diff --git a/federationapi/routing/devices.go b/federationapi/routing/devices.go
index 07862451..b397c546 100644
--- a/federationapi/routing/devices.go
+++ b/federationapi/routing/devices.go
@@ -37,12 +37,28 @@ func GetUserDevices(
 		return jsonerror.InternalServerError()
 	}
 
+	sigReq := &keyapi.QuerySignaturesRequest{
+		TargetIDs: map[string][]gomatrixserverlib.KeyID{},
+	}
+	sigRes := &keyapi.QuerySignaturesResponse{}
+	for _, dev := range res.Devices {
+		sigReq.TargetIDs[userID] = append(sigReq.TargetIDs[userID], gomatrixserverlib.KeyID(dev.DeviceID))
+	}
+	keyAPI.QuerySignatures(req.Context(), sigReq, sigRes)
+
 	response := gomatrixserverlib.RespUserDevices{
 		UserID:   userID,
 		StreamID: res.StreamID,
 		Devices:  []gomatrixserverlib.RespUserDevice{},
 	}
 
+	if masterKey, ok := sigRes.MasterKeys[userID]; ok {
+		response.MasterKey = &masterKey
+	}
+	if selfSigningKey, ok := sigRes.SelfSigningKeys[userID]; ok {
+		response.SelfSigningKey = &selfSigningKey
+	}
+
 	for _, dev := range res.Devices {
 		var key gomatrixserverlib.RespUserDeviceKeys
 		err := json.Unmarshal(dev.DeviceKeys.KeyJSON, &key)
@@ -56,6 +72,20 @@ func GetUserDevices(
 			DisplayName: dev.DisplayName,
 			Keys:        key,
 		}
+
+		if targetUser, ok := sigRes.Signatures[userID]; ok {
+			if targetKey, ok := targetUser[gomatrixserverlib.KeyID(dev.DeviceID)]; ok {
+				for sourceUserID, forSourceUser := range targetKey {
+					for sourceKeyID, sourceKey := range forSourceUser {
+						if _, ok := device.Keys.Signatures[sourceUserID]; !ok {
+							device.Keys.Signatures[sourceUserID] = map[gomatrixserverlib.KeyID]gomatrixserverlib.Base64Bytes{}
+						}
+						device.Keys.Signatures[sourceUserID][sourceKeyID] = sourceKey
+					}
+				}
+			}
+		}
+
 		response.Devices = append(response.Devices, device)
 	}
 
diff --git a/keyserver/api/api.go b/keyserver/api/api.go
index f46a9ee2..520562e7 100644
--- a/keyserver/api/api.go
+++ b/keyserver/api/api.go
@@ -20,6 +20,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/matrix-org/dendrite/keyserver/types"
 	userapi "github.com/matrix-org/dendrite/userapi/api"
 	"github.com/matrix-org/gomatrixserverlib"
 )
@@ -38,6 +39,7 @@ type KeyInternalAPI interface {
 	QueryKeyChanges(ctx context.Context, req *QueryKeyChangesRequest, res *QueryKeyChangesResponse)
 	QueryOneTimeKeys(ctx context.Context, req *QueryOneTimeKeysRequest, res *QueryOneTimeKeysResponse)
 	QueryDeviceMessages(ctx context.Context, req *QueryDeviceMessagesRequest, res *QueryDeviceMessagesResponse)
+	QuerySignatures(ctx context.Context, req *QuerySignaturesRequest, res *QuerySignaturesResponse)
 }
 
 // KeyError is returned if there was a problem performing/querying the server
@@ -242,6 +244,24 @@ type QueryDeviceMessagesResponse struct {
 	Error    *KeyError
 }
 
