[HTTP Signatures](https://datatracker.ietf.org/doc/html/draft-cavage-http-signatures-08) is a specification for signing HTTP messages by using a `Signature:` header with your HTTP request. Mastodon requires the use of HTTP Signatures in order to validate that any activity received was authored by the actor generating it. When secure mode is enabled, all GET requests require HTTP signatures as well.
The `keyId` should correspond to the actor and the key being used to generate the `signature`, whose value is equal to all parameters in `headers` concatenated together and signed by the key, then Base64-encoded. See [ActivityPub > Public key]({{< relref "activitypub#publicKey" >}}) for more information on actor keys. An example key looks like this:
"publicKeyPem": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAvXc4vkECU2/CeuSo1wtn\nFoim94Ne1jBMYxTZ9wm2YTdJq1oiZKif06I2fOqDzY/4q/S9uccrE9Bkajv1dnkO\nVm31QjWlhVpSKynVxEWjVBO5Ienue8gND0xvHIuXf87o61poqjEoepvsQFElA5ym\novljWGSA/jpj7ozygUZhCXtaS2W5AD5tnBQUpcO0lhItYPYTjnmzcc4y2NbJV8hz\n2s2G8qKv8fyimE23gY1XrPJg+cRF+g4PqFXujjlJ7MihD9oqtLGxbu7o1cifTn3x\nBfIdPythWu5b4cujNsB3m3awJjVmx+MHQ9SugkSIYXV0Ina77cTNS0M2PYiH1PFR\nTwIDAQAB\n-----END PUBLIC KEY-----\n"
},
```
See also: [https://blog.joinmastodon.org/2018/07/how-to-make-friends-and-verify-requests/](https://blog.joinmastodon.org/2018/07/how-to-make-friends-and-verify-requests/)
The signature string is constructed using the values of the HTTP headers defined in `headers`, joined by newlines. Typically, you will want to include the request target, as well as the host and the date. Mastodon assumes `Date:` header if none are provided. For the above GET request, to generate a `Signature:` with `headers="(request-target) host date"` we would generate the following string:
The signature string is then hashed with RSA-SHA256 (RSASSA-PKCS1-v1_5 with SHA-256) and signed with the actor's private key. The resulting value is attached as `signature` within the `Signature:` header. The final request looks like this:
This request is functionally equivalent to saying that `https://my-example.com/actor` is requesting `https://mastodon.example/users/username/inbox` and is proving that they sent this request by signing `(request-target)`, `Host:`, and `Date:` with their private key linked at `keyId`, resulting in the provided `signature`.
When making a POST request to Mastodon, you must calculate the RSA-SHA256 digest hash of your request's body and include this hash within the `Digest:` header. The `Digest:` header must also be included within the `headers` parameter of the `Signature:` header. For example:
Mastodon's current implementation of LD Signatures is outdated due to a change in the JSON-LD `@context` between the drafting stage and finalization stage of the specification. Mastodon expects a `type` of `RsaSignature2017` while later drafts instead define `RsaSignature2018` via the namespace `https://w3id.org/security/v2`. Furthermore, the LD Signatures specification as a whole has been superseded by the [Verifiable Credential Data Integrity 1.0](https://w3c.github.io/vc-data-integrity/) specification, which is largely incompatible with the earlier LD Signature spec. For this reason, it is not advised to implement support for LD Signatures.
{{</hint>}}
[Linked Data Signatures 1.0](https://web.archive.org/web/20170923124140/https://w3c-dvcg.github.io/ld-signatures/) was a draft specification for attaching cryptographic signatures to JSON-LD documents. LD Signatures are not used widely within Mastodon, but they are used in the following situations:
- When running a [self-destruct]({{< relref "admin/tootctl#tootctl-self-destruct" >}}) sequence to send Delete activities to all known peers, the payload will use LD Signatures because HTTP Signatures will not be available. Receiving servers will process the signature by validating it against the locally cached actor key, since the HTTP server will no longer be hosting old actor information.
- When accepting activities from a relay. Public activities can optionally be sent to a relay with LD Signatures, and any server subscribing to a relay does not have to manually refetch the activity from the origin. This prevents having potentially infinite servers attempt to load the status from your instance.
To create a signature, Mastodon uses the keypair attached to an actor at `https://mastodon.example/users/username#main-key`. It then creates an RSA-SHA256 hash of the document, signs it with the keypair, and Base64-strict-encodes the resulting output to derive a `signatureValue`. The following hash is merged into the JSON-LD document:
