PairOnce

SECURITY · ENCRYPTED TRANSFER

End-to-end encrypted file transfer in your browser, with no account

PairOnce encrypts everything on the sending device and decrypts it on the receiving one: AES-256-GCM, with keys agreed between the two devices (P-256 ECDH). There is no account and no app.

When you pair, both screens show the same 13-digit safety code. If they match, nobody sits between the two devices.

The file goes directly between the two browsers; if a network blocks that, an encrypted relay passes it on without being able to read it. PairOnce keeps no copy.

Step by step

  1. Open PairOnce on both devices. paironce.com in any current browser on a phone or a computer.
  2. Connect. On the same Wi-Fi, the other device appears under On this Wi-Fi: tap Connect. On any other network, scan the QR code or type the six-digit code. A page opened from a QR code or link asks you to confirm first: tap Connect.
  3. Compare the safety code. Both screens show the same 13-digit code. If they match, click Approve device; if they differ, stop and start again.
  4. Send. Tap Choose files or Message or link; the other side accepts each file and saves it.

What is encrypted, and who can read it

File contents, file names, text and links are encrypted on one device and decrypted on the other. A network observer sees only encrypted traffic, and a relay that forwards the data sees IP addresses and how many bytes pass, not the contents.

The PairOnce service introduces the two devices. It sees IP addresses, when devices connect and device names, and for relayed transfers their size; it holds no key that opens a file, a name or a message.

The safety code

With a QR code or an invitation link, the host’s public key travels in the part of the link that browsers never send to a server, so the joining device already knows it is talking to the right host. With a six-digit code, the safety code is what guards the pairing: it is computed from the keys each device holds, so two screens that show the same code mean the same two keys. Compare it every time you pair a new device.

After the first pairing each device remembers the other’s key and refuses a different one, so later connections need no new code.

Open and checkable

How PairOnce pairs devices and moves files, and what the services in between can and cannot see, is published as the PairOnce protocol and threat model, under a CC BY 4.0 licence. The reference implementation, @paironce/core, is on GitHub, and the command-line tool is open source (Apache-2.0).

Limits, and when this is not the right tool

  • A file can be up to 2 GB over a direct connection when the receiving browser saves straight to disk (Chrome or Edge on a computer or Android, Firefox on a computer, Safari on iOS 26 or later). Other browsers take up to 50 MB per file and hold 100 MB of received files at a time.
  • Files over 2 GB need PairOnce Pro on one of the two devices. Then a file can be up to 10 GB, received the same way, and the relay carries 5 GB a month.
  • When the two devices cannot reach each other directly, an encrypted relay carries the data. Each device can send and receive 200 MB a month that way, counted together; a direct connection has no monthly limit.
  • Both pages must stay open until everything has arrived: keep the phone’s screen on and the browser in front.
  • PairOnce moves things between two devices that are open at the same time. It is not cloud storage: nothing waits on a server for later.

Other ways to do it

  • A link-based encrypted service. Fits when the receiver is offline: the file waits on a server, encrypted, until they download it.
  • PairDrop. Free and open source in the browser; finds devices on the same network and pairs devices across networks. See the Snapdrop and PairDrop comparison.
  • LocalSend. A free open-source app that sends files over the local network with HTTPS, no internet needed. See the LocalSend comparison.
  • AirDrop. Built in, between Apple devices.

Common questions

Which encryption does PairOnce use?

AES-256-GCM for the files, text and control messages, with keys derived from a P-256 ECDH exchange between the two devices. The direct connection adds its own DTLS layer on top.

Can PairOnce read my files?

No. Contents, names and text are encrypted on your device before they leave it, and the service and the relay do not have the keys.

Do I need an account?

No. Sending files and messages is free, with no sign-up and no app.

Where is the file stored?

Nowhere on a server. It goes from one browser to the other, and waits in the receiving page until you save it.