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securitycritical PriorityID: webauthn-l3-prf-symmetric-key-derivation-standard

WebAuthn Level 3 PRF Extension Deterministic Zero-Knowledge Key Derivation Standard

Enforces strict implementation invariants for WebAuthn Level 3 PRF (Pseudo-Random Function) extension symmetric key derivation. Guarantees that client-side deterministic encryption keys remain hardware-bound, non-extractable within the Web Crypto API subsystem, and that all intermediate memory buffers are zeroized immediately upon derivation.

Prohibited Anti-Pattern: Exposing Raw PRF Material, Permitting Extractable CryptoKeys, or Allowing Insecure Degradation on PRF-Incompatible Authenticators

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---
description: WebAuthn Level 3 PRF Extension Deterministic Zero-Knowledge Key Derivation Standard - Enforces strict implementation invariants for WebAuthn Level 3 PRF (Pseudo-Random Function) extension symmetric key derivation. Guarantees that client-side deterministic encryption keys remain hardware-bound, non-extractable within the Web Crypto API subsystem, and that all intermediate memory buffers are zeroized immediately upon derivation.
globs: src/**/auth/**/*.{ts,tsx}, src/**/crypto/**/*.{ts,tsx}, src/**/security/**/*.{ts,tsx}, src/**/vault/**/*.{ts,tsx}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/webauthn-l3-prf-symmetric-key-derivation-standard"
---

# WebAuthn Level 3 PRF Extension Deterministic Zero-Knowledge Key Derivation Standard

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: Exposing Raw PRF Material, Permitting Extractable CryptoKeys, or Allowing Insecure Degradation on PRF-Incompatible Authenticators

## System Constraints
You are a Principal Security Architect enforcing the WebAuthn Level 3 PRF Extension Symmetric Key Derivation Standard. You must reject any implementation where derived authentication or encryption keys are extractable, stored in persistent or unmanaged heap memory, transmitted over network transports, or allowed to silently degrade when authenticator hardware lacks PRF capability.

## Strict Guidelines
- Engineers MUST ALWAYS verify authenticator PRF capability by asserting `clientExtensionResults.prf.enabled === true` during credential creation before persisting registration metadata.
- Engineers MUST ALWAYS ingest raw PRF evaluator outputs directly into `crypto.subtle.importKey` as non-extractable master keys (`extractable: false`) with strict usage restrictions (`['deriveKey', 'deriveBits']`).
- Engineers MUST ALWAYS zeroize the transient `ArrayBuffer` holding raw PRF output immediately after WebCrypto ingestion using `new Uint8Array(buffer).fill(0)` inside a deterministic `finally` block.
- Engineers MUST NOT configure derived target symmetric keys (e.g., `AES-GCM`, `HKDF`) as `extractable: true` under any circumstance.
- Code MUST NEVER serialize, convert to strings (hex/base64), or emit raw PRF output bytes into console logs, error contexts, telemetry, or network payloads.
- Services MUST FAIL CLOSED and throw a domain-specific `PrfNotSupportedHardwareError` if `results.extensions?.prf?.results?.first` is absent or undefined during assertion, completely barring silent software fallback.

## Bad vs Good Implementations
### WebAuthn L3 PRF Credential Assertion and Non-Extractable Key Derivation

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: The anti-pattern assumes PRF evaluation always succeeds without validation, serializes raw cryptographic secret material into a JavaScript heap-allocated string, persists it to localStorage, and fails to scrub transient buffers, exposing deterministic keys directly to XSS attacks and memory inspection.
```typescript
export async function deriveVaultKey(credentialId: Uint8Array, salt: Uint8Array): Promise<string> {
  // Flaw 1: No verification of authenticator PRF support
  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 results = assertion.getClientExtensionResults();
  // Flaw 2: Unsafe cast without fallback/hardware assertion
  const rawPrf = results.prf?.results?.first as ArrayBuffer;

  // Flaw 3: Extracting raw bytes to hex string and retaining in JS heap memory
  const hexKey = Array.from(new Uint8Array(rawPrf))
    .map(b => b.toString(16).padStart(2, '0'))
    .join('');

  // Flaw 4: Vulnerable to heap dumps and XSS exfiltration
  localStorage.setItem('raw_vault_key', hexKey);
  return hexKey;
}
```

