GoToSocial/docs/federation/federating_with_gotosocial/http_signatures.md

HTTP Signatures

GoToSocial requires all GET and POST requests to ActivityPub s2s endpoints to be accompanied by a valid http signature.

GoToSocial will also sign all outgoing GET and POST requests that it makes to other servers.

This behavior is the equivalent of Mastodon's AUTHORIZED_FETCH / "secure mode".

GoToSocial uses the go-fed/httpsig library for signing outgoing requests, and for parsing and validating the signatures of incoming requests. This library strictly follows the Cavage http signature RFC, which is the same RFC used by other implementations like Mastodon, Pixelfed, Akkoma/Pleroma, etc. (This RFC has since been superceded by the httpbis http signature RFC, but this is not yet widely implemented.)

Incoming Requests

GoToSocial request signature validation is implemented in internal/federation.

GoToSocial will attempt to parse the signature using the following algorithms (in order), stopping at the first success:

RSA_SHA256
RSA_SHA512
ED25519

Outgoing Requests

GoToSocial request signing is implemented in internal/transport.

When assembling signatures:

• outgoing GET requests use (request-target) host date
• outgoing POST requests use (request-target) host date digest

GoToSocial uses the RSA_SHA256 algorithm for signing requests, which is in line with other ActivityPub implementations.

Quirks

The keyId used by GoToSocial in the Signature header will look something like the following:

https://example.org/users/example_user/main-key

This is different from most other implementations, which usually use a fragment (#) in the keyId uri. For example, on Mastodon the user's key would instead be found at:

https://example.org/users/example_user#main-key

For Mastodon, the public key of a user is served as part of that user's Actor representation. GoToSocial mimics this behavior when serving the public key of a user, but instead of returning the entire Actor at the main-key endpoint (which may contain sensitive fields), will return only a partial stub of the actor. This looks like the following:

{
  "@context": [
    "https://w3id.org/security/v1",
    "https://www.w3.org/ns/activitystreams"
  ],
  "id": "https://example.org/users/example_user",
  "preferredUsername": "example_user",
  "publicKey": {
    "id": "https://example.org/users/example_user/main-key",
    "owner": "https://example.org/users/example_user",
    "publicKeyPem": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAzGB3yDvMl+8p+ViutVRG\nVDl9FO7ZURYXnwB3TedSfG13jyskoiMDNvsbLoUQM9ajZPB0zxJPZUlB/W3BWHRC\nNFQglE5DkB30GjTClNZoOrx64vLRT5wAEwIOjklKVNk9GJi1hFFxrgj931WtxyML\nBvo+TdEblBcoru6MKAov8IU4JjQj5KUmjnW12Rox8dj/rfGtdaH8uJ14vLgvlrAb\neQbN5Ghaxh9DGTo1337O9a9qOsir8YQqazl8ahzS2gvYleV+ou09RDhS75q9hdF2\nLI+1IvFEQ2ZO2tLk3umUP1ioa+5CWKsWD0GAXbQu9uunAV0VoExP4+/9WYOuP0ei\nKwIDAQAB\n-----END PUBLIC KEY-----\n"
  },
  "type": "Person"
}

Remote servers federating with GoToSocial should extract the public key from the publicKey field. Then, they should use the owner field of the public key to further dereference the full version of the Actor, using a signed GET request.

This behavior was introduced as a way of avoiding having remote servers make unsigned GET requests to the full Actor endpoint. However, this may change in future as it is not compliant and causes issues. Tracked in this issue.