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securitycritical PriorityID: webauthn-prf-zk-client-key-derivation-invariant

Zero-Knowledge Hardware-Bound Key Derivation via WebAuthn PRF Extension

Enforces strict cryptographic boundaries when deriving symmetric client-side encryption keys from WebAuthn Pseudo-Random Function (PRF) extension outputs (HMAC-secret). Raw PRF outputs must never be persisted in browser storage, transmitted across network boundaries, or held long-term in JavaScript heap memory, requiring ephemeral Web Crypto HKDF derivation into non-extractable CryptoKeys.

Prohibited Anti-Pattern: Exposing Raw Authenticator PRF Outputs or Storing Unsalted Symmetric Master Keys in Browser Storage

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---
description: Zero-Knowledge Hardware-Bound Key Derivation via WebAuthn PRF Extension - Enforces strict cryptographic boundaries when deriving symmetric client-side encryption keys from WebAuthn Pseudo-Random Function (PRF) extension outputs (HMAC-secret). Raw PRF outputs must never be persisted in browser storage, transmitted across network boundaries, or held long-term in JavaScript heap memory, requiring ephemeral Web Crypto HKDF derivation into non-extractable CryptoKeys.
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-client-key-derivation-invariant"
---

# Zero-Knowledge Hardware-Bound Key Derivation via WebAuthn PRF Extension

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: Exposing Raw Authenticator PRF Outputs or Storing Unsalted Symmetric Master Keys in Browser Storage

## System Constraints
You are a Principal Security Architect enforcing zero-knowledge client-side encryption paradigms using the WebAuthn PRF (Pseudo-Random Function) extension (RFC 8812 / FIDO2 hmac-secret). You must strictly ensure that PRF seed outputs are consumed instantaneously inside the browser's SubtleCrypto subsystem to generate non-extractable symmetric keys (AES-GCM-256), immediately zeroing out intermediate typed arrays and preventing raw key material from leaking into memory leaks, telemetry, or unencrypted storage tiers.

## Strict Guidelines
- Engineers MUST ALWAYS declare WebCrypto PRF outputs as non-extractable (`extractable: false`) when importing and deriving symmetric keys via `crypto.subtle.deriveKey` or `crypto.subtle.importKey`.
- Engineers MUST NOT serialize, log, or persist raw authenticator PRF outputs (`first` or `second` buffers from `results.prf.results`) into `localStorage`, `sessionStorage`, `IndexedDB`, or cookies.
- Code MUST explicitly overwrite the raw PRF output `ArrayBuffer` or `Uint8Array` memory with zeros (`rawPrfBytes.fill(0)`) immediately after importing it into the Web Crypto subsystem.
- Services MUST supply a cryptographically secure, tenant- or domain-separated salt (minimum 32 bytes) along with a deterministic `info` parameter to HKDF during the PRF-to-AES derivation step.
- Components NEVER transmit raw PRF outputs or derived client-side master keys over network APIs; all zero-knowledge encryption/decryption operations must execute strictly on the client host device.

## Bad vs Good Implementations
### WebAuthn PRF Derivation: Insecure Storage vs. Non-Extractable HKDF Derivation with Memory Scrubbing

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Exposes the raw hardware PRF output to JavaScript memory, converts it into an exportable string, persists it in plain text within localStorage (accessible to any XSS exploit), and fails to sanitize heap-allocated cryptographic material.
```typescript
export async function deriveClientKeyBad(credentialId: Uint8Array): Promise<string> {
  // Anti-Pattern: Extracting PRF output directly, storing in localStorage, and reusing raw seed
  const assertion = (await navigator.credentials.get({
    publicKey: {
      challenge: crypto.getRandomValues(new Uint8Array(32)),
      allowCredentials: [{ id: credentialId, type: 'public-key' }],
      extensions: {
        prf: {
          eval: { first: new Uint8Array(32).fill(1) } // Hardcoded / non-isolated salt
        }
      }
    }
  })) as PublicKeyCredential;

  const results = assertion.getClientExtensionResults();
  const rawPrf = new Uint8Array(results.prf!.results!.first);

  // CRITICAL VULNERABILITY: Raw cryptographic key material converted to string and persisted
  const base64Key = btoa(String.fromCharCode(...rawPrf));
  window.localStorage.setItem('auth_master_key', base64Key);
  return base64Key;
}
```

