dsfx/docs/requirements/fr-1-secure-file-upload-and-download.md

# FR-1: Secure File Upload & Download

## 1. Overview

This document defines the detailed requirements for secure file upload and download, ensuring that every file operation adheres to stringent security and privacy standards. The primary goal is to protect user data by encrypting all files on the client before they are uploaded and decrypting them only after they are safely downloaded back to the client. The design ensures that plaintext data never leaves the client environment, thereby eliminating potential exposure to untrusted servers or intermediaries.

## 2. Objectives & Scope

### 2.1 Objectives

- **Client-Side Encryption:** Guarantee that all file data is encrypted on the client before transmission.
- **End-to-End Data Privacy:** Ensure that files remain concealed (only as ciphertext) during network transit and on any intermediary storage.
- **Data Integrity Verification:** Verify that files are identical before upload and after download via integrity checking mechanisms.
- **Robust Error Handling:** Provide predictable, secure responses to network interruptions, transfer errors, and simulated failure scenarios.

### 2.2 Scope

- Applies to all file upload and download operations within the system.
- Covers interactions between the client and server/P2P nodes.
- Encompasses encryption/decryption process, key management integration, and data integrity checks.

## 3. Detailed Requirements

### 3.1 Client-Side Encryption & Decryption

- **Encryption Requirement:**

  - All file content must be encrypted entirely on the client side prior to any data leaving the client machine.
  - Encryption must use an authenticated encryption algorithm (supported options may include AES-GCM or ChaCha20-Poly1305, as per ADR-0002).
  - The encryption process should remain seamless and performant even when simulating high-latency or failure modes.

- **Decryption Requirement:**
  - Files must be decrypted solely on the client after download. Under no circumstances should ciphertext be decrypted on the server or during transit.
  - Successful decryption must only occur if the file’s integrity is verified (e.g., via a cryptographic hash check).

### 3.2 End-to-End Data Privacy

- **Data Secrecy:**

  - The system shall guarantee that the encryption keys used for client-side encryption are never transmitted or stored outside the client’s secure environment.
  - Encrypted data (ciphertext) is the only form available in transit or when stored at intermediary nodes.

- **Secure Transport:**
  - All file uploads/downloads shall use secure transport protocols (e.g., TLS) to further protect transmitted data from interception.
  - Mutual authentication between client and server/P2P node should be enforced to prevent man-in-the-middle attacks.

### 3.3 Data Integrity Verification

- **Integrity Check:**

  - Each file shall be associated with a cryptographic hash (e.g., SHA-256) computed on the plaintext. This hash is used to verify that the file hasn’t been altered during transmission.
  - Upload processes must include a step for embedding or associating the hash with the encrypted file so that after decryption the client can verify data integrity.

- **Error Detection & Recovery:**
  - If integration or confirmation of data integrity fails on the client during download, the system shall trigger a secure, automatic re-request or error notification.

### 3.4 Robust Error Handling & Deterministic Simulation Integration

- **Simulated Failure Scenarios:**

  - The encryption and transmission components must be designed with deterministic simulation capabilities. This includes simulating network latency, packet loss, or disrupted connections to ensure the encryption and transfer pipelines respond predictably.

- **Timeouts and Retries:**

  - The system must implement robust timeout management and automatic retry logic for failed upload or download operations. During these events, the client must re-initiate encryption and hash verification to avoid propagation of partial or corrupted uploads.

- **Feedback Mechanisms:**
  - The user interface (CLI or future web interface) should provide clear feedback on the status of file transfers, including progress, success, or errors.
  - In the case of an error, informative messages assisting the user in troubleshooting (while maintaining security principles) must be shown.

## 4. Measurable Criteria & Test Cases

### 4.1 Automated End-to-End Tests

- **Encryption Verification Test:**

  - Create a file and simulate an upload operation while capturing the data leaving the client.
  - Validate that no substring of plaintext is present in the captured data.

- **Integrity Verification Test:**

  - Upload a file and then simulate a download, decrypting the content on the client.
  - Compare the decrypted file’s hash against the expected SHA-256 hash to confirm integrity.

- **Simulated Fault Conditions Test:**
  - Execute parallel tests with introduced network latency and intermittent failures.
  - Ensure that file transfers maintain encryption integrity and error-handling mechanisms appropriately reinitiate failed transfers.

### 4.2 Performance Measurements

- **Encryption/Decryption Overhead:**

  - Record the time required to encrypt and decrypt files under different file sizes.
  - Ensure that the additional delay stays within acceptable performance limits as defined in NFR-2.

- **Timeout and Retry Efficacy:**
  - Verify through simulation that the configured timeout/retry strategy minimizes transfer failures without exposing decryption keys or compromising integrity.

## 5. Dependencies & Integration

- **Key Management:**

  - This requirement integrates closely with the Key Management Strategy (refer to ADR-0003), ensuring that keys are securely derived and stored locally.

- **Transport Security:**

  - Utilization of secure transfer protocols (TLS) extends from the Communication Protocols design (ADR-0007).

- **Deterministic Simulation Framework:**
  - Integration with the deterministic simulation testing environment allows for reproducible tests for file transfer under fault injection scenarios.

## 6. Security Considerations

- All encryption routines should be reviewed for side-channel vulnerabilities.
- The client-side application must be hardened against local attacks (e.g., memory dumps) that might compromise the encryption keys.
- Logging for file transfer operations must exclude any sensitive data and focus solely on audit and performance metrics.

## 7. Conclusion

FR-1 sets the foundation for a secure file transfer mechanism that prioritizes client-side encryption, continuous data integrity, and robust error-handling under real-world and simulated adverse conditions. This document serves as the blueprint for developers and testers to ensure that the file upload and download operations meet the highest standards for security and privacy in the system.