior to transmitting the authentication response to the backend.

Engineers MUST ALWAYS strip PRF client extension outputs from the PublicKeyCredential assertion object prior to transmitting the authentication response to the backend.

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 NOT transmit raw PRF byte outputs (`results.first` or `results.second`) or intermediate key material over the network or log them in monitoring buffers.

Engineers MUST NOT transmit raw PRF byte outputs (`results.first` or `results.second`) or intermediate key material over the network or log them in monitoring buffers.

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

Code MUST derive deterministic symmetric keys using the client-side Web Crypto API (`SubtleCrypto.importKey` with `HKDF`, followed by `SubtleCrypto.deriveKey` for `AES-GCM-256`).

Code MUST derive deterministic symmetric keys using the client-side Web Crypto API (`SubtleCrypto.importKey` with `HKDF`, followed by `SubtleCrypto.deriveKey` for `AES-GCM-256`).

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

Services MUST enforce that the server remains completely blind to symmetric encryption keys, storing only application ciphertext, IVs, and public cryptographic salts.

Services MUST enforce that the server remains completely blind to symmetric encryption keys, storing only application ciphertext, IVs, and public cryptographic salts.

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

Components NEVER store derived master keys or raw PRF outputs in unencrypted browser storage (localStorage, sessionStorage, or unencrypted IndexedDB).

Components NEVER store derived master keys or raw PRF outputs in unencrypted browser storage (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.
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 evaluation of WebAuthn Pseudo-Random Function (PRF) extension inputs to derive deterministic symmetric encryption keys directly within client memory. Mandates that raw PRF outputs and derived key material are never exposed to the authentication backend, ensuring hardware-bound zero-knowledge encryption guarantees. 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 WebAuthn PRF extension results to backend authentication services or deriving encryption keys server-side.) 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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