Release Signing

Syncthing releases are signed in various ways to enable users and automatic systems to determine that it is in fact a genuine release.

Checksum Files

Two checksum files are created during the release process. These are sha1sum.txt and sha256sum.txt. They contain the SHA1 and SHA256 checksums of the release archives, respectively. To protect against tampering the checksum files are signed by the Syncthing Release Management GPG key and thus gain a .asc extension. To verify that a download is geniuine, first verify the signature on the checksum file is correct, then that the checksum matches the release archive.

Binary Signing

New in version 0.12.0.

In a genuine release archive you expect to find the syncthing binary (syncthing.exe on Windows) and an accompanying signature syncthing.sig (syncthing.exe.sig on Windows). The signature file contains the ECDSA signature of the binary, computed at the time the release was made and signed by the Syncthing Release Management private key. The keys and signature are PEM encoded for ease of transmission - the details of the signature and encoding handling are in the signature package The public key is included in the source code and compiled into Syncthing.

When Syncthing performs an automatic upgrade, it verifies the included signature using the actual binary and the public key. If these match, we know that the binary has not been tampered with and the release is genuine - the upgrade proceeds. If there is a mismatch, Syncthing deletes any temporary files and aborts the upgrade.

Creating and Verifying Binary Signatures Manually

As a developer, you may need to verify and create signatures manually. Syncthing provides a tool to perform these operations in the same manner as the built in verification: stsigtool. To get this tool, ensure that you have Go installed and run:

$ go install github.com/syncthing/syncthing/cmd/stsigtool

After installation you can test it on an arbitrary release (from v0.12.0 and onwards):

$ stsigtool verify syncthing.sig syncthing
correct signature
$ echo >> syncthing  # append a newline to the binary
$ stsigtool verify syncthing.sig syncthing
incorrect signature

To create signatures of your own, you need a private key. The Syncthing private key is a closely guarded secret, but you can generate your own using stsigtool gen. The gen command generates and outputs a new private and public key pair to stdout; you’ll need to paste them into a PEM file each for storage. You can then sign binaries with the private key using stsigtool sign, verify them with the public key using stsigtool verify, and have Syncthing accept these signatures by replacing the compiled in public key. This may be useful in an enterprise setting, for example.