gotosocial/internal/federation/util.go

/*
   GoToSocial
   Copyright (C) 2021 GoToSocial Authors admin@gotosocial.org

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU Affero General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU Affero General Public License for more details.

   You should have received a copy of the GNU Affero General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

package federation

import (
	"context"
	"crypto/x509"
	"encoding/json"
	"encoding/pem"
	"errors"
	"fmt"
	"net/http"
	"net/url"

	"github.com/go-fed/activity/pub"
	"github.com/go-fed/activity/streams"
	"github.com/go-fed/activity/streams/vocab"
	"github.com/go-fed/httpsig"
	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
	"github.com/superseriousbusiness/gotosocial/internal/typeutils"
)

/*
	publicKeyer is BORROWED DIRECTLY FROM https://github.com/go-fed/apcore/blob/master/ap/util.go
	Thank you @cj@mastodon.technology ! <3
*/
type publicKeyer interface {
	GetW3IDSecurityV1PublicKey() vocab.W3IDSecurityV1PublicKeyProperty
}

/*
	getPublicKeyFromResponse is adapted from https://github.com/go-fed/apcore/blob/master/ap/util.go
	Thank you @cj@mastodon.technology ! <3
*/
func getPublicKeyFromResponse(c context.Context, b []byte, keyID *url.URL) (vocab.W3IDSecurityV1PublicKey, error) {
	m := make(map[string]interface{})
	if err := json.Unmarshal(b, &m); err != nil {
		return nil, err
	}

	t, err := streams.ToType(c, m)
	if err != nil {
		return nil, err
	}

	pker, ok := t.(publicKeyer)
	if !ok {
		return nil, fmt.Errorf("ActivityStreams type cannot be converted to one known to have publicKey property: %T", t)
	}

	pkp := pker.GetW3IDSecurityV1PublicKey()
	if pkp == nil {
		return nil, errors.New("publicKey property is not provided")
	}

	var pkpFound vocab.W3IDSecurityV1PublicKey
	for pkpIter := pkp.Begin(); pkpIter != pkp.End(); pkpIter = pkpIter.Next() {
		if !pkpIter.IsW3IDSecurityV1PublicKey() {
			continue
		}
		pkValue := pkpIter.Get()
		var pkID *url.URL
		pkID, err = pub.GetId(pkValue)
		if err != nil {
			return nil, err
		}
		if pkID.String() != keyID.String() {
			continue
		}
		pkpFound = pkValue
		break
	}

	if pkpFound == nil {
		return nil, fmt.Errorf("cannot find publicKey with id: %s", keyID)
	}

	return pkpFound, nil
}

// AuthenticateFederatedRequest authenticates any kind of incoming federated request from a remote server. This includes things like
// GET requests for dereferencing our users or statuses etc, and POST requests for delivering new Activities. The function returns
// the URL of the owner of the public key used in the http signature.
//
// Authenticate in this case is defined as just making sure that the http request is actually signed by whoever claims
// to have signed it, by fetching the public key from the signature and checking it against the remote public key. This function
// *does not* check whether the request is authorized, only whether it's authentic.
//
// The provided username will be used to generate a transport for making remote requests/derefencing the public key ID of the request signature.
// Ideally you should pass in the username of the user *being requested*, so that the remote server can decide how to handle the request based on who's making it.
// Ie., if the request on this server is for https://example.org/users/some_username then you should pass in the username 'some_username'.
// The remote server will then know that this is the user making the dereferencing request, and they can decide to allow or deny the request depending on their settings.
//
// Note that it is also valid to pass in an empty string here, in which case the keys of the instance account will be used.
//
// Also note that this function *does not* dereference the remote account that the signature key is associated with.
// Other functions should use the returned URL to dereference the remote account, if required.
func (f *federator) AuthenticateFederatedRequest(username string, r *http.Request) (*url.URL, error) {
	verifier, err := httpsig.NewVerifier(r)
	if err != nil {
		return nil, fmt.Errorf("could not create http sig verifier: %s", err)
	}

	// The key ID should be given in the signature so that we know where to fetch it from the remote server.
	// This will be something like https://example.org/users/whatever_requesting_user#main-key
	requestingPublicKeyID, err := url.Parse(verifier.KeyId())
	if err != nil {
		return nil, fmt.Errorf("could not parse key id into a url: %s", err)
	}

	transport, err := f.GetTransportForUser(username)
	if err != nil {
		return nil, fmt.Errorf("transport err: %s", err)
	}

