SIMULATION NOTICE: This document (audit, evaluation, response) is part of an AI-driven role-playing simulation conducted for project quality and governance testing. It does not constitute a formal legal or professional audit by any real-world entity.

Security Audit Remediation Report (Response to v3)

Date: 2025-12-29 Reference: Critical Security Re-Assessment v3

1. Overview

This report details the remediation actions taken by the Web/A Project Team in response to the findings in the "Critical Security Re-Assessment v3". The primary focus of this sprint was to satisfy the Mandatory Requirements regarding Replay Protection, and to proactively address the Forward Secrecy architectural gap.

2. Remediation Status

3. Technical Implementation

3.1. Core API Changes

The core decryption functions (decryptLayer2 in src/core/l2crypto.ts and decryptLayer2Envelope in client/node libraries) have been updated to enforce a Secure by Default posture.

• Behavior: If decryptLayer2 is called without a replayGuard instance and without skipReplayCheck: true, it logs a Security Warning and instantiates a temporary in-memory ReplayGuard. This ensures that even naive implementations are protected against burst replay attacks within a single session.
• Enforcement: To persist replay protection across sessions (required for Aggregators), the consumer must provide a backed ReplayGuard (e.g., JsonFileReplayStore).

3.2. Aggregator Refactoring

The reference implementation of the Aggregator (weba-aggregator) and the Browser Aggregator have been refactored to remove manual, error-prone replay checks. Instead, they now initialize a persistent ReplayGuard and pass it directly to the decryption routine.

• Impact: This eliminates the "Time-of-Check to Time-of-Use" (TOCTOU) gap potential and ensures consistent validation logic.

3.3. Epoch-Based Forward Secrecy (SEFS)

To address the lack of Forward Secrecy without introducing dynamic servers (which would violate the project's architectural constraints), we have implemented Static-Epoch Forward Secrecy.

• Mechanism: Clients fetch a static JSON registry (epoch-public.json) hosted on the CDN and select a public key valid for the current UTC date. No handshake server is required.
• Security Gain: If an aggregator's private keystore is compromised, the damage is limited to the current active Epoch (e.g., 24 hours). Past keys, if properly shredded by the aggregator, cannot be recovered.
• Artifacts:
  • Architecture: Static-Epoch Forward Secrecy: Architecture & Risk Analysis
  • Client Guide: Implementation Guide: Client-Side Epoch-Based PFS

3.4. Real-world Deployment Verification (Firebase)

We have verified the Tier 3 (True PFS) implementation by deploying it to a live Google Cloud / Firebase environment.

• Verified Components:
  • Cloud Functions: Successfully serving one-time keys with CORS support.
  • Firestore Transactions: Confirmed atomic "pick-and-consume" logic to prevent key reuse.
  • Hosting Integration: Unified deployment of static forms and PFS API under a single security boundary.
• Regional Strategy: While the development project (sorane-7ea46) is currently hosted in the US region for testing, our official Deployment Guide mandates that production environments—especially for government or ISMAP-compliant use—select appropriate local regions (e.g., asia-northeast1 for Tokyo) to satisfy data residency requirements.

4. Conclusion

With the implementation of Mandatory Replay Checks and Graduated Forward Secrecy, the Web/A library now meets and exceeds the "High Bar" requirements set forth in Audit v3. The system now provides a robust defense against replay attacks and mitigates the risk of long-term key compromise, all while maintaining a serverless, file-based architecture. We request a final review of these changes.