+type QuerySignaturesRequest struct {
+	// A map of target user ID -> target key/device IDs to retrieve signatures for
+	TargetIDs map[string][]gomatrixserverlib.KeyID `json:"target_ids"`
+}
+
+type QuerySignaturesResponse struct {
+	// A map of target user ID -> target key/device ID -> origin user ID -> origin key/device ID -> signatures
+	Signatures map[string]map[gomatrixserverlib.KeyID]types.CrossSigningSigMap
+	// A map of target user ID -> cross-signing master key
+	MasterKeys map[string]gomatrixserverlib.CrossSigningKey
+	// A map of target user ID -> cross-signing self-signing key
+	SelfSigningKeys map[string]gomatrixserverlib.CrossSigningKey
+	// A map of target user ID -> cross-signing user-signing key
+	UserSigningKeys map[string]gomatrixserverlib.CrossSigningKey
+	// The request error, if any
+	Error *KeyError
+}
+
 type InputDeviceListUpdateRequest struct {
 	Event gomatrixserverlib.DeviceListUpdateEvent
 }
diff --git a/keyserver/internal/cross_signing.go b/keyserver/internal/cross_signing.go
index 802d02b3..141a4eb2 100644
--- a/keyserver/internal/cross_signing.go
+++ b/keyserver/internal/cross_signing.go
@@ -17,6 +17,7 @@ package internal
 import (
 	"context"
 	"crypto/ed25519"
+	"database/sql"
 	"encoding/json"
 	"fmt"
 	"strings"
@@ -104,7 +105,7 @@ func (a *KeyInternalAPI) PerformUploadDeviceKeys(ctx context.Context, req *api.P
 	// If the user hasn't given a new master key, then let's go and get their
 	// existing keys from the database.
 	if !hasMasterKey {
-		existingKeys, err := a.DB.CrossSigningKeysForUser(ctx, req.UserID)
+		existingKeys, err := a.DB.CrossSigningKeysDataForUser(ctx, req.UserID)
 		if err != nil {
 			res.Error = &api.KeyError{
 				Err: "Retrieving cross-signing keys from database failed: " + err.Error(),
@@ -177,21 +178,24 @@ func (a *KeyInternalAPI) PerformUploadDeviceKeys(ctx context.Context, req *api.P
 	}
 	for purpose, key := range toVerify {
 		// Collect together the key IDs we need to verify with. This will include
-		// all of the key IDs specified in the signatures. We don't do this for
-		// the master key because we have no means to verify the signatures - we
-		// instead just need to store them.
-		if purpose != gomatrixserverlib.CrossSigningKeyPurposeMaster {
-			// Marshal the specific key back into JSON so that we can verify the
-			// signature of it.
-			keyJSON, err := json.Marshal(key)
-			if err != nil {
-				res.Error = &api.KeyError{
-					Err: fmt.Sprintf("The JSON of the key section is invalid: %s", err.Error()),
-				}
-				return
+		// all of the key IDs specified in the signatures.
+		keyJSON, err := json.Marshal(key)
+		if err != nil {
+			res.Error = &api.KeyError{
+				Err: fmt.Sprintf("The JSON of the key section is invalid: %s", err.Error()),
 			}
+			return
+		}
 
-			// Now check if the subkey is signed by the master key.
+		switch purpose {
+		case gomatrixserverlib.CrossSigningKeyPurposeMaster:
+			// The master key might have a signature attached to it from the
+			// previous key, or from a device key, but there's no real need
+			// to verify it. Clients will perform key checks when the master
+			// key changes.
+
+		default:
+			// Sub-keys should be signed by the master key.
 			if err := gomatrixserverlib.VerifyJSON(req.UserID, masterKeyID, ed25519.PublicKey(masterKey), keyJSON); err != nil {
 				res.Error = &api.KeyError{
 					Err:                fmt.Sprintf("The %q sub-key failed master key signature verification: %s", purpose, err.Error()),
@@ -212,10 +216,44 @@ func (a *KeyInternalAPI) PerformUploadDeviceKeys(ctx context.Context, req *api.P
 		res.Error = &api.KeyError{
 			Err: fmt.Sprintf("a.DB.StoreCrossSigningKeysForUser: %s", err),
 		}
+		return
+	}
+
+	// Now upload any signatures that were included with the keys.
+	for _, key := range toVerify {
+		var targetKeyID gomatrixserverlib.KeyID
+		for targetKey := range key.Keys { // iterates once, see sanityCheckKey
+			targetKeyID = targetKey
+		}
+		for sigUserID, forSigUserID := range key.Signatures {
+			if sigUserID != req.UserID {
+				continue
+			}
+			for sigKeyID, sigBytes := range forSigUserID {
+				if err := a.DB.StoreCrossSigningSigsForTarget(ctx, sigUserID, sigKeyID, req.UserID, targetKeyID, sigBytes); err != nil {
+					res.Error = &api.KeyError{
+						Err: fmt.Sprintf("a.DB.StoreCrossSigningSigsForTarget: %s", err),
+					}
+					return
+				}
+			}
+		}
 	}
 }
 
