Introduction
SecureLync is an end-to-end encrypted file sharing service. Files are encrypted in your browser before upload and can only be decrypted by the recipient with the secure download token.
Key Features
- AES-256-GCM encryption — Client-side encryption, server never sees plaintext
- HMAC-signed tokens — Tamper-proof, time-limited download links
- Gate links — Download-count-limited links that close automatically after N downloads
- Security scanning — Multi-engine hash checks (MalwareBazaar, CIRCL, VirusTotal)
- Geo visualization — See where uploads and downloads originate
- Self-destructing files — Automatic expiry, manual revocation
Quickstart
1. Upload a File
Drag and drop files onto the upload area on the home page or app dashboard. You can upload up to 10 files (max 50MB each, 500MB total).
2. Get Your Link
Once uploaded, you'll receive a unique token link. Copy it and share it with your recipient. The link includes an encrypted key that only the recipient can use.
3. Recipient Downloads
The recipient clicks the link, acknowledges the disclaimer, and downloads. Files remain encrypted until they unlock with the embedded key.
Encryption happens entirely in your browser. The server never has access to your encryption keys or unencrypted file contents.
How to Upload
Select Files
Drag files onto the dropzone or click to open the file picker. Multiple files are supported.
Set Expiry (Optional)
Choose a custom expiration time or leave it at the default (24 hours).
Upload & Encrypt
Files are encrypted in your browser using AES-256-GCM before being sent to the server.
Copy Link
Receive your secure token link. Copy and share it with your intended recipient(s).
Token Links
When you generate a token, SecureLync gives you a share link in this format:
The share link contains only a token identifier — no decryption keys, no file contents, and no personal data. The token is stored server-side and maps to your encrypted files.
How the decryption key is protected
The AES decryption key is never embedded in the URL. It is stored encrypted in the database and only released to a recipient under strict conditions:
Token validation
The server verifies the token exists, has not expired, and has not hit its download limit.
Terms of Service acknowledgement
The recipient must read and accept the Terms of Service. This is enforced server-side — the file list and decryption keys are not sent until a valid acknowledgement token is issued by the server. This gate cannot be bypassed by editing the page in browser tools.
Key delivery
Only then does the server return the encrypted file metadata and decryption keys over an authenticated, time-limited request. The keys are held in browser memory only for the duration of the download and are never written to disk or stored in the URL.
Client-side decryption
The encrypted blob is fetched and decrypted entirely in the recipient's browser. The server never sees plaintext file contents at any point.
Embedding decryption keys in URLs exposes them in browser history, server access logs, referrer headers, and proxy logs. SecureLync's server-gated key delivery means the key is never in the URL at any point — even if a share link is intercepted, it cannot be used to decrypt files without also passing the server-side acknowledgement gate.
Expiration & Revocation
All download tokens have an expiration time. Default: 24 hours from creation. You can set a custom duration when uploading:
You can also manually revoke a token at any time from your dashboard. Once revoked, the token cannot be used again, even if it hasn't expired.
Gate Links
A Gate link automatically closes after a set number of downloads rather than a time window. Once the limit is hit, the link deactivates immediately — regardless of expiry date — and any further download attempts are rejected.
Gate links are ideal for controlled releases: early access to a product, the first 50 attendees of an event, a limited distribution of a document, or any scenario where you want access to end at an exact count.
How to Create a Gate Link
Open the Gate tab
In the Generate Token window, switch to the Gate tab.
Upload files and set a limit
Add your files, enter a label (e.g. "Product Launch"), and set the maximum download count.
Set a safety expiry
Choose a fallback time limit. The link closes at whichever comes first — the download count or the expiry date.
Launch and share
Copy the link and distribute it. Once the limit is reached, recipients see a 🏁 Download Limit Reached message — the page disables all download buttons automatically.
Tracking Gate Links
Gate links appear in the Gated tab with a live progress bar showing downloads used vs. the limit. Closed links (limit reached) are visually distinct from active ones. You can enable auto-clear in Settings to automatically remove closed Gate links once their limit is hit.
If a recipient downloads a file and the link has now hit its limit, all download buttons on their page are immediately disabled without requiring a page refresh. A banner message notifies them the link is closed.
End-to-End Encryption
SecureLync uses AES-256-GCM (Galois/Counter Mode) for authenticated encryption. Here's how it works:
Key points:
- Encryption happens entirely in your browser using the Web Crypto API
- The server never sees your encryption keys or unencrypted file contents
- Each file gets a unique random AES key and IV
- The AES key is encrypted and embedded in the download URL
HMAC Signatures
Download tokens are cryptographically signed using HMAC-SHA256. The signature covers:
- The token identifier
- The user ID (owner)
- The expiration timestamp
When a download request arrives, the server recomputes the HMAC and verifies it matches the stored signature. Any tampering with the token, user, or expiry invalidates the signature.
