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

by unprovable • 2 months ago

168 65 2 months ago

unprovable7• 2 months ago

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.

pajamasam• 2 months ago

Interesting idea! A demo video would be great :)

HanClinto• 2 months ago

If the phone had broken comms, how did you get the code onto the phone to run?

Chant-I-CRW• 2 months ago

In ye olden days we used cables to sync all of our apps and data to our phones.

rmunn• 2 months ago

Not OP, but I'm guessing by running the code on itself, i.e. turning the code into a QR code (or a series of them), then scanning those QR codes on the phone and reassembling them using a text-editing app on the phone.

unprovable7• 2 months ago

With great difficulty...

harkaniemi2• 2 months ago

This is wild. I made an identical program 24 hours ago. Mine just transfers text files

harkaniemi2• 2 months ago

Okei, I tested yours and it works. I would change one thing: You could start receiving chunks at any point. I had total chunk amount and chunk order in every chunk, so you can start receiving at any point.

lukew3• 2 months ago

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

hootz3• 2 months ago

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.

xnx• 2 months ago

a convoluted mess of audio frequency modulation

Like a modem

hootz3• 2 months ago

I guess lmao, but much more rudimentary, less reliable and with loads of issues, as it had to blast piercing sounds through a speaker and then capture those with a microphone. But it was pretty cool when it worked!

villgax• 2 months ago

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

hoansdz• 2 months ago

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?

bensyverson• 2 months ago

I've done a POC with the native QR reading code on iOS. The short answer is: it's not a problem at all, and you can drive very large QR codes for more efficient transfer.

rirze• 2 months ago

You could also do it the menial way; create multiple QR codes that can connect together. Not very practical for everyone except the very-motivated.

0cf8612b2e1e• 2 months ago

Why not record with a standard high quality webcam and do the QR processing later? That’s how I would exfiltrate a large volume of data.

anhldbk• 2 months ago

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

tripflag• 2 months ago

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.

MattCruikshank4• 2 months ago

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.

skinfaxi3• 2 months ago

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.

Why do you need a separate device for this and not just an airgapped computer?

MattCruikshank4• 2 months ago

Me, in my life, I have a PC that's connected to the internet. I have a phone that's connected to the internet.

I want another device, which I imagine to be a Pi or Esp32 or something with a camera and a touchscreen display, and capacitive charging. After I program it and give it the public/private keypair and the OTP, I imagine physically breaking off a USB port, or sealing one with some hardening resin.

I don't want an entire airgapped computer. Maybe you do, that's fine. For me, I'd love it to be a credit-card sized doodad.

reaperducer3• 2 months ago

Congratulations. You just invented IrDA: https://en.wikipedia.org/wiki/IrDA

MattCruikshank4• 2 months ago

I specifically want this device to have no input or output hardware that could be used without my knowledge. IrDA could absolutely be used without my knowledge.

unprovable7• 2 months ago

So two parts to a reply - first is, you don't need the encryption per se, but you can add that in the case that you give it some key and then it's encrypted. I don't see the value unless you're using this to generate frames for a video, which isn't current functionality but totally doable.

Second part, Charlie Bennet said "the only entropy source is one you can trust" and the best entropy source is quantum fluctuations, so we built a fully open source phase diffusion QRNG at Quantum Village and released it. Link: https://github.com/QuantumVillage/EntropyLoop

alex_suzuki• 2 months ago

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

unprovable7• 2 months ago

Cheers!

thedougd3• 2 months ago

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.

skinfaxi3• 2 months ago

If you can connect to Github pages couldn't you exfil that way? This takes 2 mins for 100KB.

thedougd3• 2 months ago

Not quietly. Uploads are commonly monitored by data loss prevention (DLP) solutions, especially when MITM is being used for corporate proxy.