 func (a *KeyInternalAPI) PerformUploadDeviceSignatures(ctx context.Context, req *api.PerformUploadDeviceSignaturesRequest, res *api.PerformUploadDeviceSignaturesResponse) {
+	// Before we do anything, we need the master and self-signing keys for this user.
+	// Then we can verify the signatures make sense.
+	queryReq := &api.QueryKeysRequest{
+		UserID:        req.UserID,
+		UserToDevices: map[string][]string{},
+	}
+	queryRes := &api.QueryKeysResponse{}
+	for userID := range req.Signatures {
+		queryReq.UserToDevices[userID] = []string{}
+	}
+	a.QueryKeys(ctx, queryReq, queryRes)
+
 	selfSignatures := map[string]map[gomatrixserverlib.KeyID]gomatrixserverlib.CrossSigningForKeyOrDevice{}
 	otherSignatures := map[string]map[gomatrixserverlib.KeyID]gomatrixserverlib.CrossSigningForKeyOrDevice{}
 
@@ -254,14 +292,14 @@ func (a *KeyInternalAPI) PerformUploadDeviceSignatures(ctx context.Context, req
 		}
 	}
 
-	if err := a.processSelfSignatures(ctx, req.UserID, selfSignatures); err != nil {
+	if err := a.processSelfSignatures(ctx, req.UserID, queryRes, selfSignatures); err != nil {
 		res.Error = &api.KeyError{
 			Err: fmt.Sprintf("a.processSelfSignatures: %s", err),
 		}
 		return
 	}
 
-	if err := a.processOtherSignatures(ctx, req.UserID, otherSignatures); err != nil {
+	if err := a.processOtherSignatures(ctx, req.UserID, queryRes, otherSignatures); err != nil {
 		res.Error = &api.KeyError{
 			Err: fmt.Sprintf("a.processOtherSignatures: %s", err),
 		}
@@ -270,7 +308,7 @@ func (a *KeyInternalAPI) PerformUploadDeviceSignatures(ctx context.Context, req
 }
 
 func (a *KeyInternalAPI) processSelfSignatures(
-	ctx context.Context, _ string,
+	ctx context.Context, _ string, queryRes *api.QueryKeysResponse,
 	signatures map[string]map[gomatrixserverlib.KeyID]gomatrixserverlib.CrossSigningForKeyOrDevice,
 ) error {
 	// Here we will process:
@@ -281,8 +319,39 @@ func (a *KeyInternalAPI) processSelfSignatures(
 		for targetKeyID, signature := range forTargetUserID {
 			switch sig := signature.CrossSigningBody.(type) {
 			case *gomatrixserverlib.CrossSigningKey:
+				// The user is signing their master key with one of their devices
+				// The QueryKeys response should contain the device key hopefully.
+				// First we need to marshal the blob back into JSON so we can verify
+				// it.
+				j, err := json.Marshal(sig)
+				if err != nil {
+					return fmt.Errorf("json.Marshal: %w", err)
+				}
+
 				for originUserID, forOriginUserID := range sig.Signatures {
+					originDeviceKeys, ok := queryRes.DeviceKeys[originUserID]
+					if !ok {
+						return fmt.Errorf("missing device keys for user %q", originUserID)
+					}
+
 					for originKeyID, originSig := range forOriginUserID {
+						originDeviceKeyID := gomatrixserverlib.KeyID("ed25519:" + originKeyID)
+
+						var originKey gomatrixserverlib.DeviceKeys
+						if err := json.Unmarshal(originDeviceKeys[string(originKeyID)], &originKey); err != nil {
+							return fmt.Errorf("json.Unmarshal: %w", err)
+						}
+
+						originSigningKey, ok := originKey.Keys[originDeviceKeyID]
+						if !ok {
+							return fmt.Errorf("missing origin signing key %q", originDeviceKeyID)
+						}
+						originSigningKeyPublic := ed25519.PublicKey(originSigningKey)
+
+						if err := gomatrixserverlib.VerifyJSON(originUserID, originDeviceKeyID, originSigningKeyPublic, j); err != nil {
+							return fmt.Errorf("gomatrixserverlib.VerifyJSON: %w", err)
+						}
+
 						if err := a.DB.StoreCrossSigningSigsForTarget(
 							ctx, originUserID, originKeyID, targetUserID, targetKeyID, originSig,
 						); err != nil {
@@ -292,8 +361,35 @@ func (a *KeyInternalAPI) processSelfSignatures(
 				}
 
