[HTTP Signatures](https://w3c-dvcg.github.io/http-signatures/) 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.
For any HTTP request incoming to Mastodon, the following header should be attached:
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.md#public-key" >}}) 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/)
To create an HTTP signature, you will have to define which headers are being hashed and signed. For example, consider the following request being sent out:
```http
GET /users/username/inbox HTTP/1.1
Host: mastodon.example
Date: 18 Dec 2019 10:08:46 GMT
Accept: application/activity+json
```
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 request, to generate a `Signature:` with `headers="(request-target) host date"` we would generate the following string:
```text
(request-target): get /users/username/inbox
host: mastodon.example
date: 18 Dec 2019 10:08:46 GMT
```
Note that we don't care about the `Accept:` header because we won't be specifying it in `headers`.
The signature string is then hashed with SHA256 and signed with the actor's public 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 public key linked at `keyId`, resulting in the provided `signature`.
[Linked Data Signatures 1.0](https://w3c-dvcg.github.io/ld-signatures/) is a 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.md#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 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:
Mastodon's current implementation of LD Signatures is somewhat 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 the current specification instead defines `RsaSignature2018` via the namespace `https://w3id.org/security/v2`.
