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securitycritical PriorityID: webauthn-prf-deterministic-key-derivation-standard

WebAuthn PRF Deterministic Zero-Knowledge Symmetric Key Derivation Standard

Enforces strict cryptographic handling when deriving hardware-backed symmetric encryption keys via the WebAuthn Pseudo-Random Function (PRF) extension (hmac-secret). Requires using high-entropy application salts, applying secondary KDFs (HKDF-SHA256) with domain separation contexts, and strictly prohibiting raw PRF output usage, plaintext key persistence, or fallback to software-derived master keys without explicit zero-knowledge degradation handling.

Prohibited Anti-Pattern: Directly Utilizing Raw PRF Output as Symmetric Keys or Storing Hardware-Derived Keys in Unprotected Persistent Browser Storage

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---
description: WebAuthn PRF Deterministic Zero-Knowledge Symmetric Key Derivation Standard - Enforces strict cryptographic handling when deriving hardware-backed symmetric encryption keys via the WebAuthn Pseudo-Random Function (PRF) extension (hmac-secret). Requires using high-entropy application salts, applying secondary KDFs (HKDF-SHA256) with domain separation contexts, and strictly prohibiting raw PRF output usage, plaintext key persistence, or fallback to software-derived master keys without explicit zero-knowledge degradation handling.
globs: src/**/*.{ts,tsx}, lib/**/*.{ts,tsx}, services/**/*.{ts,tsx}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/webauthn-prf-deterministic-key-derivation-standard"
---

# WebAuthn PRF Deterministic Zero-Knowledge Symmetric Key Derivation Standard

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: Directly Utilizing Raw PRF Output as Symmetric Keys or Storing Hardware-Derived Keys in Unprotected Persistent Browser Storage

## System Constraints
You are a Principal Security Architect enforcing the WebAuthn PRF Extension Deterministic Key Derivation Standard. Mandate that all client-side encryption keys derived from authenticators strictly leverage the WebAuthn PRF extension via an explicit secondary HKDF-SHA256 step with strong domain separation. Reject any implementation that uses raw PRF buffers directly as cipher keys, caches raw keys in unencrypted web storage (localStorage, IndexedDB), or lacks validation for the authenticator's PRF extension output.

## Strict Guidelines
- Engineers MUST ALWAYS verify that the `prf` extension output is present in `assertion.getClientExtensionResults().prf` and reject authentication flows if the expected PRF result is missing.
- Engineers MUST ALWAYS pipe the raw PRF `first` (or `second`) salt output into a secondary standard Key Derivation Function (specifically HKDF-SHA256 via Web Crypto API) using an explicit, immutable context string (info parameter) and application-level salt for domain separation.
- Engineers MUST NOT utilize the 32-byte raw PRF output directly as an AES-GCM symmetric key.
- Engineers MUST NEVER persist the derived CryptoKey or raw PRF secrets to `localStorage`, `sessionStorage`, or unencrypted `IndexedDB`; derived symmetric keys must remain strictly in memory or non-extractable CryptoKey references.
- Services MUST supply static, unique, high-entropy 32-byte salts per cryptographic domain (e.g., storage encryption vs. signature signing) during the WebAuthn `get()` assertion request.
- Code SHOULD enforce `extractable: false` when importing derived material into the Web Crypto API `SubtleCrypto.importKey` or `SubtleCrypto.deriveKey` interfaces.

## Bad vs Good Implementations
### Cryptographic Key Derivation from WebAuthn PRF Output

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Directly using raw PRF output breaches cryptographic domain separation, permits cross-protocol collisions, creates extractable cryptographic keys, and exposes zero-knowledge secrets via unencrypted localStorage.
```typescript
// Anti-pattern: Using raw PRF output directly as AES-GCM key and storing derived raw bytes in localStorage
async function deriveStorageKeyVulnerable(credential: PublicKeyCredential): Promise<CryptoKey> {
  const extensionResults = credential.getClientExtensionResults();
  // Flaw 1: No validation if PRF actually succeeded
  const rawPrf = extensionResults.prf?.results?.first as ArrayBuffer;

  // Flaw 2: Direct importation of PRF output as symmetric cipher key without HKDF domain separation
  const key = await window.crypto.subtle.importKey(
    "raw",
    rawPrf,
    { name: "AES-GCM" },
    true, // Flaw 3: Extractable key
    ["encrypt", "decrypt"]
  );

  // Flaw 4: Storing raw key material insecurely
  const exported = await window.crypto.subtle.exportKey("raw", key);
  localStorage.setItem("storage_master_key", btoa(String.fromCharCode(...new Uint8Array(exported))));
  return key;
}
```

#### Production Standard (Correct)
> **Standard**: Validates PRF evaluation presence, imports PRF entropy strictly as intermediate IKM, enforces HKDF-SHA256 with application salt and context info for domain separation, and sets extractable to false.
```typescript
// Production Standard: Strict HKDF domain separation with non-extractable CryptoKey
const STORAGE_INFO_CONTEXT = new TextEncoder().encode("app:v1:zk-client-storage-aes-gcm");
const APP_SALT = new Uint8Array([/* 32 deterministic application-domain bytes */]);

export async function deriveDeterministicStorageKey(
  credential: PublicKeyCredential
): Promise<CryptoKey> {
  const extensionResults = credential.getClientExtensionResults();
  const prfResults = extensionResults.prf?.results;

  if (!prfResults?.first) {
    throw new Error("Authenticator does not support or failed to evaluate WebAuthn PRF extension");
  }

  // Step 1: Import raw PRF output as raw HKDF Initial Keying Material (IKM)
  const ikm = await window.crypto.subtle.importKey(
    "raw",
    prfResults.first,
    { name: "HKDF" },
    false, // Non-extractable
    ["deriveKey"]
  );

  // Step 2: Derive final AES-GCM 256-bit key with explicit domain separation
  return await window.crypto.subtle.deriveKey(
    {
      name: "HKDF",
      hash: "SHA-256",
      salt: APP_SALT,
      info: STORAGE_INFO_CONTEXT
    },
    ikm,
    { name: "AES-GCM", length: 256 },
    false, // Non-extractable: cannot be read or leaked via memory scraping
    ["encrypt", "decrypt"]
  );
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

WebAuthn & WebCrypto Inspector MCP
Analyzes client-side WebCrypto pipelines and WebAuthn extension configurations for cryptographic vulnerabilities.
Recommended Agent Skills (askills.dev)

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

Crypto Audit CLI
Static AST analyzer verifying correct parameters, domain separation, and non-extractability in WebCrypto API invocations.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: WebAuthn PRF Deterministic Zero-Knowledge Symmetric Key Derivation Standard

WebAuthn PRF Deterministic Zero-Knowledge Symmetric Key Derivation Standard defines an industrial-grade behavioral standard for security engineering. Enforces strict cryptographic handling when deriving hardware-backed symmetric encryption keys via the WebAuthn Pseudo-Random Function (PRF) extension (hmac-secret). Requires using high-entropy application salts, applying secondary KDFs (HKDF-SHA256) with domain separation contexts, and strictly prohibiting raw PRF output usage, plaintext key persistence, or fallback to software-derived master keys without explicit zero-knowledge degradation 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: Directly Utilizing Raw PRF Output as Symmetric Keys or Storing Hardware-Derived Keys in Unprotected Persistent 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 DriftDirectly Utilizing Raw PRF Output as Symmetric Keys or Storing Hardware-Derived Keys in Unprotected Persistent 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 verify that the `prf` extension output is present in `assertion.getClientExtensionResults().prf` and reject authentication flows if the expected PRF result is missing.

Engineers MUST ALWAYS verify that the `prf` extension output is present in `assertion.getClientExtensionResults().prf` and reject authentication flows if the expected PRF result is missing.

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 pipe the raw PRF `first` (or `second`) salt output into a secondary standard Key Derivation Function (specifically HKDF-SHA256 via Web Crypto API) using an explicit, immutable context string (info parameter) and application-level salt for domain separation.

Engineers MUST ALWAYS pipe the raw PRF `first` (or `second`) salt output into a secondary standard Key Derivation Function (specifically HKDF-SHA256 via Web Crypto API) using an explicit, immutable context string (info parameter) and application-level salt for domain separation.

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 utilize the 32-byte raw PRF output directly as an AES-GCM symmetric key.

Engineers MUST NOT utilize the 32-byte raw PRF output directly as an AES-GCM symmetric key.

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 NEVER persist the derived CryptoKey or raw PRF secrets to `localStorage`, `sessionStorage`, or unencrypted `IndexedDB`

Engineers MUST NEVER persist the derived CryptoKey or raw PRF secrets to `localStorage`, `sessionStorage`, or unencrypted `IndexedDB`; derived symmetric keys must remain strictly in memory or non-extractable CryptoKey references.

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

Services MUST supply static, unique, high-entropy 32-byte salts per cryptographic domain (e.g., storage encryption vs. signature signing) during the WebAuthn `get()` assertion request.

Services MUST supply static, unique, high-entropy 32-byte salts per cryptographic domain (e.g., storage encryption vs. signature signing) during the WebAuthn `get()` assertion request.

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

Code SHOULD enforce `extractable

Code SHOULD enforce `extractable: false` when importing derived material into the Web Crypto API `SubtleCrypto.importKey` or `SubtleCrypto.deriveKey` interfaces.

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

Enforces strict cryptographic handling when deriving hardware-backed symmetric encryption keys via the WebAuthn Pseudo-Random Function (PRF) extension (hmac-secret). Requires using high-entropy application salts, applying secondary KDFs (HKDF-SHA256) with domain separation contexts, and strictly prohibiting raw PRF output usage, plaintext key persistence, or fallback to software-derived master keys without explicit zero-knowledge degradation 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 (Directly Utilizing Raw PRF Output as Symmetric Keys or Storing Hardware-Derived Keys in Unprotected Persistent 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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