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-prf-zk-isolation-boundary

WebAuthn PRF Zero-Knowledge Key Derivation & Ephemeral Memory Isolation

Enforces architectural isolation boundaries for symmetric keys derived via WebAuthn PRF (Pseudo-Random Function) extensions. Requires non-exportable WebCrypto keys, immediate manual buffer zeroization in scoped execution blocks, and strict prevention of key persistence across DOM storage, global state, and network layers.

Prohibited Anti-Pattern: Persisting raw PRF material, using exportable CryptoKeys, or abandoning unwiped key buffers to garbage collection

Export Ready-to-Drop Configuration

Select format below to copy code or download file directly.

---
description: WebAuthn PRF Zero-Knowledge Key Derivation & Ephemeral Memory Isolation - Enforces architectural isolation boundaries for symmetric keys derived via WebAuthn PRF (Pseudo-Random Function) extensions. Requires non-exportable WebCrypto keys, immediate manual buffer zeroization in scoped execution blocks, and strict prevention of key persistence across DOM storage, global state, and network layers.
globs: src/**/{auth,crypto,security}/**/*.{ts,tsx,js}, src/**/*.worker.{ts,js}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/webauthn-prf-zk-isolation-boundary"
---

# WebAuthn PRF Zero-Knowledge Key Derivation & Ephemeral Memory Isolation

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: Persisting raw PRF material, using exportable CryptoKeys, or abandoning unwiped key buffers to garbage collection

## System Constraints
You are a Principal Security Architect enforcing zero-knowledge WebAuthn PRF key-derivation invariants. Whenever generating or consuming WebAuthn PRF outputs, you must enforce non-exportable CryptoKey constraints, immediate memory sanitization (zeroization) inside finally blocks, and complete isolation from browser storage APIs and observability sinks.

## Strict Guidelines
- Engineers MUST ALWAYS import derived symmetric keys into WebCrypto using subtle.importKey with `extractable: false`.
- Engineers MUST ALWAYS manually overwrite raw PRF outputs and intermediate HKDF salt/key buffers using `.fill(0)` inside a deterministic `finally` block.
- Engineers MUST NOT serialize, clone, or persist raw PRF results into `localStorage`, `sessionStorage`, `IndexedDB`, browser cookies, or global frontend state containers.
- Code SHOULD isolate key derivation and decryption logic within dedicated Web Workers to mitigate main-thread DOM-based memory inspection vectors.
- Components NEVER expose raw PRF outputs or derived key material to error messages, console logs, or client-side telemetry payloads.

## Bad vs Good Implementations
### Insecure: Storing PRF Output and Importing Exportable Key

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: The raw PRF output is saved into sessionStorage where any main-thread script/XSS can read it. The CryptoKey is imported with extractable: true, allowing memory-scraping attacks, and the raw ArrayBuffer remains floating in V8 heap memory waiting for non-deterministic garbage collection.
```typescript
async function decryptUserData(assertion: AuthenticationCredential, ciphertext: ArrayBuffer, iv: Uint8Array) {
  const extensionResults = assertion.getClientExtensionResults();
  const prfRaw = extensionResults.prf?.results?.first;
  if (!prfRaw) throw new Error("PRF output unavailable");

  // CRITICAL SECURITY FLAW: Persisting raw PRF material in unencrypted session storage
  sessionStorage.setItem("user_master_seed", Array.from(new Uint8Array(prfRaw)).toString());

  // CRITICAL SECURITY FLAW: extractable: true allows unauthorized exfiltration via subtle.exportKey
  const key = await window.crypto.subtle.importKey(
    "raw",
    prfRaw,
    { name: "AES-GCM" },
    true,
    ["decrypt"]
  );

  return window.crypto.subtle.decrypt({ name: "AES-GCM", iv }, key, ciphertext);
}
```