	// The actual http call to the remote server is made right here in the Dereference function.
	b, err := transport.Dereference(context.Background(), requestingPublicKeyID)
	if err != nil {
		return nil, fmt.Errorf("error deferencing key %s: %s", requestingPublicKeyID.String(), err)
	}

	// if the key isn't in the response, we can't authenticate the request
	requestingPublicKey, err := getPublicKeyFromResponse(context.Background(), b, requestingPublicKeyID)
	if err != nil {
		return nil, fmt.Errorf("error getting key %s from response %s: %s", requestingPublicKeyID.String(), string(b), err)
	}

	// we should be able to get the actual key embedded in the vocab.W3IDSecurityV1PublicKey
	pkPemProp := requestingPublicKey.GetW3IDSecurityV1PublicKeyPem()
	if pkPemProp == nil || !pkPemProp.IsXMLSchemaString() {
		return nil, errors.New("publicKeyPem property is not provided or it is not embedded as a value")
	}

	// and decode the PEM so that we can parse it as a golang public key
	pubKeyPem := pkPemProp.Get()
	block, _ := pem.Decode([]byte(pubKeyPem))
	if block == nil || block.Type != "PUBLIC KEY" {
		return nil, errors.New("could not decode publicKeyPem to PUBLIC KEY pem block type")
	}

	p, err := x509.ParsePKIXPublicKey(block.Bytes)
	if err != nil {
		return nil, fmt.Errorf("could not parse public key from block bytes: %s", err)
	}
	if p == nil {
		return nil, errors.New("returned public key was empty")
	}

	// do the actual authentication here!
	algo := httpsig.RSA_SHA256 // TODO: make this more robust
	if err := verifier.Verify(p, algo); err != nil {
		return nil, fmt.Errorf("error verifying key %s: %s", requestingPublicKeyID.String(), err)
	}

	// all good! we just need the URI of the key owner to return
	pkOwnerProp := requestingPublicKey.GetW3IDSecurityV1Owner()
	if pkOwnerProp == nil || !pkOwnerProp.IsIRI() {
		return nil, errors.New("publicKeyOwner property is not provided or it is not embedded as a value")
	}
	pkOwnerURI := pkOwnerProp.GetIRI()

	return pkOwnerURI, nil
}

func (f *federator) DereferenceRemoteAccount(username string, remoteAccountID *url.URL) (typeutils.Accountable, error) {

	transport, err := f.GetTransportForUser(username)
	if err != nil {
		return nil, fmt.Errorf("transport err: %s", err)
	}

	b, err := transport.Dereference(context.Background(), remoteAccountID)
	if err != nil {
		return nil, fmt.Errorf("error deferencing %s: %s", remoteAccountID.String(), err)
	}

	m := make(map[string]interface{})
	if err := json.Unmarshal(b, &m); err != nil {
		return nil, fmt.Errorf("error unmarshalling bytes into json: %s", err)
	}

	t, err := streams.ToType(context.Background(), m)
	if err != nil {
		return nil, fmt.Errorf("error resolving json into ap vocab type: %s", err)
	}

	switch t.GetTypeName() {
	case string(gtsmodel.ActivityStreamsPerson):
		p, ok := t.(vocab.ActivityStreamsPerson)
		if !ok {
			return nil, errors.New("error resolving type as activitystreams person")
		}
		return p, nil
	case string(gtsmodel.ActivityStreamsApplication):
		// TODO: convert application into person
	}

	return nil, fmt.Errorf("type name %s not supported", t.GetTypeName())
}

func (f *federator) GetTransportForUser(username string) (pub.Transport, error) {
	// We need an account to use to create a transport for dereferecing the signature.
	// If a username has been given, we can fetch the account with that username and use it.
	// Otherwise, we can take the instance account and use those credentials to make the request.
	ourAccount := &gtsmodel.Account{}
	var u string
	if username == "" {
		u = f.config.Host
	} else {
		u = username
	}
	if err := f.db.GetLocalAccountByUsername(u, ourAccount); err != nil {
		return nil, fmt.Errorf("error getting account %s from db: %s", username, err)
	}

	transport, err := f.transportController.NewTransport(ourAccount.PublicKeyURI, ourAccount.PrivateKey)
	if err != nil {
		return nil, fmt.Errorf("error creating transport for user %s: %s", username, err)
	}
	return transport, nil
}