 			case *gomatrixserverlib.DeviceKeys:
+				// The user is signing one of their devices with their self-signing key
+				// The QueryKeys response should contain the master key hopefully.
+				// First we need to marshal the blob back into JSON so we can verify
+				// it.
+				j, err := json.Marshal(sig)
+				if err != nil {
+					return fmt.Errorf("json.Marshal: %w", err)
+				}
+
 				for originUserID, forOriginUserID := range sig.Signatures {
 					for originKeyID, originSig := range forOriginUserID {
+						originSelfSigningKeys, ok := queryRes.SelfSigningKeys[originUserID]
+						if !ok {
+							return fmt.Errorf("missing self-signing key for user %q", originUserID)
+						}
+
+						var originSelfSigningKeyID gomatrixserverlib.KeyID
+						var originSelfSigningKey gomatrixserverlib.Base64Bytes
+						for keyID, key := range originSelfSigningKeys.Keys {
+							originSelfSigningKeyID, originSelfSigningKey = keyID, key
+							break
+						}
+
+						originSelfSigningKeyPublic := ed25519.PublicKey(originSelfSigningKey)
+
+						if err := gomatrixserverlib.VerifyJSON(originUserID, originSelfSigningKeyID, originSelfSigningKeyPublic, j); err != nil {
+							return fmt.Errorf("gomatrixserverlib.VerifyJSON: %w", err)
+						}
+
 						if err := a.DB.StoreCrossSigningSigsForTarget(
 							ctx, originUserID, originKeyID, targetUserID, targetKeyID, originSig,
 						); err != nil {
@@ -312,7 +408,7 @@ func (a *KeyInternalAPI) processSelfSignatures(
 }
 
 func (a *KeyInternalAPI) processOtherSignatures(
-	ctx context.Context, userID string,
+	ctx context.Context, userID string, queryRes *api.QueryKeysResponse,
 	signatures map[string]map[gomatrixserverlib.KeyID]gomatrixserverlib.CrossSigningForKeyOrDevice,
 ) error {
 	// Here we will process:
@@ -331,20 +427,14 @@ func (a *KeyInternalAPI) crossSigningKeysFromDatabase(
 			continue
 		}
 
-		for keyType, keyData := range keys {
-			b64 := keyData.Encode()
-			keyID := gomatrixserverlib.KeyID("ed25519:" + b64)
-			key := gomatrixserverlib.CrossSigningKey{
-				UserID: userID,
-				Usage: []gomatrixserverlib.CrossSigningKeyPurpose{
-					keyType,
-				},
-				Keys: map[gomatrixserverlib.KeyID]gomatrixserverlib.Base64Bytes{
-					keyID: keyData,
-				},
+		for keyType, key := range keys {
+			var keyID gomatrixserverlib.KeyID
+			for id := range key.Keys {
+				keyID = id
+				break
 			}
 
-			sigs, err := a.DB.CrossSigningSigsForTarget(ctx, userID, keyID)
+			sigMap, err := a.DB.CrossSigningSigsForTarget(ctx, userID, keyID)
 			if err != nil {
 				logrus.WithError(err).Errorf("Failed to get cross-signing signatures for user %q key %q", userID, keyID)
 				continue
@@ -360,7 +450,7 @@ func (a *KeyInternalAPI) crossSigningKeysFromDatabase(
 				key.Signatures[originUserID][originKeyID] = signature
 			}
 