Downloading a tiny JS from a CDN, or accessing a GitHub page is mostly noise, especially if obfuscated well.

skeptic_ai• 2 months ago

Npm install qr-made-up-name Can show qr in console. How do you stop that?

thedougd3• 2 months ago

I'm likely being overly specific, but blocking npm downloads, installation on corporate devices, etc is trivial in a restrictive corporate environment.

divan• 2 months ago

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.

acrophiliac• 2 months ago

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

unprovable7• 2 months ago

QR code has limits, use the file transfer to chunk it. :)

Levitating2• 2 months ago

https://gist.github.com/SMUsamaShah/fd6e275e44009b72f64d0570...

aliirz• 2 months ago

very nice list! Can I suggest adding my https://phntm.sh as well? itis a zero-knowledge file transmission system.

Levitating2• 2 months ago

I am not the author, you could leave a comment on the gist instead.

Aleesha_hacker• 2 months ago

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

jaysyrk• 2 months ago

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

encom• 2 months ago

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.

kig4• 2 months ago

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.)

cl3misch• 2 months ago

That sounds amazing!

The WebRTC "fallback" basically means the QR code is just a handshake when both devices are on the same network?

kig4• 2 months ago

The handshake details are passed alongside the data so that if a direct connection is successful, the network races the optical transfer.

dschep2• 2 months ago

This looks like a nice polished implementation of the idea, but when I try and use it, I get to "file complete" but then.. nothing? And I see no way to report a problem or bug.

kig4• 2 months ago

Thanks for trying it, sounds like a bug I've been running into for a couple days where the wasm stream decoder does a bad memory access. I made some buffer lifetime fixes, next up a more complete stress test and fuzzing to see if I can pinpoint it.

rao-v2• 2 months ago

What is the fastest you can transfer data from ~10 meters away using a modern phone front camera and screen? Surely 100 kB/s is slow?

kig4• 2 months ago

Depends on the zoom. With this setup you can transfer about 0.1 B/s per pixel of 60 FPS video. So a 65" screen and 1080p camera at 10 meters away would max out at 2 kB/s with the normal lens (26mm equiv) or 45 kB/s with the tele lens (120mm equiv.)

I'm cooking something faster but depends on the job situation and funding whether I have time to spend on it.

Napkin math: QR codes encode 0.75 bits per module, each module needs about 3 pixels of camera resolution, and the temporal resolution is quite dodgy as well, maybe 0.25 * min(cameraHz, screenHz). So if everything is perfect, 44 kB/s at 60Hz per a 500x500 pixel patch. I've seen ~250 kB/s when a 1920x1080@60 transfer is working well. At 4k@30, you might reach 0.5 MB/s. If you throw in the 2x subsampled UV channels to transfer data as well, you might get an extra 50%.

m_m_carvalho• 2 months ago

Clever use of QR for file transfer. How does it handle larger files? Is there chunking + reassembly on the receiving end?

econ• 2 months ago

Finally real one way data transfer.

okandship• 2 months ago

animated qr + erasure coding is a neat fit here. the interesting ux question is how clearly the receiver shows progress and recovery

unprovable7• 2 months ago

There is a toggle where you can show what chunks have been received. This is also where the 'show specific chunk' function comes in... the receiver can see "oh, I'm only missing chunk 125, so just show me that" etc. etc.

davidcollantes• 2 months ago

Is there a client that needs to be used to be able to receive files this way? I tried the dedicated QR reader on my iPhone, and the Camera app, and couldn’t get it to assemble the chunks. It will show the encodings one after the other, but nothing else.

teravor• 2 months ago

i feel like it should be possible to push this to extremely high bandwidth.

use a 1D code variant with very high FPS to work around the rolling shutter.

msafi04• 2 months ago

I love the focus on giving a second life to old hardware. We often talk about 'local-first' for privacy, but this shows how local-first is also great for hardware longevity. It’s refreshing to see a tool that doesn't require a cloud connection or a functional Bluetooth stack to be useful.

unprovable7• 2 months ago

Update - put it on a site for folk to use: https://shadowcat.online/

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

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