#### Production Standard (Correct)
> **Standard**: The raw PRF bytes are imported with extractable: false to prevent exfiltration via subtle.exportKey. The raw buffer is explicitly cleared with .fill(0) inside a deterministic finally block, and zero key data is committed to persistent or semi-persistent storage.
```typescript
async function decryptUserData(assertion: AuthenticationCredential, ciphertext: ArrayBuffer, iv: Uint8Array): Promise<ArrayBuffer> {
  const extensionResults = assertion.getClientExtensionResults();
  const prfRawBuffer = extensionResults.prf?.results?.first;
  if (!prfRawBuffer) {
    throw new Error("Authenticator PRF evaluation failed");
  }

  const rawSecret = new Uint8Array(prfRawBuffer);
  let ephemeralKey: CryptoKey | null = null;

  try {
    // Import as strictly non-exportable key to isolate from execution context
    ephemeralKey = await window.crypto.subtle.importKey(
      "raw",
      rawSecret,
      { name: "AES-GCM", length: 256 },
      false, // non-exportable boundary
      ["decrypt"]
    );

    return await window.crypto.subtle.decrypt(
      { name: "AES-GCM", iv },
      ephemeralKey,
      ciphertext
    );
  } finally {
    // Explicitly zeroize the sensitive key material from memory
    rawSecret.fill(0);
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

Web Crypto Auditor
Static AST analyzer for detecting leaked crypto keys and invalid WebCrypto extractability parameters
Recommended Agent Skills (askills.dev)

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

WebAuthn PRF Boundary Linter
Static enforcement checks that fail builds if WebAuthn extension results are assigned to non-ephemeral scopes
Architectural Specification & Invariant Rationale

Architectural Invariant Design: WebAuthn PRF Zero-Knowledge Key Derivation & Ephemeral Memory Isolation

WebAuthn PRF Zero-Knowledge Key Derivation & Ephemeral Memory Isolation defines an industrial-grade behavioral standard for security engineering. Enforces architectural isolation boundaries for symmetric keys derived via WebAuthn PRF (Pseudo-Random Function) extensions. Requires non-exportable WebCrypto keys, immediate manual buffer zeroization in scoped execution blocks, and strict prevention of key persistence across DOM storage, global state, and network layers. 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: Persisting raw PRF material, using exportable CryptoKeys, or abandoning unwiped key buffers to garbage collection 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 DriftPersisting raw PRF material, using exportable CryptoKeys, or abandoning unwiped key buffers to garbage collectionCritical
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 import derived symmetric keys into WebCrypto using subtle.importKey with `extractable

Engineers MUST ALWAYS import derived symmetric keys into WebCrypto using subtle.importKey with `extractable: false`.

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 manually overwrite raw PRF outputs and intermediate HKDF salt/key buffers using `.fill(0)` inside a deterministic `finally` block.

Engineers MUST ALWAYS manually overwrite raw PRF outputs and intermediate HKDF salt/key buffers using `.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.
3

Engineers MUST NOT serialize, clone, or persist raw PRF results into `localStorage`, `sessionStorage`, `IndexedDB`, browser cookies, or global frontend state containers.

Engineers MUST NOT serialize, clone, or persist raw PRF results into `localStorage`, `sessionStorage`, `IndexedDB`, browser cookies, or global frontend state containers.

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

Code SHOULD isolate key derivation and decryption logic within dedicated Web Workers to mitigate main-thread DOM-based memory inspection vectors.

Code SHOULD isolate key derivation and decryption logic within dedicated Web Workers to mitigate main-thread DOM-based memory inspection vectors.

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 expose raw PRF outputs or derived key material to error messages, console logs, or client-side telemetry payloads.

Components NEVER expose raw PRF outputs or derived key material to error messages, console logs, or client-side telemetry 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.
Technical Questions & Answers

Frequently Asked Technical Questions

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

Q:Why is WebAuthn PRF Zero-Knowledge Key Derivation & Ephemeral Memory Isolation critical for AI coding assistants?

Enforces architectural isolation boundaries for symmetric keys derived via WebAuthn PRF (Pseudo-Random Function) extensions. Requires non-exportable WebCrypto keys, immediate manual buffer zeroization in scoped execution blocks, and strict prevention of key persistence across DOM storage, global state, and network layers. 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 (Persisting raw PRF material, using exportable CryptoKeys, or abandoning unwiped key buffers to garbage collection) 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