Manifest Format¶

The manifest lists all available image versions and their SHA256 hashes. The format is human-readable for convenience.

Structure¶

The manifest starts with a line Tako Manifest 1 that identifies the file as a manifest.

After the header is a blank line. Then follows one image version per line, formatted as the version number, a space, the file size in bytes, a space, and the hexadecimally encoded SHA256 of the image. This makes it easy to use sha256sum as a sanity check. Versions are sorted by version number.

After the image versions is again a blank line, followed by the base64-encoded Ed25519 signature of all of the preceding content (including newlines).

Newlines are a single line feed (U+000A). Version numbers should be ASCII. Hence the entire file is valid ASCII, and also valid UTF-8.

Example¶

Tako Manifest 1

1.0.0 10092569 b101acf3c4870594bb4363090d5ab966c193fb329e2f2db2096708e08c4913e2
1.1.0 11239411 9641a49d02e90cbb6213f202fb632da70cdc59073d42283cfcdc1d786454f17f
2.0.0 11862029 b7b01c6f6772529c66b945e559cb1f46546ef62063e44c1d1068725157ae1cda

fQK92C/tPnH0uqxrTEnU+LEE4jnSpQPbOItph4kGAEfWEmn6wPXiQsSdXlDmoneaJkG6KLvInTvB7FlELoeQFg==

Rationale¶

The manifest format is inspired by the well-established practice of distributing a GPG-signed SHASUMS file.

• We include a version line, so future versions of Tako can still read older manifests.
• The signature is embedded, rather than external, to avoid race conditions when uploading a new manifest to the server. One would still need to upload new images before uploading a new manifest.
• The signature is an Ed25519 signature rather than a GPG signature, because GPG involves a stateful trust store that is difficult to provision in an automated way. In practice people use GPG only for signature verification. For authentication, rather than relying on GPG’s web of trust, people announce the fingerprint of their key in a trusted location (Twitter, Github, an https-protected website). Ed25519 public keys are small enough that the full public key can be announced in places where we would normally announce a fingerprint.
• The manifest does not include timestamps, to ensure that it is reproducible. Timestamps belong in a changelog or audit log.
• Entries should never be removed from the manifest. There are reasons to stop providing an image (for instance because it contained a critical bug that causes data loss). In that case the image itself can be removed from the server, but it should still be listed in the manifest. This prevents accidentally releasing different images under the same version number. It also ensures that clients which did download the image can still identify it, so they do not end up running a mysterous image without record of existence.
• The size of each image is included (and therefore signed) so malicious mirrors cannot cause clients to download large files that would fill up their disks.