The HMAC signing key is stored in a secure environment variable and never exposed to clients.
Security Scanning
SecureLync automatically scans every file for known threats before upload. Scanning is hash-based — only a SHA-256 fingerprint of each file is checked. File contents are never transmitted to any scanning service.
When you add files to an upload, SecureLync computes each file's SHA-256 hash in your browser and checks it against multiple threat intelligence databases. Results are shown as security badges next to each file. The actual file is only uploaded after scanning completes.
Scanning services
SecureLync checks each file hash against the following services in parallel:
- MalwareBazaar — Threat intelligence database of confirmed malware samples maintained by abuse.ch.
- CIRCL Hashlookup — Known-safe file reference database maintained by the Computer Incident Response Center Luxembourg (circl.lu). A match here reduces suspicion.
- VirusTotal — Aggregates results from over 70 antivirus engines. Active when configured.
Results are cached for 30 days so re-uploading a previously scanned file returns instantly without re-querying.
Hash-based scanning detects known threats only. Novel malware or heavily modified files may not be flagged. Always scan downloaded files with your own antivirus before opening, regardless of badge status.
Security badges
Only the file's SHA-256 hash is transmitted — never the file itself. A hash cannot be reversed to reconstruct the original file.
Download Rules by Token Type
SecureLync has two distinct token types with different download behaviour. Understanding the difference is important for both senders and recipients.
| Behaviour | Standard Token | Gate Token |
|---|---|---|
| Expires by | Time (1 hour – 90 days) | Download count or time (whichever first) |
| Downloads per session | Unlimited while active | One per browser session |
| Re-download after refresh | Yes, until expiry | Yes, if count not exhausted |
| After limit hit | N/A | Link closes immediately for everyone |
| Scan coverage | Full 5-layer pipeline | Full 5-layer pipeline (identical) |
| Dashboard tab | Tokens | Gated |
Once a recipient downloads any file from a Gate link during a browser session, all download buttons on that page are permanently disabled for that session. This is a client-side safety measure that prevents accidental re-downloads within the same session. Refreshing the page re-validates the link — if the download count has not been exhausted, files remain available.
If you need recipients to be able to download each file exactly once across all sessions (not just per session), set your Gate limit to match your expected audience size. For example, a limit of 50 means the 51st person to visit the link will see a "Download Limit Reached" page.
Data Retention & Cleanup
SecureLync automatically manages storage and data cleanup through a combination of user-controlled preferences and a server-side hourly process. Here is exactly what gets deleted, when, and why.
The three layers of cleanup
| Layer | Trigger | What is deleted | User controlled? |
|---|---|---|---|
| Encrypted blob removal | Hourly, automatic | Encrypted file bytes for expired tokens. Download history and token records are kept. | No — always runs |
| Token auto-clear | Hourly if enabled, or when you open the Tokens/Gated tabs | Expired token records and capped Gate token records. Download history is preserved. | Yes — toggle in Settings |
| 30-day inactive cleanup | Hourly, when account has had no activity for 30 days | All tokens, files, and download history. Full account data purge. | No — last resort only |
When this setting is on, expired token records are removed from your dashboard automatically. Your download history is never touched — the globe, terminal, and race graph continue to show all past downloads even after the token that generated them is cleared.
When a token's expiry time passes, the encrypted file bytes are removed from storage on the next hourly run. This is automatic for all users regardless of the auto-clear setting — it is a storage hygiene measure, not a data deletion event from your perspective. The token record and download history remain in your dashboard until you or the auto-clear setting removes them.
If you have not logged in or made any authenticated request for 30 consecutive days, the next hourly run will permanently delete all your tokens, files, and download history regardless of your auto-clear preferences. This is a last-resort cleanup for abandoned accounts. You will not receive a warning. If you use SecureLync regularly this rule will never apply to you.
Auto-clear settings are saved to your account, not just your browser. This means the hourly server process respects your preferences even when you are not logged in.
Upload Limits
SecureLync enforces the following limits per upload session:
Attempting to upload beyond these limits will result in an error. Consider splitting large batches into separate uploads.
Security Badge Meanings
Each file receives a security badge based on its scan status:
Frequently Asked Questions
No. Files are encrypted in your browser before upload using AES-256-GCM. The server only stores encrypted blobs and has no ability to decrypt them.
The download link becomes permanently invalid. The encrypted file remains in storage but cannot be accessed. You would need to re-upload to generate a new token.
Yes. All files are scanned against multiple threat intelligence sources (MalwareBazaar, CIRCL, and VirusTotal) before upload. Only the hash is sent — never the actual file contents.
No. Expired tokens cannot be revived. You would need to upload the file again to generate a fresh token with a new expiration.
From your dashboard, revoke the token (in Admin panel or User token list). This immediately invalidates the token and prevents further downloads. The encrypted file remains in R2 storage until manually purged by the server operator.