-			for originUserID, forOrigin := range sigs {
+			for originUserID, forOrigin := range sigMap {
 				for originKeyID, signature := range forOrigin {
 					switch {
 					case req.UserID != "" && originUserID == req.UserID:
@@ -387,3 +477,70 @@ func (a *KeyInternalAPI) crossSigningKeysFromDatabase(
 		}
 	}
 }
+
+func (a *KeyInternalAPI) QuerySignatures(ctx context.Context, req *api.QuerySignaturesRequest, res *api.QuerySignaturesResponse) {
+	for targetUserID, forTargetUser := range req.TargetIDs {
+		for _, targetKeyID := range forTargetUser {
+			keyMap, err := a.DB.CrossSigningKeysForUser(ctx, targetUserID)
+			if err != nil {
+				if err == sql.ErrNoRows {
+					continue
+				}
+				res.Error = &api.KeyError{
+					Err: fmt.Sprintf("a.DB.CrossSigningKeysForUser: %s", err),
+				}
+			}
+
+			for targetPurpose, targetKey := range keyMap {
+				switch targetPurpose {
+				case gomatrixserverlib.CrossSigningKeyPurposeMaster:
+					if res.MasterKeys == nil {
+						res.MasterKeys = map[string]gomatrixserverlib.CrossSigningKey{}
+					}
+					res.MasterKeys[targetUserID] = targetKey
+
+				case gomatrixserverlib.CrossSigningKeyPurposeSelfSigning:
+					if res.SelfSigningKeys == nil {
+						res.SelfSigningKeys = map[string]gomatrixserverlib.CrossSigningKey{}
+					}
+					res.SelfSigningKeys[targetUserID] = targetKey
+
+				case gomatrixserverlib.CrossSigningKeyPurposeUserSigning:
+					if res.UserSigningKeys == nil {
+						res.UserSigningKeys = map[string]gomatrixserverlib.CrossSigningKey{}
+					}
+					res.UserSigningKeys[targetUserID] = targetKey
+				}
+			}
+
+			sigMap, err := a.DB.CrossSigningSigsForTarget(ctx, targetUserID, targetKeyID)
+			if err != nil {
+				if err == sql.ErrNoRows {
+					continue
+				}
+				res.Error = &api.KeyError{
+					Err: fmt.Sprintf("a.DB.CrossSigningSigsForTarget: %s", err),
+				}
+				return
+			}
+
+			for sourceUserID, forSourceUser := range sigMap {
+				for sourceKeyID, sourceSig := range forSourceUser {
+					if res.Signatures == nil {
+						res.Signatures = map[string]map[gomatrixserverlib.KeyID]types.CrossSigningSigMap{}
+					}
+					if _, ok := res.Signatures[targetUserID]; !ok {
+						res.Signatures[targetUserID] = map[gomatrixserverlib.KeyID]types.CrossSigningSigMap{}
+					}
+					if _, ok := res.Signatures[targetUserID][targetKeyID]; !ok {
+						res.Signatures[targetUserID][targetKeyID] = types.CrossSigningSigMap{}
+					}
+					if _, ok := res.Signatures[targetUserID][targetKeyID][sourceUserID]; !ok {
+						res.Signatures[targetUserID][targetKeyID][sourceUserID] = map[gomatrixserverlib.KeyID]gomatrixserverlib.Base64Bytes{}
+					}
+					res.Signatures[targetUserID][targetKeyID][sourceUserID][sourceKeyID] = sourceSig
+				}
+			}
+		}
+	}
+}
diff --git a/keyserver/internal/internal.go b/keyserver/internal/internal.go
index c5711e73..28638c29 100644
--- a/keyserver/internal/internal.go
+++ b/keyserver/internal/internal.go
@@ -300,12 +300,38 @@ func (a *KeyInternalAPI) QueryKeys(ctx context.Context, req *api.QueryKeysReques
 
 	// attempt to satisfy key queries from the local database first as we should get device updates pushed to us
 	domainToDeviceKeys = a.remoteKeysFromDatabase(ctx, res, domainToDeviceKeys)
-	if len(domainToDeviceKeys) == 0 && len(domainToCrossSigningKeys) == 0 {
-		return // nothing to query
+	if len(domainToDeviceKeys) > 0 || len(domainToCrossSigningKeys) > 0 {
+		// perform key queries for remote devices
+		a.queryRemoteKeys(ctx, req.Timeout, res, domainToDeviceKeys, domainToCrossSigningKeys)
 	}
 
