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Security Public key cryptography and supported signature schemes over HTTP and JSON-LD.
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HTTP Signatures

{{< caption-link url="https://github.com/tootsuite/mastodon/blob/master/app/lib/request.rb" caption="app/lib/request.rb" >}}

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:

Signature: keyId="https://my-example.com/actor#main-key",headers="(request-target) host date",signature="Y2FiYW...IxNGRiZDk4ZA=="

The three parts of the Signature: header can be broken down like so:

Signature:
  keyId="https://my-example.com/actor#main-key",
  headers="(request-target) host date",
  signature="Y2FiYW...IxNGRiZDk4ZA=="

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:

"publicKey": {
    "id": "https://my-example.com/actor#main-key",
    "owner": "https://my-example.com/actor",
    "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/

Creating HTTP signatures

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:

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:

(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:

GET /users/username/inbox HTTP/1.1
Host: mastodon.example
Date: 18 Dec 2019 10:08:46 GMT
Accept: application/activity+json
Signature: keyId="https://my-example.com/actor#main-key",headers="(request-target) host date",signature="Y2FiYW...IxNGRiZDk4ZA=="

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.

Verifying HTTP signatures

{{< caption-link url="https://github.com/tootsuite/mastodon/blob/master/app/controllers/concerns/signature_verification.rb" caption="app/controllers/concerns/signature_verification.rb" >}}

Consider the following request:

GET /users/username/inbox HTTP/1.1
Host: mastodon.example
Date: 18 Dec 2019 10:08:46 GMT
Accept: application/activity+json
Signature: keyId="https://my-example.com/actor#main-key",headers="(request-target) host date",signature="Y2FiYW...IxNGRiZDk4ZA=="

Mastodon verifies the signature using the following algorithm:

  • Split Signature: into its separate parameters.
  • Construct the signature string from the value of headers.
  • Fetch the keyId and resolve to an actor's publicKey.
  • SHA256 hash the signature string and compare to the Base64-decoded signature as decrypted by publicKey[publicKeyPem].
  • Use the Date: header to check that the signed request was made within the past 12 hours.

Linked Data Signatures

{{< caption-link url="https://github.com/tootsuite/mastodon/blob/master/app/lib/activitypub/linked_data_signature.rb" caption="app/lib/activitypub/linked_data_signature.rb" >}}

Linked Data Signatures 1.0 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.

Creating LD signatures

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:

"signature": {
    "type": "RsaSignature2017",
    "creator": "https://mastodon.example/users/username#main-key",
    "created": "2019-12-08T03:48:33.901Z",
    "signatureValue": "s69F3mfddd99dGjmvjdjjs81e12jn121Gkm1"
}

{{< hint style="warning" >}} 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. {{< /hint >}}

Verifying LD signatures

To verify a signature, Mastodon uses the following algorithm:

  • Make sure that a signature exists and is a hash.
  • Make sure that signature[type] is RsaSignature2017.
  • Fetch the signature[creator] URI. Make sure the creator exists.
  • Strip type, id, and signatureValue from the signature, leaving only signature[creator] and signature[created].
  • Base64-decode the signatureValue and verify it against the public key in signature[creator].