Remote instance dereferencing (#70)

Remote instances are now dereferenced when they post to an inbox on a GtS instance.

    Dereferencing will be done first by checking the /api/v1/instance endpoint of an instance.
    If that doesn't work, /.well-known/nodeinfo will be checked.
    If that doesn't work, only a minimal representation of the instance will be stored.

A new field was added to the Instance database model. To create it:

alter table instances add column contact_account_username text;

1 files changed, 244 insertions, 0 deletions

diff --git a/internal/federation/authenticate.go b/internal/federation/authenticate.go
new file mode 100644
index 000000000..c4183144b
--- /dev/null
+++ b/internal/federation/authenticate.go
@@ -0,0 +1,244 @@
+/*
+   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"
+	"strings"
+
+	"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/db"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+)
+
+/*
+	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 requesting http signature.
+//
+// Authenticate in this case is defined as 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.
+//
+// To avoid making unnecessary http calls towards blocked domains, this function *does* bail early if an instance-level domain block exists
+// for the request from the incoming domain. However, it does not check whether individual blocks exist between the requesting user or domain
+// and the requested user: this should be done elsewhere.
+//
+// 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(requestedUsername string, r *http.Request) (*url.URL, error) {
+
+	var publicKey interface{}
+	var pkOwnerURI *url.URL
+	var err error
+
+	// set this extra field for signature validation
+	r.Header.Set("host", f.config.Host)
+
+	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)
+	}
+
+	// if the domain is blocked we want to make as few calls towards it as possible, so already bail here if that's the case!
+	blockedDomain, err := f.blockedDomain(requestingPublicKeyID.Host)
+	if err != nil {
+		return nil, fmt.Errorf("could not tell if domain %s was blocked or not: %s", requestingPublicKeyID.Host, err)
+	}
+	if blockedDomain {
+		return nil, fmt.Errorf("host %s was domain blocked, aborting auth", requestingPublicKeyID.Host)
+	}
+
+	requestingRemoteAccount := &gtsmodel.Account{}
+	requestingLocalAccount := &gtsmodel.Account{}
+	requestingHost := requestingPublicKeyID.Host
+	if strings.EqualFold(requestingHost, f.config.Host) {
+		// LOCAL ACCOUNT REQUEST
+		// the request is coming from INSIDE THE HOUSE so skip the remote dereferencing
+		if err := f.db.GetWhere([]db.Where{{Key: "public_key_uri", Value: requestingPublicKeyID.String()}}, requestingLocalAccount); err != nil {
+			return nil, fmt.Errorf("couldn't get local account with public key uri %s from the database: %s", requestingPublicKeyID.String(), err)
+		}
+		publicKey = requestingLocalAccount.PublicKey
+		pkOwnerURI, err = url.Parse(requestingLocalAccount.URI)
+		if err != nil {
+			return nil, fmt.Errorf("error parsing url %s: %s", requestingLocalAccount.URI, err)
+		}
+	} else if err := f.db.GetWhere([]db.Where{{Key: "public_key_uri", Value: requestingPublicKeyID.String()}}, requestingRemoteAccount); err == nil {
+		// REMOTE ACCOUNT REQUEST WITH KEY CACHED LOCALLY
+		// this is a remote account and we already have the public key for it so use that
+		publicKey = requestingRemoteAccount.PublicKey
+		pkOwnerURI, err = url.Parse(requestingRemoteAccount.URI)
+		if err != nil {
+			return nil, fmt.Errorf("error parsing url %s: %s", requestingRemoteAccount.URI, err)
+		}
+	} else {
+		// REMOTE ACCOUNT REQUEST WITHOUT KEY CACHED LOCALLY
+		// the request is remote and we don't have the public key yet,
+		// so we need to authenticate the request properly by dereferencing the remote key
+		transport, err := f.GetTransportForUser(requestedUsername)
+		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")
+		}
+
+		publicKey, err = x509.ParsePKIXPublicKey(block.Bytes)
+		if err != nil {
+			return nil, fmt.Errorf("could not parse public key from block bytes: %s", 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()
+	}
+	if publicKey == 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(publicKey, algo); err != nil {
+		return nil, fmt.Errorf("error verifying key %s: %s", requestingPublicKeyID.String(), err)
+	}
+
+	return pkOwnerURI, nil
+}
+
+func (f *federator) blockedDomain(host string) (bool, error) {
+	b := &gtsmodel.DomainBlock{}
+	err := f.db.GetWhere([]db.Where{{Key: "domain", Value: host, CaseInsensitive: true}}, b)
+	if err == nil {
+		// block exists
+		return true, nil
+	}
+
+	if _, ok := err.(db.ErrNoEntries); ok {
+		// there are no entries so there's no block
+		return false, nil
+	}
+
+	// there's an actual error
+	return false, err
+}