AgentRules.org
RulesStacksClientsAuditorPrunerSelf-HealRFCsBenchmarkCLICI/CDAuthorsSubmit
This specification is maintained by the AgentRules Working Group. Have enhancements or want to claim maintainership?Submit Specification →
securitycritical PriorityID: webauthn-l3-prf-deterministic-vaulting-security

WebAuthn Level 3 PRF Zero-Knowledge Deterministic Key Vaulting Standard

Enforces strict client-side isolation and rigorous cryptographic derivation standards for WebAuthn Level 3 PRF (Pseudo-Random Function) extensions. Protects zero-knowledge client architectures by forbidding server-bound transmission of PRF outputs, mandating HKDF domain separation, and enforcing non-exportable CryptoKey lifecycle handling.

Prohibited Anti-Pattern: Exposing Raw Authenticator PRF Outputs to Servers or Omitting Cryptographic HKDF Context Binding

Export Ready-to-Drop Configuration

Select format below to copy code or download file directly.

---
description: WebAuthn Level 3 PRF Zero-Knowledge Deterministic Key Vaulting Standard - Enforces strict client-side isolation and rigorous cryptographic derivation standards for WebAuthn Level 3 PRF (Pseudo-Random Function) extensions. Protects zero-knowledge client architectures by forbidding server-bound transmission of PRF outputs, mandating HKDF domain separation, and enforcing non-exportable CryptoKey lifecycle handling.
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-l3-prf-deterministic-vaulting-security"
---

# WebAuthn Level 3 PRF Zero-Knowledge Deterministic Key Vaulting Standard

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: Exposing Raw Authenticator PRF Outputs to Servers or Omitting Cryptographic HKDF Context Binding

## System Constraints
You are a Principal Security Architect enforcing WebAuthn Level 3 Passkey PRF Deterministic Key Vaulting. You must enforce zero-knowledge invariants where authenticators derive deterministic secrets locally. Reject any code that leaks PRF evaluation outputs to server endpoints, directly uses PRF outputs as AES keys without HKDF expansion, or retains unzeroed raw secret buffers in JavaScript execution contexts.

## Strict Guidelines
- Engineers MUST ALWAYS pass the authenticator PRF output directly into an HKDF (RFC 5869) pipeline with explicit domain-separated 'info' tags before key usage.
- Engineers MUST NOT transmit raw PRF output buffers, salt pre-images, or derived root symmetric keys to the Relying Party (RP) backend or telemetry under any circumstances.
- Code MUST verify the presence and byte-length of 'results.prf.results.first' (precisely 32 bytes) from 'credential.getClientExtensionResults()' prior to downstream cryptographic consumption.
- Services MUST ensure that WebCrypto 'importKey' sets 'extractable: false' when importing the HKDF-derived vault encryption key into the WebCrypto runtime.
- Components NEVER store PRF outputs, intermediate key material, or plaintext master keys in Web Storage (localStorage, sessionStorage) or unmanaged global variables.

## Bad vs Good Implementations
### Zero-Knowledge PRF Derivation with HKDF and Non-Exportable CryptoKey

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: The anti-pattern violates zero-knowledge boundaries by sending raw PRF evaluation bytes to the server, imports the key as extractable: true, and fails to use HKDF domain separation, creating cross-context key-reuse vulnerabilities.
```typescript
// ANTI-PATTERN: Leaks PRF output to server and uses raw bytes directly as an AES key
async function unlockVaultWithPasskey(credentialId: string, salt: Uint8Array) {
  const credential = (await navigator.credentials.get({
    publicKey: {
      challenge: new Uint8Array(32),
      allowCredentials: [{ id: Uint8Array.from(atob(credentialId), c => c.charCodeAt(0)), type: 'public-key' }],
      extensions: { prf: { eval: { first: salt } } }
    }
  })) as PublicKeyCredential;

  const extResults = credential.getClientExtensionResults();
  const rawPrf = extResults.prf?.results?.first;

  // CRITICAL: Transmitting PRF secret to backend destroys zero-knowledge architecture
  await fetch('/api/vault/sync', {
    method: 'POST',
    body: JSON.stringify({ prfSecret: Array.from(new Uint8Array(rawPrf!)) })
  });

  // CRITICAL: Direct use of PRF output as AES-GCM key without HKDF context binding
  return crypto.subtle.importKey('raw', rawPrf!, 'AES-GCM', true, ['decrypt']);
}
```

