This File Transfer Uses No Internet

How do you send a file without a network, a cable, or even a USB stick? Evan Crowley’s Decimen, a groundbreaking browser-native tool, provides an elegant answer. It enables optical file transfer between devices using nothing more than a screen and a camera, completely circumventing the traditional network stack or physical storage. This means no Wi-Fi, no Bluetooth, and no Internet.

Decimen’s most compelling application lies in bridging the air gap for secure systems. Environments physically isolated from external networks, like critical infrastructure or classified research facilities, rely on this separation for security. Yet, transferring data to or from them remains a challenge, as conventional methods like USB drives pose significant risks, famously exploited by malware like Stuxnet. Decimen offers a crucial, isolated pathway.

The mechanism is deceptively simple: one device displays a rapid, continuous stream of visually encoded data, similar to high-density QR codes. These frames, potentially leveraging structures like QR code version 40 to pack 2,953 bytes each, flash across the screen. Another device, pointing its camera, records and decodes these optical signals, meticulously reconstructing the original file without any electronic connection.

Optical file transfer faces a fundamental technical hurdle: synchronizing a screen's refresh rate with a camera's capture rate. Screens update at speeds like 60 Hz, but cameras might capture at 30 frames per second, creating a mismatch. This desynchronization inevitably leads to missed frames or, worse, exposure windows that straddle two different QR codes, blending them into undecipherable data. This results in corrupted frames, rendering traditional sequential transfers unreliable.

Decimen overcomes this with an ingenious solution: fountain coding. Instead of transmitting discrete, sequential data chunks—like expecting frame one, then frame two, then frame three to arrive intact—it generates an effectively unlimited stream of mathematically blended frames. Each frame isn't a specific, irreplaceable segment; rather, it combines fragments of the original file's entire content. This technique, often leveraging Luby Transform (LT) codes, fundamentally changes how data integrity is approached.

This method has a profound impact: no single frame is essential for file reconstruction. The receiver simply collects frames until it has 'enough' information, regardless of which specific frames arrive or their order. Missed, duplicated, or out-of-order frames don't corrupt the file; they merely extend the collection time. This makes the process incredibly robust against errors, with the receiver typically needing about 115% of the minimum required data to reconstruct the original file.

Optical file transfer hinges on a tripartite balancing act: the data density encoded in each frame, the sender's frame rate, and the receiving camera's resolution and focus. Decimen, by default, optimizes for close-range phone-to-phone transfers, pushing 2,953 bytes per frame (QR version 40) at 60 frames per second. This yields a respectable 128 kilobytes per second (KB/s) in ideal conditions.

However, applying these defaults to a laptop-to-phone setup at arm's length reveals significant limitations. The video's test achieved a mere 3 KB/s, decoding only 1-2% of transmitted frames. Multiple factors conspired against it.

A primary culprit is frame rate mismatch: the laptop sent 60 FPS, but the phone camera captured at 30 FPS, leading to blended frames where the exposure window straddled two different QR codes. Furthermore, LCD ghosting on the sender's screen blurred transitions, and the high data density of QR version 40 (requiring 600+ pixels across) proved too challenging for a distant phone camera.

The solution proves counterintuitive. To dramatically increase actual throughput, reconfigure the sender. Drop the data density to 1,465 bytes per frame (a coarser QR version) and the transmit FPS to 24. This deliberate reduction ensures the receiver's camera captures clean, distinct frames, boosting decode rates.

This highlights the necessity of fine-tuning Decimen for specific hardware and use cases. For more on this innovative approach to data transfer, visit Decimen Optical Transfer — transfer files with light.

Decimen's true strength lies in its perfect network isolation, establishing a genuine air-gap between devices. This security model, however, presents a critical trade-off: the tool offers zero built-in encryption. Visual eavesdropping remains a real threat; any observer with a camera can capture and potentially decode the data stream. Users handling sensitive files must encrypt them externally before initiating a Decimen transfer.

Beyond high-security air-gapped networks, Decimen unlocks solutions for numerous niche challenges. Imagine transferring diagnostic logs to legacy hardware lacking network ports, or updating embedded systems in isolated environments. It serves as an invaluable bridge in ad-hoc scenarios where a screen and camera represent the only common interfaces between disparate devices.

Decimen doesn't aim to replace ubiquitous wireless protocols like Wi-Fi. Instead, Evan Crowley's clever engineering offers a powerful, elegant solution for specific, often intractable data transfer problems. This browser-native tool expands the possibilities for secure, off-grid data exchange, proving the ingenuity of optical communication for specialized tasks.

What is Decimen?

Decimen is an open-source, browser-based tool for transferring files between devices using a stream of flashing QR codes, requiring no internet, Wi-Fi, or physical connection.

How does fountain coding help Decimen work?

It encodes the file into an endless stream of redundant, blended data frames. This allows the receiver to reconstruct the file even if many frames are missed, blurred, or captured out of order, making the transfer highly resilient.

Is transferring files with Decimen secure?

It provides perfect network isolation, which is highly secure against network-based attacks. However, the data itself is not encrypted, so anyone with a camera and a direct line-of-sight to the screen can intercept the transfer.

What is the maximum file size Decimen supports?

Decimen currently supports individual files up to 64 MB and pasted text snippets up to 4 MB.