diff --git a/src/github.com/matrix-org/dendrite/clientapi/writers/membership.go b/src/github.com/matrix-org/dendrite/clientapi/writers/membership.go
index 40ca4ea77..679a7a367 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/writers/membership.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/writers/membership.go
@@ -15,7 +15,14 @@
 package writers
 
 import (
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
 	"net/http"
+	"net/url"
+	"strings"
+	"time"
 
 	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
 	"github.com/matrix-org/dendrite/clientapi/auth/storage/accounts"
@@ -31,7 +38,7 @@ import (
 	"github.com/matrix-org/util"
 )
 
-type requestBody struct {
+type membershipRequestBody struct {
 	UserID   string `json:"user_id"`
 	Reason   string `json:"reason"`
 	IDServer string `json:"id_server"`
@@ -46,11 +53,15 @@ func SendMembership(
 	roomID string, membership string, cfg config.Dendrite,
 	queryAPI api.RoomserverQueryAPI, producer *producers.RoomserverProducer,
 ) util.JSONResponse {
-	var body requestBody
+	var body membershipRequestBody
 	if reqErr := httputil.UnmarshalJSONRequest(req, &body); reqErr != nil {
 		return *reqErr
 	}
 
+	if res := checkAndProcess3PIDInvite(req, device, &body, roomID); res != nil {
+		return *res
+	}
+
 	stateKey, reason, reqErr := getMembershipStateKey(body, device, membership)
 	if reqErr != nil {
 		return *reqErr
@@ -120,7 +131,7 @@ func SendMembership(
 // In the latter case, if there was an issue retrieving the user ID from the request body,
 // returns a JSONResponse with a corresponding error code and message.
 func getMembershipStateKey(
-	body requestBody, device *authtypes.Device, membership string,
+	body membershipRequestBody, device *authtypes.Device, membership string,
 ) (stateKey string, reason string, response *util.JSONResponse) {
 	if membership == "ban" || membership == "unban" || membership == "kick" || membership == "invite" {
 		// If we're in this case, the state key is contained in the request body,
@@ -142,3 +153,146 @@ func getMembershipStateKey(
 
 	return
 }
+
+func checkAndProcess3PIDInvite(
+	req *http.Request, device *authtypes.Device, body *membershipRequestBody,
+	roomID string,
+) *util.JSONResponse {
+	if body.Address == "" && body.IDServer == "" && body.Medium == "" {
+		// If none of the 3PID-specific fields are supplied, it's a standard invite
+		// so return nil for it to be processed as such
+		return nil
+	} else if body.Address == "" || body.IDServer == "" || body.Medium == "" {
+		// If at least one of the 3PID-specific fields is supplied but not all
+		// of them, return an error
+		return &util.JSONResponse{
+			Code: 400,
+			JSON: jsonerror.BadJSON("'address', 'id_server' and 'medium' must all be supplied"),
+		}
+	}
+
+	resp, _, err := queryIDServer(req, body)
+	if err != nil {
+		resErr := httputil.LogThenError(req, err)
+		return &resErr
+	}
+
+	if resp.MXID != "" {
+		// Set the Matrix user ID from the body request and let the process
+		// continue to create a "m.room.member" event
+		body.UserID = resp.MXID
+	}
+	return nil
+}
+
+type idServerLookupResponse struct {
+	TS         int64                        `json:"ts"`
+	NotBefore  int64                        `json:"not_before"`
+	NotAfter   int64                        `json:"not_after"`
+	Medium     string                       `json:"medium"`
+	Address    string                       `json:"address"`
+	MXID       string                       `json:"mxid"`
+	Signatures map[string]map[string]string `json:"signatures"`
+}
+
+func queryIDServer(req *http.Request, body *membershipRequestBody) (res *idServerLookupResponse, token string, err error) {
+	res, err = queryIDServerLookup(body)
+	if err != nil {
+		return
+	}
+
+	if res.MXID == "" {
+		// TODO: Store the invite and send a 3PID invite event
+	}
+
+	// Get timestamp in milliseconds to compare it
+	now := time.Now().UnixNano() / 1000000
+	if res.NotBefore > now || now > res.NotAfter {
+		// If the current timestamp isn't in the time frame in which the association
+		// is known to be valid, re-run the query
+		return queryIDServer(req, body)
+	}
+
+	ok, err := checkIDServerSignatures(body, res)
+	if err != nil {
+		return
+	}
+	if !ok {
+		err = errors.New("The identity server's identity could not be verified")
+		return
+	}
+
+	return
+}
+
+func queryIDServerLookup(body *membershipRequestBody) (res *idServerLookupResponse, err error) {
+	address := url.QueryEscape(body.Address)
+	url := fmt.Sprintf("https://%s/_matrix/identity/api/v1/lookup?medium=%s&address=%s", body.IDServer, body.Medium, address)
+	resp, err := http.Get(url)
+	if err != nil {
+		return
+	}
+	// TODO: Check status code
+	res = new(idServerLookupResponse)
+	err = json.NewDecoder(resp.Body).Decode(res)
+	return
+}
+
+func queryIDServerStoreInvite(device *authtypes.Device, body *membershipRequestBody, roomID string) (*http.Response, error) {
+	client := http.Client{}
+
+	data := url.Values{}
+	data.Add("medium", body.Medium)
+	data.Add("address", body.Address)
+	data.Add("room_id", roomID)
+	data.Add("sender", device.UserID)
+
+	url := fmt.Sprintf("https://%s/_matrix/identity/api/v1/store-invite", body.IDServer)
+	req, err := http.NewRequest("POST", url, strings.NewReader(data.Encode()))
+	if err != nil {
+		return nil, err
+	}
+
+	req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
+
+	return client.Do(req)
+}
+
+func queryIDServerPubKey(body *membershipRequestBody, keyID string) (publicKey []byte, err error) {
+	url := fmt.Sprintf("https://%s/_matrix/identity/api/v1/pubkey/%s", body.IDServer, keyID)
+	resp, err := http.Get(url)
+	if err != nil {
+		return
+	}
+
+	var pubKeyRes struct {
+		PublicKey string `json:"public_key"`
+	}
+	if err = json.NewDecoder(resp.Body).Decode(&pubKeyRes); err != nil {
+		return nil, err
+	}
+	// TODO: Store the public key in the database and, if there's one stored, retrieve
+	// it and verify its validity (/isvalid) instead of fetching it
+	return base64.RawStdEncoding.DecodeString(pubKeyRes.PublicKey)
+}
+
+func checkIDServerSignatures(body *membershipRequestBody, res *idServerLookupResponse) (ok bool, err error) {
+	marshalledBody, err := json.Marshal(*res)
+	if err != nil {
+		return
+	}
+
+	for domain, signatures := range res.Signatures {
+		for keyID := range signatures {
+			pubKey, err := queryIDServerPubKey(body, keyID)
+			if err != nil {
+				return false, err
+			}
+			if err = gomatrixserverlib.VerifyJSON(domain, gomatrixserverlib.KeyID(keyID), pubKey, marshalledBody); err != nil {
+				return false, nil
+			}
+		}
+	}
+
+	return true, nil
+}