#### Production Standard (Correct)
> **Standard**: Adheres to zero-knowledge guarantees: validates PRF availability, scopes the PRF output through HKDF with contextual domain separation ('info'), enforces 'extractable: false', and securely scrubs the plaintext PRF buffer via finally { prfBuffer.fill(0); }.
```typescript
// PRODUCTION STANDARD: Client-side HKDF derivation with domain separation and memory zeroing
async function deriveVaultKeyWithPrf(
  credentialId: BufferSource,
  vaultSalt: Uint8Array,
  vaultId: string
): Promise<CryptoKey> {
  const credential = (await navigator.credentials.get({
    publicKey: {
      challenge: crypto.getRandomValues(new Uint8Array(32)),
      allowCredentials: [{ id: credentialId, type: 'public-key' }],
      extensions: { prf: { eval: { first: vaultSalt } } }
    }
  })) as PublicKeyCredential | null;

  if (!credential) {
    throw new Error('Passkey authentication cancelled or failed.');
  }

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

  if (!prfResult || prfResult.byteLength !== 32) {
    throw new Error('Authenticator does not support PRF extension or returned invalid output.');
  }

  const prfBuffer = new Uint8Array(prfResult);
  try {
    // Import PRF output as HKDF Initial Keying Material (IKM)
    const ikmKey = await crypto.subtle.importKey(
      'raw',
      prfBuffer,
      'HKDF',
      false,
      ['deriveKey']
    );

    const info = new TextEncoder().encode(`org.production.vault:v1:aes-gcm-256:${vaultId}`);
    const subSalt = new Uint8Array(32); // Constant derivation salt when PRF output is already uniformly random

    return await crypto.subtle.deriveKey(
      {
        name: 'HKDF',
        hash: 'SHA-256',
        salt: subSalt,
        info: info
      },
      ikmKey,
      { name: 'AES-GCM', length: 256 },
      false,
      ['encrypt', 'decrypt']
    );
  } finally {
    // Scrub transient buffer from memory immediately
    prfBuffer.fill(0);
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

WebAuthn & FIDO2 Security Inspector MCP
Validates client extension inputs and checks relying party configurations against FIDO2/W3C Level 3 specs.
Recommended Agent Skills (askills.dev)

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

WebCrypto Static Analysis Security Linter
Scans client-side code for key extraction vulnerabilities, missing HKDF domain bindings, and sensitive buffer leaks.
Architectural Specification & Invariant Rationale

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

WebAuthn Level 3 PRF Zero-Knowledge Deterministic Key Vaulting Standard defines an industrial-grade behavioral standard for security engineering. Enforces strict client-side isolation and rigorous cryptographic derivation standards for WebAuthn Level 3 PRF (Pseudo-Random Function) extensions. Protects zero-knowledge client architectures by forbidding server-bound transmission of PRF outputs, mandating HKDF domain separation, and enforcing non-exportable CryptoKey lifecycle handling. 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 to Servers or Omitting Cryptographic HKDF Context Binding 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 to Servers or Omitting Cryptographic HKDF Context BindingCritical
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 pass the authenticator PRF output directly into an HKDF (RFC 5869) pipeline with explicit domain-separated 'info' tags before key usage.

Engineers MUST ALWAYS pass the authenticator PRF output directly into an HKDF (RFC 5869) pipeline with explicit domain-separated 'info' tags before key usage.

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 output buffers, salt pre-images, or derived root symmetric keys to the Relying Party (RP) backend or telemetry under any circumstances.

Engineers MUST NOT transmit raw PRF output buffers, salt pre-images, or derived root symmetric keys to the Relying Party (RP) backend or telemetry under any circumstances.

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 verify the presence and byte-length of 'results.prf.results.first' (precisely 32 bytes) from 'credential.getClientExtensionResults()' prior to downstream cryptographic consumption.

Code MUST verify the presence and byte-length of 'results.prf.results.first' (precisely 32 bytes) from 'credential.getClientExtensionResults()' prior to downstream cryptographic consumption.

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 ensure that WebCrypto 'importKey' sets 'extractable

Services MUST ensure that WebCrypto 'importKey' sets 'extractable: false' when importing the HKDF-derived vault encryption key into the WebCrypto runtime.

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 PRF outputs, intermediate key material, or plaintext master keys in Web Storage (localStorage, sessionStorage) or unmanaged global variables.

Components NEVER store PRF outputs, intermediate key material, or plaintext master keys in Web Storage (localStorage, sessionStorage) or unmanaged global variables.

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 Zero-Knowledge Deterministic Key Vaulting Standard critical for AI coding assistants?

Enforces strict client-side isolation and rigorous cryptographic derivation standards for WebAuthn Level 3 PRF (Pseudo-Random Function) extensions. Protects zero-knowledge client architectures by forbidding server-bound transmission of PRF outputs, mandating HKDF domain separation, and enforcing non-exportable CryptoKey lifecycle handling. 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 to Servers or Omitting Cryptographic HKDF Context Binding) 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.

Open Specification · CC BY 4.0·Free for Personal, Team & Commercial In-Repo Use

Citation & Open Source Attribution Guidelines

This specification is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0). You are completely free to adapt and share this rule in your workspace. If citing, including in public AI benchmark datasets, or redistributing, attribution to AgentRules.org is required.

Related SECURITY Rules

View all rules →
security
105

OpenFGA Graph Invariants: Cyclic Relation Prevention & Contextual Tuple Hardening

Enforces strict directed acyclic graph (DAG) topology validation during relationship mutation writes and mandates strict server-side schema verification for ephemeral contextual tuples in OpenFGA evaluations. Prohibits persistent storage of ephemeral request attributes and guards against recursive relation loops causing graph expansion denial-of-service.

security
81

WebAuthn Conditional UI Mediation, Nonce Atomicity, and Assertion Cryptographic Verification

Enforces zero-trust cryptographic validation for FIDO2/WebAuthn Passkey authentication with Conditional UI autofill. Requires atomic single-use challenge consumption to eliminate replay attacks, strict RP ID hash and origin binding, authenticatorData flag verification (User Presence and User Verification), and monotonic signature counter enforcement to detect authenticator cloning.

security
111

Enforce Sigstore Keyless OIDC Attestation and Admission Controller Verification

Enforces immutable supply-chain security using Sigstore Cosign keyless signing via ephemeral OIDC tokens issued by Fulcio and verified against the Rekor transparency log. Container workloads must be signed and attested at build time by digest, and admission controllers (such as Kyverno or Sigstore Policy Controller) must strictly validate the OIDC issuer and subject identity before allowing image execution.

1 Rule in Stack

WebAuthn