Lenzeye File Transfer¶

The fastest, most secure way to send large files — no account needed on the sender's side.

Lenzeye File Transfer is the flagship feature of Lenzeye Technologies. It allows anyone — a client, a vendor, a collaborator — to upload files directly into a registered user's secure storage without needing to create an account or install any software. The receiver (registered user) controls everything: storage limits, encryption, and access.

What Problem Does It Solve?¶

Traditional file transfer in India is fragmented:

WhatsApp compresses photos and videos, destroying quality.
Google Drive / WeTransfer require both sender and receiver to have accounts.
Email attachments are capped at 25 MB.
Physical drives / pen drives are error-prone and not scalable.

Lenzeye File Transfer eliminates all of these problems. A photographer's client can send 2,000+ RAW files in a single session — no account, no app, no compression — directly into the photographer's encrypted cloud storage.

How It Works — User Perspective¶

flowchart TD
    A[Receiver shares upload link] --> B[Sender opens link in browser]
    B --> C[Sender enters email
accepts guidelines]
    C --> D{Encryption enabled?}
    D -- Yes --> E[Files encrypted
then uploaded]
    D -- No --> F[Files uploaded
directly to cloud]
    E --> G[Download link generated]
    F --> G
    G --> H[Sender gets token + OTP]

The receiver (registered Lenzeye user) shares their upload page link.
The sender opens the link, enters their email, and reviews the upload guidelines — no account creation required.
The sender selects files (any type, any size within storage limits).
Files are uploaded in secure chunks directly to the receiver's cloud storage.
A shareable download link with a one-time OTP is generated automatically.
The receiver can access, download, or manage files from their Lenzeye dashboard.

Key Capabilities¶

Capability	Detail
No sender account required	Sender only needs an email address
Large file support	Tested with 2,600+ files in a single session (6h 56min sustained)
Any file type	RAW photos, videos, PDFs, ZIPs — no restrictions
Chunked transfer	Files split into 10 MB parts — resumable, reliable
Server-side encryption	AES-256-CTR encryption per file if receiver has it enabled
Unique folder naming	Duplicate folder names auto-versioned (`_v1`, `_v2`, etc.)
Storage awareness	Checks available storage before upload begins
OTP-protected download link	Every upload generates a secure, shareable link
Live storage stats	Sender sees real-time Wasabi storage usage on the upload page

Upload Flow — Technical Overview¶

Phase 1 — Pre-Upload Checks¶

Before a single byte is transferred, Lenzeye performs three checks:

Guidelines check — Has the sender reviewed and accepted the current upload guidelines? The guidelines are hashed (SHA-256) and the accepted version stored per user — if guidelines are updated, re-acceptance is required.
Storage check — Does the receiver have enough available storage for the selected files? The check is done in GB and blocks upload if insufficient space exists.
Folder name check — If a folder with the same name already exists in the receiver's storage, Lenzeye automatically assigns a unique name (folder_v1, folder_v2, etc.) to prevent collisions.

Phase 2 — Multipart Upload¶

Lenzeye uses S3 Multipart Upload (via Wasabi S3-compatible API):

Each file is split into 10 MB chunks (parts).
Each part is uploaded independently via a presigned URL — a time-limited, cryptographically signed URL that allows the browser to upload directly to cloud storage.
Parts are assembled on the cloud side — the Lenzeye server never buffers the full file in memory.
If a part fails, only that part is retried — not the entire file.

Phase 3 — Encryption (When Enabled)¶

If the receiver has server-side encryption enabled, the upload path changes:

flowchart LR
    A[Browser sends
raw chunk] --> B[Lenzeye Server]
    B --> C[AES-256-CTR
encrypt chunk]
    C --> D[Upload encrypted
chunk to Wasabi S3]
    D --> E[HMAC accumulated
per chunk]
    E --> F[HMAC stored in
S3 metadata]

Each chunk is encrypted on the server before reaching cloud storage.
The encryption uses AES-256-CTR mode — a seekable stream cipher that allows each chunk to be encrypted at its correct byte offset independently.
An HMAC-SHA256 integrity tag is accumulated chunk-by-chunk during upload and stored in S3 metadata on completion — no re-download needed to compute it.
The server processes at most 4 concurrent encrypt+upload operations at any time, keeping peak memory usage bounded (≈80 MB for encryption buffers).

Phase 4 — Link Generation¶

After all files are uploaded:

A GuestDownloadLink record is created in the database with a unique cryptographic token.
A one-time OTP is generated and bound to the link.
The shareable URL format: /guest/transfer-manager?token=<token>&otp=<otp>
The sender receives this link immediately after upload completes.

Storage Layout on Cloud¶

Every file uploaded via Lenzeye File Transfer is stored at:

<receiver_email>/<folder_name>/<filename>

Example: studio@example.com/Wedding_Album_v1/DSC_0001.RAW studio@example.com/Wedding_Album_v1/DSC_0002.RAW

This keeps each receiver's files completely isolated by email namespace.

Encryption Details¶

Property	Value
Algorithm	AES-256-CTR (Counter Mode)
Key size	256-bit (32 bytes) per user
IV size	128-bit (16 bytes), randomly generated per file
Integrity	HMAC-SHA256 (Encrypt-then-MAC)
Key wrapping	Per-user key encrypted with AES-256-GCM master key
Storage format	`[16B IV][ciphertext]` + HMAC in S3 metadata
Key derivation	Master key loaded from environment variable, never stored in code

Why AES-256-CTR?¶

CTR mode converts AES (a block cipher) into a stream cipher. This makes it seekable — any byte position in the encrypted stream can be computed independently without decrypting from the beginning. This property is essential for multipart uploads: each 10 MB chunk can be encrypted at its correct byte offset without knowing the previous chunk's state.

Performance & Reliability¶

Validated: 2,630+ files transferred in a single session over 6 hours 56 minutes with zero crashes or data loss.
RAM efficiency: Peak server RAM usage during sustained large uploads stayed within 243–398 MB on a 512 MB server.
Concurrency: Up to 4 simultaneous encrypt+upload streams per server instance, queuing additional requests without rejection.
Resumability: Multipart upload sessions can be validated and resumed — stale sessions are detected and aborted cleanly.

Security Properties¶

Property	How Lenzeye Achieves It
Data confidentiality	AES-256-CTR encryption before data reaches cloud storage
Data integrity	HMAC-SHA256 verified on every download
Access control	OTP-protected download links, token-based session validation
Sender verification	Email + guidelines acceptance before any upload begins
Storage isolation	Each user's files stored under their own email namespace
No credential exposure	Presigned URLs expire — no permanent S3 credentials shared with browser
Tamper detection	HMAC verification raises `InvalidSignature` on any modification

Comparison With Other Services¶

Feature	Lenzeye File Transfer	WeTransfer	Google Drive	WhatsApp
No account for sender	✅	✅	❌	❌
End-to-end encryption	✅	❌	❌	✅ (messaging only)
File size limit	Storage plan based	2 GB (free)	15 GB total	2 GB
Quality preservation	✅ No compression	✅	✅	❌ Compressed
Stored in receiver's account	✅	❌	❌	❌
Indian data compliance focus	✅	❌	❌	❌
Bulk file transfer (1000+)	✅ Validated	Limited	Limited	❌

Current Status¶

Lenzeye File Transfer is live and validated in production. It has been tested with real users across sustained large-volume upload sessions. The feature is stable, production-ready, and forms the core of the Lenzeye platform.

Upcoming Improvements¶

Email notification to receiver when a guest upload completes
Upload progress tracking across browser refresh (session persistence)
Android app support for guest uploads
Configurable per-link expiry for download links