Of course. Here is the new, clean PROBLEM.md.

This version is tightly focused on defining the problem, context, and constraints. It removes all implementation details and historical analysis, setting the stage perfectly for your other documents (SOLUTION.md, SECURITY.md) to describe the "how."

This document defines a critical security vulnerability within a public-facing travel booking application. The core issue is the exposure of payment gateway "fingerprints," which allows for automated harvesting by malicious actors. Any proposed solution must mitigate this vulnerability while strictly adhering to the business requirement of maintaining publicly shareable, unauthenticated booking pages.

• Vulnerability: Mass harvesting of single-use payment fingerprints.
• Business Impact: High risk of payment processor fraud alerts and potential merchant account suspension.
• Critical Constraint: The system must not require user login or registration.

The application allows public users to browse travel tours, view details (including pricing), and proceed to payment without creating an account or logging in. The business model relies on making this process as frictionless as possible. To facilitate sharing, direct links to tour/booking pages are designed to be sent via email or messages and must be accessible to anyone with the link.

The original implementation exposed the payment gateway's unique payment token (the "fingerprint") in the client-side source code upon page load.

• Attack Vector: An attacker can programmatically access a booking page URL and repeatedly reload or request it. With each request, a new, valid payment fingerprint is generated and exposed. The attacker can harvest thousands of these fingerprints without ever attempting a payment.
• Business Impact: Payment processors like Zenpay monitor fingerprint generation and usage rates. A high volume of generated but unused fingerprints is a strong indicator of fraudulent activity or a security breach. This can lead to:
  1. The processor automatically blocking fingerprint generation.
  2. The merchant account being flagged and potentially suspended.
  3. Legitimate customers being unable to complete payments.

Any viable solution must operate within the following strict constraints.

• No User Authentication: The system must not require any form of user registration, login, or password. This is a non-negotiable business requirement.
• Publicly Shareable Links: Tour and booking pages must remain accessible to anyone who has the URL. The solution cannot rely on traditional, long-lived user sessions that would break this functionality.

The payment gateway's behavior imposes two critical constraints on the solution design:

1. Fingerprints are Single-Use & Consumed Immediately: A payment fingerprint is invalidated after any payment attempt, regardless of whether it succeeds or fails. A customer who needs to retry a payment due to a typo or insufficient funds requires a completely new fingerprint.
2. Callback Authentication via ValidationCode: The gateway sends a server-to-server POST request (a "callback") to report the transaction status. To ensure the authenticity and integrity of this callback, the gateway includes a ValidationCode. This code is a SHA3-512 hash of the transaction details combined with secret credentials. Any solution must verify this ValidationCode to securely confirm transaction status.

• Platform: The application is deployed on Val Town, a serverless Deno environment.
• Database: The primary data store is SQLite.

A successful solution will be measured by its ability to meet the following criteria:

1. Prevent Mass Harvesting: The solution's primary goal is to make the automated, large-scale harvesting of payment fingerprints computationally and logistically infeasible for an attacker.
2. Maintain Public Access Model: The core business requirement of unauthenticated, shareable pages must not be compromised.
3. Provide a Clear Retry Path: The user interface and system flow must gracefully handle payment failures and allow a user to easily attempt payment again.
4. Operate Statelessly: The solution must not depend on long-lived, server-side user sessions that would conflict with the public access model.
5. Integrate Securely with Gateway: The solution must correctly generate all required parameters for the payment gateway and securely validate incoming callbacks using the ValidationCode mechanism.