Implement Secure and Anonymous QR Code Payment Handles

### Description

**Description:**

This epic is to design and implement a system that generates QR code payment handles for users to present to merchants. These payment handles will facilitate secure and anonymous transactions, ensuring user privacy while enabling merchants to process payments and issue reimbursements.

The payment handle embedded in the QR code must meet the following criteria:

- **Anonymity:** The handle should not contain any personally identifiable information that can be directly linked back to the user by the merchant. Merchants should not be able to retrace the handle to the user after the transaction.
- **Uniqueness:** Each handle must uniquely identify the user's bank account for payment processing.
- **Expiration:** Handles should have a limited lifespan to enhance security and prevent misuse.
- **Reusability for Reimbursement:** The handle must be reusable by the merchant to initiate refunds to the user's bank account.
- **Security:** To ensure confidentiality and prevent unauthorized access, the payment handle will be generated by encrypting the user's bank account ID using the target server's public key. Only the bank's server will possess the private key required to decrypt and process the payment.

This feature aims to provide a secure and privacy-focused payment experience for users while maintaining the necessary functionality for merchants to conduct transactions and manage refunds.

**Acceptance Criteria:**

- [ ] A QR code generation service is implemented that generates QR codes containing encrypted payment handles.
- [ ] Payment handles are encrypted using the target server's public key.
- [ ] Payment handles are unique to the user's bank account.
- [ ] Payment handles include an expiration mechanism (e.g., timestamp).
- [ ] Merchants can successfully use the payment handle to initiate payment processing.
- [ ] Merchants can reuse the payment handle to initiate reimbursements to the user.
- [ ] The system demonstrably ensures anonymity, preventing merchants from directly identifying the user from the payment handle.
- [ ] The system is tested for security vulnerabilities and privacy compliance.
- [ ] Comprehensive documentation for developers and merchants on how to use the QR code payment handle system is created.

**User Stories:**

- As a user, I want to be able to generate a QR code payment handle so that I can pay merchants without revealing my personal bank account details, protecting my privacy.
- As a user, I want my payment handle to expire automatically after a short period, enhancing the security and reducing the risk of unauthorized use if intercepted.
- As a user, I want to be able to receive reimbursements from merchants easily using the same payment handle I used for the initial payment, simplifying the refund process.
- As a merchant, I want to be able to quickly scan a QR code payment handle presented by a user to initiate a payment, making the checkout process efficient.
- As a merchant, I want to be able to reuse the payment handle provided by the user to process a reimbursement, ensuring smooth handling of returns and refunds.
- As the Bank, we need to be able to decrypt the payment handle to securely identify the user's bank account and process the transaction, maintaining the integrity of the payment system.

**Tasks:**

- [ ] **Backend Development:**
- [ ] Design the structure of the payment handle to include necessary information (bank account ID, expiration timestamp, etc.).
- [ ] Implement the encryption service to encrypt the payment handle using the target server's public key.
- [ ] Develop the QR code generation service to encode the encrypted payment handle into a QR code.
- [ ] Implement the decryption service on the bank server to decrypt the payment handle using the corresponding private key.
- [ ] Develop API endpoints for:
- [ ] User application to request and receive a QR code payment handle.
- [ ] Merchant application to submit a payment using the QR code payment handle.
- [ ] Merchant application to initiate a reimbursement using the QR code payment handle.
- [ ] Implement logic for payment handle expiration and validation.
- [ ] Implement robust error handling and logging for all services.
- [ ] Ensure secure storage and management of encryption keys.

- [ ] **Frontend Development (User Application):**
- [ ] Develop UI for users to request and display their QR code payment handle.
- [ ] Implement QR code display functionality within the user application.
- [ ] Ensure seamless integration with the backend API for handle generation.

- [ ] **Frontend Development (Merchant Application):**
- [ ] Implement QR code scanning functionality within the merchant application.
- [ ] Develop UI for merchants to initiate payments and reimbursements using the scanned QR code payment handle.
- [ ] Ensure seamless integration with the backend API for payment and reimbursement processing.

- [ ] **Security and Testing:**
- [ ] Conduct thorough security testing, including penetration testing and vulnerability assessments, to ensure the anonymity and security of the payment handle system.
- [ ] Implement unit and integration tests for all backend and frontend components.
- [ ] Perform end-to-end testing to validate the complete payment and reimbursement flow.
- [ ] Conduct privacy compliance checks to ensure adherence to relevant data protection regulations.

- [ ] **Documentation:**
- [ ] Create detailed developer documentation for all backend and frontend components, including API specifications and integration guidelines.
- [ ] Develop merchant-facing documentation outlining how to use the QR code payment handle system for payments and reimbursements.
- [ ] Prepare user-facing documentation or guides on how to generate and use QR code payment handles.

**Dependencies:**

- Key Management System for secure storage and retrieval of public and private keys.
- QR Code generation library.
- Encryption library.
- Payment processing API infrastructure.

### Discussion

https://github.com/ADORSYS-GIS/webank-OnlineBanking/issues/58

### Issues

_No response_

### Motivation

_No response_