Untrusted (Encrypted) Devices

It is possible to set a password on a folder when it’s shared with another device. Data sent will be encrypted by this password, and data received will be decrypted by the same password.

As an example, lets assume a trusted device T1, maybe your laptop. You have sensitive documents here but they are in cleartext from Syncthing’s point of view (perhaps protected by full disk encryption). There is also an untrusted device U1, perhaps a cloud server, where we want to sync data but in unreadable form.

We set a folder password on T1 when sharing the folder with U1. Data on disk on T1 is not affected, but data sent to U1 becomes encrypted – garbage, if you don’t know the password.

digraph g { rankdir=LR "T1" [label="T1\n(Clear text)", style=filled, color="/accent3/1"] "U1" [label="U1\n(Encrypted)", style=filled, color="/accent3/2"] T1 -> U1 [label="Encrypted by T1"] U1 -> T1 [label="Decrypted by T1"] }

From this setup it’s also possible to add further trusted devices, say T2, and have these sync the data from the untrusted device U1 without being in contact with T1. Using the same folder password on T2 makes the existing data on U2 intelligeble and the plaintext data becomes available.

digraph g { rankdir=LR "T1" [style=filled, color="/accent3/1"] "U1" [style=filled, color="/accent3/2"] "T2" [style=filled, color="/accent3/1"] T1 -> U1 [label="Encrypted by T1"] U1 -> T1 [label="Decrypted by T1"] U1 -> T2 [label="Decrypted by T2"] T2 -> U1 [label="Encrypted by T2"] }

Similarly, it’s fine to add “normal mode” synchronization between untrusted devices.

digraph g { rankdir=LR "T1" [style=filled, color="/accent3/1"] "U1" [style=filled, color="/accent3/2"] "T2" [style=filled, color="/accent3/1"] "U2" [style=filled, color="/accent3/2"] T1 -> U1 [label="Encrypted by T1"] U1 -> T1 [label="Decrypted by T1"] T2 -> U2 [label="Encrypted by T2"] U2 -> T2 [label="Decrypted by T2"] U1 -> U2 [dir="both"]; subgraph U { rank="same" U1 U2 } }





This is the configuration on a trusted device. Here the folder default is shared with three devices. The device 373HSRP is a traditional trusted peer. The other two devices CJBIJBJ and I6KAH76 are both untrusted and will get encrypted folder data, using different passwords.

<folder id="default" ...>
    <device id="373HSRP-..."></device>
    <device id="CJBIJBJ-..." encryptionPassword="foo"></device>
    <device id="I6KAH76-..." encryptionPassword="bar"></device>

There is no specific configuration required on the untrusted devices; they will simply accept the encrypted data as is.


The following things are encrypted / hidden / protected on untrusted devices:

  • File data

  • File metadata, including name, times and hashes

  • File structure (your directory structure is not replicated, even in encrypted-name form)

The following things are not protected:

  • Folder ID and label

  • File sizes (files grow a little on the encrypted side compared to the plaintext original, but it’s still easy to derive the original size from the encrypted file)

Encryption is AES-256-GCM with a key derived from the password and folder ID using scrypt.

Untrusted Side File Structure

The directory structure on the untrusted side doesn’t follow that of the plaintext hierarchy. Instead file names are encrypted as a whole, with their full path within the folder, and then split into a logical structure. As an example, the name foo.txt (in the folder root) might encrypt to S21K3P1VJO08DEQJ1DQJE0DLOMT068JJFD857L8ODM2TAKI3CC. On disk this gets split into a top level directory with a file extension, a second level directory, and the rest:

graph { "folder" [shape=folder] "S.syncthing-enc" [shape=folder] "21" [shape=folder] "K3P1VJO0..." [shape=file] "folder" -- "S.syncthing-enc" "S.syncthing-enc" -- "21" "21" -- "K3P1VJO0..." }

The upper level directories serve to combine files, avoiding issues that might arise with having too many files in a single directory.

Similarly, a file with the name Documents/Project/My project.docx might encrypt to IKFEDO9653D8ON1L776EUI286CPD1C.... With the same system as above this file gets placed as:

graph { "folder" [shape=folder] "I.syncthing-enc" [shape=folder] "KF" [shape=folder] "EDO9653D..." [shape=file] "S.syncthing-enc" [shape=folder, color=grey] "21" [shape=folder, color=grey] "K3P1VJO0..." [shape=file, color=grey] "folder" -- "I.syncthing-enc" "I.syncthing-enc" -- "KF" "KF" -- "EDO9653D..." "folder" -- "S.syncthing-enc" [color=grey] "S.syncthing-enc" -- "21" [color=grey] "21" -- "K3P1VJO0..." [color=grey] }

Even longer files get split into deeper directories, and files sharing (encrypted) first characters in the name will end up beside each other – regardless of their original placement in the folder.