OwlTransfer

A folder that is the same on your computer and your phone. Drop a file in on one and it is on the other within a second, over your own Wi-Fi, with no account and no server.

Download for Android Download for Linux Download for Windows

The AppImage runs anywhere: chmod +x and go. A .deb for Debian and Ubuntu and an .msi for anyone who deploys with one are on the releases page. The Linux builds are made on Ubuntu 22.04 so they run on older systems too. The Windows installer is not code-signed, so SmartScreen asks once.

The OwlTransfer desktop app: a sidebar with the sync folder and two paired devices each showing a status dot, a file list in the middle with a sync badge beside every name and one file part way through, and a status strip along the bottom. The OwlTransfer desktop app: a sidebar with the sync folder and two paired devices each showing a status dot, a file list in the middle with a sync badge beside every name and one file part way through, and a status strip along the bottom.
The same folder on a phone: a header with the owl mark and the OwlTransfer wordmark beside buttons for adding files and making a folder, a file list with a green sync badge and a size beside every name, a status strip along the bottom reading Up to date, and tabs for Files, Devices and Settings under it. The same folder on a phone: a header with the owl mark and the OwlTransfer wordmark beside buttons for adding files and making a folder, a file list with a green sync badge and a size beside every name, a status strip along the bottom reading Up to date, and tabs for Files, Devices and Settings under it.

How it works

Two devices, one network, nothing in between

Nothing leaves your network.
The two devices find each other with a broadcast on your own Wi-Fi and then talk directly. Files never reach a relay, a cloud account or anyone's server, because there is no server anywhere in this. The app is the whole system.
The connection is TLS 1.3, pinned on both ends.
Each device generates its own certificate the first time it runs and keeps the private key. A paired device is a certificate fingerprint you have already approved, so only a device you paired with gets a session. Anything else is allowed to ask to pair and to do nothing else.
Both devices keep the whole folder.
This is a copy on each device, not a window onto storage somewhere else. Open a file on the plane with the phone in flight mode. Changes made while the two were apart are exchanged the next time they see each other, and an edit on both sides keeps both, one renamed rather than thrown away.

Pairing

Six digits, once, with both devices in front of you

Open Devices on the phone, pick the computer from the list, and both screens show the same six-digit number. Check that they match and press Accept. That number is derived from the two certificates and a random value from each device, and neither side can pick its value after seeing the other's, so a device in the middle cannot work out a pair of values that makes two different handshakes show the same code.

After that the two remember each other and reconnect on their own whenever they are on the same network. Forget a device on the Devices screen and its certificate stops being trusted at once, and the next connection from it is told it is not paired, so it forgets you back instead of redialling. If the network blocks broadcasts, or you are pairing against an emulator, type the other device's address instead: one side dialling is enough, and the connection carries the folder in both directions.

What it costs

Nothing, and there is nowhere for a bill to come from

There is no hosted service to pay for, no account to create, and no storage quota, because your files only ever sit on hardware you already own. The only third party involved is GitHub, which builds the releases and hosts the downloads above, and that is free and unmetered on a public repository.

It is MIT licensed and the source is public, so you can read what it does with your files and build it yourself. The architecture document describes the wire protocol, the merge rules and the threat model in full, for anyone who wants to audit it before trusting it with a folder.