#### Production Standard (Correct)
> **Standard**: Enforces user verification, strictly asserts the presence of PRF output bytes, encapsulates the raw seed in a non-extractable WebCrypto key structure, and guarantees memory sanitization by explicitly zeroizing the transient buffer in a finally block.
```typescript
export class PrfHardwareUnsupportedError extends Error {
  constructor() {
    super('Authenticator hardware does not support the WebAuthn PRF extension.');
    this.name = 'PrfHardwareUnsupportedError';
  }
}

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

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

  const extResults = assertion.getClientExtensionResults();
  const prfResult = extResults.prf?.results?.first;

  if (!prfResult || !(prfResult instanceof ArrayBuffer)) {
    throw new PrfHardwareUnsupportedError();
  }

  let rawBufferView: Uint8Array | null = new Uint8Array(prfResult);
  try {
    // Import as non-extractable HKDF master derivation key
    const baseKey = await crypto.subtle.importKey(
      'raw',
      rawBufferView,
      { name: 'HKDF' },
      false,
      ['deriveKey']
    );

    // Derive zero-knowledge AES-GCM-256 target key (completely non-extractable)
    const derivedKey = await crypto.subtle.deriveKey(
      {
        name: 'HKDF',
        hash: 'SHA-256',
        salt: prfSalt,
        info: new TextEncoder().encode('deterministic-zk-client-vault-v1')
      },
      baseKey,
      { name: 'AES-GCM', length: 256 },
      false,
      ['encrypt', 'decrypt']
    );

    return derivedKey;
  } finally {
    // Deterministic volatile memory sanitization
    if (rawBufferView) {
      rawBufferView.fill(0);
      rawBufferView = null;
    }
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

fetch
Verifies implementation against authoritative W3C WebAuthn Level 3 PRF specifications and security errata.
Recommended Agent Skills (askills.dev)

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

Semgrep Cryptographic Ruleset
Detects exposed raw ArrayBuffers, extractable Web Crypto keys, and leaked cryptographic handles in AST.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: WebAuthn Level 3 PRF Extension Deterministic Zero-Knowledge Key Derivation Standard

WebAuthn Level 3 PRF Extension Deterministic Zero-Knowledge Key Derivation Standard defines an industrial-grade behavioral standard for security engineering. Enforces strict implementation invariants for WebAuthn Level 3 PRF (Pseudo-Random Function) extension symmetric key derivation. Guarantees that client-side deterministic encryption keys remain hardware-bound, non-extractable within the Web Crypto API subsystem, and that all intermediate memory buffers are zeroized immediately upon derivation. 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 PRF Material, Permitting Extractable CryptoKeys, or Allowing Insecure Degradation on PRF-Incompatible Authenticators 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 DriftExposing Raw PRF Material, Permitting Extractable CryptoKeys, or Allowing Insecure Degradation on PRF-Incompatible AuthenticatorsCritical
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 capability by asserting `clientExtensionResults.prf.enabled === true` during credential creation before persisting registration metadata.

Engineers MUST ALWAYS verify authenticator PRF capability by asserting `clientExtensionResults.prf.enabled === true` during credential creation before persisting registration metadata.

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 ingest raw PRF evaluator outputs directly into `crypto.subtle.importKey` as non-extractable master keys (`extractable

Engineers MUST ALWAYS ingest raw PRF evaluator outputs directly into `crypto.subtle.importKey` as non-extractable master keys (`extractable: false`) with strict usage restrictions (`['deriveKey', 'deriveBits']`).

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 zeroize the transient `ArrayBuffer` holding raw PRF output immediately after WebCrypto ingestion using `new Uint8Array(buffer).fill(0)` inside a deterministic `finally` block.

Engineers MUST ALWAYS zeroize the transient `ArrayBuffer` holding raw PRF output immediately after WebCrypto ingestion using `new Uint8Array(buffer).fill(0)` inside a deterministic `finally` block.

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 configure derived target symmetric keys (e.g., `AES-GCM`, `HKDF`) as `extractable

Engineers MUST NOT configure derived target symmetric keys (e.g., `AES-GCM`, `HKDF`) as `extractable: true` under any circumstance.

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

Code MUST NEVER serialize, convert to strings (hex/base64), or emit raw PRF output bytes into console logs, error contexts, telemetry, or network payloads.

Code MUST NEVER serialize, convert to strings (hex/base64), or emit raw PRF output bytes into console logs, error contexts, telemetry, or network payloads.

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

Services MUST FAIL CLOSED and throw a domain-specific `PrfNotSupportedHardwareError` if `results.extensions?.prf?.results?.first` is absent or undefined during assertion, completely barring silent software fallback.

Services MUST FAIL CLOSED and throw a domain-specific `PrfNotSupportedHardwareError` if `results.extensions?.prf?.results?.first` is absent or undefined during assertion, completely barring silent software fallback.

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 Level 3 PRF Extension Deterministic Zero-Knowledge Key Derivation Standard critical for AI coding assistants?

Enforces strict implementation invariants for WebAuthn Level 3 PRF (Pseudo-Random Function) extension symmetric key derivation. Guarantees that client-side deterministic encryption keys remain hardware-bound, non-extractable within the Web Crypto API subsystem, and that all intermediate memory buffers are zeroized immediately upon derivation. 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 (Exposing Raw PRF Material, Permitting Extractable CryptoKeys, or Allowing Insecure Degradation on PRF-Incompatible Authenticators) 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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