The mainstream cloud sync model is a hub. Every device uploads to the provider's servers, and every other device downloads from them. The provider sits at the center, sees the file names and usually the contents, holds the keys for any encryption it applies, and can be compelled to hand over what it holds. You get reliability and easy sharing in exchange for the provider being a permanent third party to your files.
Syncthing is built on the opposite shape. It is open-source, it has been in active development since 2013, and it synchronizes folders directly between devices you own or trust. There is no Syncthing account. There is no Syncthing server storing your files. When two of your machines sync, the bytes move from one to the other, encrypted end to end, without a copy resting anywhere in between.
Device identity as a cryptographic key
The foundation of Syncthing is how devices identify each other. When you install it, it generates a TLS certificate, and your device ID is derived directly from that certificate's public key. The ID is a long string you can read off and share. Two devices will only sync once each has been told the other's device ID, which you exchange out of band: you copy it from one machine and paste it into the other, or scan a QR code.
This is a deliberate design. Because the device ID is bound to the key, adding a device by its ID means you are pinning that exact key. An impostor cannot present a different key under the same ID, since the ID is a fingerprint of the key. It is the same trust model described in trust-on-first-use verification: the first exchange establishes the identity, and every connection afterward is checked against it.
There is nothing to log into. Your identity in Syncthing is a keypair on each device, not a username and password stored on a server. That removes an entire category of risk. There is no central credential database to breach, no account to phish, and no provider that can lock you out or be served with a request for your data.
How the sync happens
Devices exchange data using the Block Exchange Protocol, Syncthing's own protocol, which runs over TLS 1.3. Every connection is mutually authenticated, meaning both ends verify each other's certificate against the expected device ID before any data flows, and the channel carries forward secrecy so a key compromised later cannot decrypt traffic captured earlier.
Files are broken into blocks, and only changed blocks are transferred, so editing one file in a large folder does not resend the whole folder. Each device advertises an index of what it has, the devices compare notes, and they pull the pieces they are missing from whichever peer has them. Conflicts, when two devices edit the same file, are handled by keeping both versions with a marked conflict copy rather than silently overwriting.
Finding each other across the internet
Two devices on the same local network find each other by broadcast, no internet required. Across the internet, Syncthing uses global discovery servers, which map a device ID to its current network address so peers can locate each other after IP addresses change. When a firewall or NAT prevents a direct connection, traffic can be routed through a relay.
The important detail is what these helpers can see. A discovery server learns that a device ID is reachable at an address. A relay forwards encrypted bytes between two devices that cannot connect directly. Neither can read your files, because the TLS session is between your two devices and terminates only on them. The relay is a dumb pipe passing ciphertext, in the same spirit as the relay separation used in privacy-preserving network designs.
The discovery server knows a device exists and where to reach it. The relay moves encrypted bytes it cannot read. The plaintext of your files never exists anywhere except on the devices you chose to sync. The trust boundary of the Syncthing infrastructure
The untrusted device option
A newer feature widens what Syncthing can do. Normally every device in a sync group can read the files, which is correct when they are all your machines. But sometimes you want a device to hold a copy for backup or availability without being able to read it, say a spare box at a friend's house or a cheap always-on server. Syncthing supports this with password-based encryption for untrusted devices: that device stores only encrypted blocks and never has the password, so it functions as a resilient off-site copy that is useless to anyone who seizes it. This is closely related to the broader topic of end-to-end encrypted cloud storage, arrived at from the peer-to-peer direction.
Syncthing next to cloud sync
| Property | Syncthing | Mainstream cloud sync |
|---|---|---|
| Provider holds your files | No | Yes |
| Account required | No | Yes |
| Works with a device powered off | No, peers must overlap | Yes, server is always on |
| Storage limit | Your own disks | Plan quota |
| Open source and auditable | Yes | Varies |
The honest weakness sits in that third row. Because there is no always-on server holding a master copy, two of your devices have to be online at the same time for changes to propagate. Sync a laptop and a phone that are never awake together and nothing moves until they overlap. Many people solve this by keeping one always-on machine in the sync group, which functions as a de facto hub without any provider ever seeing plaintext. It is a real trade-off, and whether it matters depends entirely on your devices' habits.
Where this fits in a private life
Syncthing will not replace every use of cloud storage, and it is not trying to. What it does is remove the provider from the cases where the provider was never adding value beyond a copy of your data on their disks. For documents, photos, notes, and backups that move between machines you already own, keeping the plaintext on your own hardware and letting the devices talk to each other directly is a cleaner arrangement than routing everything through a company's servers.
It reflects the same principle that guides how we build Haven: the fewer parties who can read your data, the smaller the set of things that have to go right for your privacy to hold. In messaging we pursue that with end-to-end encryption so that not even the service can read your messages. Syncthing pursues it for files by removing the service altogether. Different layer, same idea, and both belong in a serious approach to keeping your own information your own. If you are assembling a broader setup, our privacy stack guide puts tools like this in context.