Show HN: ShadowCat – file transfer through QR Codes in a Browser

This is cool and minimalistic!

I've been noodling on https://qr-send.com which is a slightly more polished version of the "erasure fountain codes + stream of QRs"-idea, inspired by divan's Txqr posts but using Wirehair FEC for the fountain code (basically: you receive ~file size bytes via QR codes and it magically assembles them into the source file regardless of missed codes).

It's an offline-first progressive web app and there are native & wasm builds for the sender. The browser-to-browser transfer falls up to WebRTC when possible because 30 MB/s over wifi beats a 100 kB/s QR stream. The QR scanner is a heavily-optimized WASM build of zbar, scanning at 60 fps on mobile & multiple QRs per frame (but it's finicky! Work in progress.)

This method of animated QR data transfer is quite efficient with fountain codes. I had PoC implementation back in the day - Txqr [1] [2]

[1] https://divan.dev/posts/animatedqr/

[2] https://divan.dev/posts/fountaincodes/

Recently I rewrote it in Dart/Flutter and finally implemented RaptorQ codes (way more efficient than Luby used in original Txqr). Testing it internally now, prepareing Appstores/GooglePlay/Web deployment and new article.

Single page file transfer using QR Codes and a browser. Sending device loads a file into the page, gets chunked. Receiver gets all the chunks through a camera, tosses lightly and reassembles, CRC to garnish. Designed to push data from an old phone that had broken comms after it took a swimming lesson in a coffee mug, it's been quite handy.

You should turn on github pages so we can see it live. Seems cool but I’m not at my pc rn

I also implemented a static web with that idea: https://github.com/anhldbk/get-beam

I've wanted to use this for an air-gapped communication device.

I have a device with a camera and a touch-screen that only uses capacitive charging. I type a message. Bytes are encrypted. I hit send. QR codes flash on my screen. I use my PC or my normal phone to receive the encrypted bytes, and transmit them to you. You have the same device. You have your PC or phone flash encrypted QR codes. You use your device to receive, and then decrypt.

I've daydreamed about also buying several different hardware random noise generators. XOR all of their bits together. Save a huge one time pad to each of our devices. And then also use public key crypto on top of it.

I'm not really sure why I want this. But, it's my answer for how to reduce attack surface as much as possible, and have truly secret messages.

I love this type of stuff. Some years ago I did something similar, but instead of QR Codes it used a convoluted mess of audio frequency modulation to send data through sound between devices. This is much more practical if you have two cameras.

Cool! Out of curiosity, since qr-codes can contain binary data -- rather than base64, have you tried inserting the file as-is? That way you could do away with the ASCII separator and have a binary header as well. This would spend less frames for the same amount of data, but I'm not sure if it would be computationally cheaper. The other alternative would be the alphanumeric mode of qr-codes, but then you lose lowercase.

I once heard someone create a QR code scanner to retrieve gigabytes of data, but the biggest problem is that cameras aren't powerful enough to handle it all. Essentially, the QR code needs to be downloaded to the device for loading; relying on the camera to retrieve it is very difficult. Am I wrong about this project? What's your solution?

I've done this exact approach before. It's a good way to exfiltrate data. Post the software on GitHub pages, or a popular CDN that co-hosts other shared libraries and you've got a very difficult to block method.

Really goes to show that it's very difficult to stop a motivated and informed actor.

What's the length limit? I tried pasting some text and got this message: code length overflow. (85700>18672)

I created a file optimizer, one single file. I was wondering if i could work with you to integrate that into your project. Lmk!!

Cool stuff. I’m fond of the “single HTML file” deployment option.

Let Ai help you research not write keeps the content human and original

We used to be able to send arbitrary files between phones using Bluetooth. Where did that go? We had a bit of a music piracy ring going at school for a time. Good times.

What would make this truly portable is being able to generate this consistently with a short prompt and generate with a local LLM. That way no network calls or file hash can prevent this