#### Production Standard (Correct)
> **Standard**: Safely consumes the PRF output by immediately transforming it into an HKDF-derived, non-extractable CryptoKey. The raw typed array memory is explicitly zeroed out in a `finally` block to prevent lingering in garbage collection heaps, and the derived key can never be read or serialized by application-level JavaScript.
```typescript
export async function deriveClientKeySecure(
  credentialId: Uint8Array,
  derivationSalt: Uint8Array
): Promise<CryptoKey> {
  if (derivationSalt.byteLength < 32) {
    throw new Error('Derivation salt must be at least 32 cryptographically secure bytes.');
  }

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

  const results = assertion.getClientExtensionResults();
  const prfResult = results.prf?.results?.first;
  if (!prfResult) {
    throw new Error('Authenticator did not return a PRF extension output.');
  }

  const rawPrfOutput = new Uint8Array(prfResult);

  try {
    // Import PRF output as HKDF key material
    const prfKeyMaterial = await crypto.subtle.importKey(
      'raw',
      rawPrfOutput,
      'HKDF',
      false,
      ['deriveKey']
    );

    // Derive a non-extractable AES-GCM data encryption key bound to application context
    const dataEncryptionKey = await crypto.subtle.deriveKey(
      {
        name: 'HKDF',
        salt: derivationSalt,
        info: new TextEncoder().encode('zero-knowledge:client-storage:v1'),
        hash: 'SHA-256'
      },
      prfKeyMaterial,
      { name: 'AES-GCM', length: 256 },
      false, // Invariant: key is non-extractable from WebCrypto subsystem
      ['encrypt', 'decrypt']
    );

    return dataEncryptionKey;
  } finally {
    // Explicitly zero out raw PRF output in heap memory immediately after import
    rawPrfOutput.fill(0);
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

WebCrypto & Security Context Inspector
Audits client-side cryptographic implementations and guarantees secure contexts (HTTPS/Window) for WebAuthn/SubtleCrypto operations.
Recommended Agent Skills (askills.dev)

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

Semgrep Cryptography Audit Skill
Runs static analysis rules against client-side TypeScript code to detect raw key extraction, insecure WebCrypto invocations, and storage of cryptographic secrets.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Zero-Knowledge Hardware-Bound Key Derivation via WebAuthn PRF Extension

Zero-Knowledge Hardware-Bound Key Derivation via WebAuthn PRF Extension defines an industrial-grade behavioral standard for security engineering. Enforces strict cryptographic boundaries when deriving symmetric client-side encryption keys from WebAuthn Pseudo-Random Function (PRF) extension outputs (HMAC-secret). Raw PRF outputs must never be persisted in browser storage, transmitted across network boundaries, or held long-term in JavaScript heap memory, requiring ephemeral Web Crypto HKDF derivation into non-extractable CryptoKeys. 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 Authenticator PRF Outputs or Storing Unsalted Symmetric Master Keys in Browser Storage 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 Authenticator PRF Outputs or Storing Unsalted Symmetric Master Keys in Browser StorageCritical
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 declare WebCrypto PRF outputs as non-extractable (`extractable

Engineers MUST ALWAYS declare WebCrypto PRF outputs as non-extractable (`extractable: false`) when importing and deriving symmetric keys via `crypto.subtle.deriveKey` or `crypto.subtle.importKey`.

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 serialize, log, or persist raw authenticator PRF outputs (`first` or `second` buffers from `results.prf.results`) into `localStorage`, `sessionStorage`, `IndexedDB`, or cookies.

Engineers MUST NOT serialize, log, or persist raw authenticator PRF outputs (`first` or `second` buffers from `results.prf.results`) into `localStorage`, `sessionStorage`, `IndexedDB`, or cookies.

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 explicitly overwrite the raw PRF output `ArrayBuffer` or `Uint8Array` memory with zeros (`rawPrfBytes.fill(0)`) immediately after importing it into the Web Crypto subsystem.

Code MUST explicitly overwrite the raw PRF output `ArrayBuffer` or `Uint8Array` memory with zeros (`rawPrfBytes.fill(0)`) immediately after importing it into the Web Crypto subsystem.

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 supply a cryptographically secure, tenant- or domain-separated salt (minimum 32 bytes) along with a deterministic `info` parameter to HKDF during the PRF-to-AES derivation step.

Services MUST supply a cryptographically secure, tenant- or domain-separated salt (minimum 32 bytes) along with a deterministic `info` parameter to HKDF during the PRF-to-AES derivation step.

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 transmit raw PRF outputs or derived client-side master keys over network APIs

Components NEVER transmit raw PRF outputs or derived client-side master keys over network APIs; all zero-knowledge encryption/decryption operations must execute strictly on the client host device.

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 Zero-Knowledge Hardware-Bound Key Derivation via WebAuthn PRF Extension critical for AI coding assistants?

Enforces strict cryptographic boundaries when deriving symmetric client-side encryption keys from WebAuthn Pseudo-Random Function (PRF) extension outputs (HMAC-secret). Raw PRF outputs must never be persisted in browser storage, transmitted across network boundaries, or held long-term in JavaScript heap memory, requiring ephemeral Web Crypto HKDF derivation into non-extractable CryptoKeys. 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 Authenticator PRF Outputs or Storing Unsalted Symmetric Master Keys in Browser Storage) 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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