-	// perform key queries for remote devices
-	a.queryRemoteKeys(ctx, req.Timeout, res, domainToDeviceKeys, domainToCrossSigningKeys)
+	// Finally, append signatures that we know about
+	// TODO: This is horrible because we need to round-trip the signature from
+	// JSON, add the signatures and marshal it again, for some reason?
+	for userID, forUserID := range res.DeviceKeys {
+		for keyID, key := range forUserID {
+			sigMap, err := a.DB.CrossSigningSigsForTarget(ctx, userID, gomatrixserverlib.KeyID(keyID))
+			if err != nil {
+				logrus.WithError(err).Errorf("a.DB.CrossSigningSigsForTarget failed")
+				continue
+			}
+			if len(sigMap) == 0 {
+				continue
+			}
+			var deviceKey gomatrixserverlib.DeviceKeys
+			if err = json.Unmarshal(key, &deviceKey); err != nil {
+				continue
+			}
+			for sourceUserID, forSourceUser := range sigMap {
+				for sourceKeyID, sourceSig := range forSourceUser {
+					deviceKey.Signatures[sourceUserID][sourceKeyID] = sourceSig
+				}
+			}
+			if js, err := json.Marshal(deviceKey); err == nil {
+				res.DeviceKeys[userID][keyID] = js
+			}
+		}
+	}
 }
 
 func (a *KeyInternalAPI) remoteKeysFromDatabase(
diff --git a/keyserver/inthttp/client.go b/keyserver/inthttp/client.go
index b685ae1a..15870571 100644
--- a/keyserver/inthttp/client.go
+++ b/keyserver/inthttp/client.go
@@ -36,6 +36,7 @@ const (
 	QueryKeyChangesPath               = "/keyserver/queryKeyChanges"
 	QueryOneTimeKeysPath              = "/keyserver/queryOneTimeKeys"
 	QueryDeviceMessagesPath           = "/keyserver/queryDeviceMessages"
+	QuerySignaturesPath               = "/keyserver/querySignatures"
 )
 
 // NewKeyServerClient creates a KeyInternalAPI implemented by talking to a HTTP POST API.
@@ -211,3 +212,20 @@ func (h *httpKeyInternalAPI) PerformUploadDeviceSignatures(
 		}
 	}
 }
+
+func (h *httpKeyInternalAPI) QuerySignatures(
+	ctx context.Context,
+	request *api.QuerySignaturesRequest,
+	response *api.QuerySignaturesResponse,
+) {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "QuerySignatures")
+	defer span.Finish()
+
+	apiURL := h.apiURL + QuerySignaturesPath
+	err := httputil.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+	if err != nil {
+		response.Error = &api.KeyError{
+			Err: err.Error(),
+		}
+	}
+}
diff --git a/keyserver/inthttp/server.go b/keyserver/inthttp/server.go
index ab096c15..ac70e3e5 100644
--- a/keyserver/inthttp/server.go
+++ b/keyserver/inthttp/server.go
@@ -124,4 +124,15 @@ func AddRoutes(internalAPIMux *mux.Router, s api.KeyInternalAPI) {
 			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
 		}),
 	)
+	internalAPIMux.Handle(QuerySignaturesPath,
+		httputil.MakeInternalAPI("querySignatures", func(req *http.Request) util.JSONResponse {
+			request := api.QuerySignaturesRequest{}
+			response := api.QuerySignaturesResponse{}
+			if err := json.NewDecoder(req.Body).Decode(&request); err != nil {
+				return util.MessageResponse(http.StatusBadRequest, err.Error())
+			}
+			s.QuerySignatures(req.Context(), &request, &response)
+			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
+		}),
+	)
 }
diff --git a/keyserver/storage/interface.go b/keyserver/storage/interface.go
index 756dc32a..b9db81ad 100644
--- a/keyserver/storage/interface.go
+++ b/keyserver/storage/interface.go
@@ -78,7 +78,8 @@ type Database interface {
 	// MarkDeviceListStale sets the stale bit for this user to isStale.
 	MarkDeviceListStale(ctx context.Context, userID string, isStale bool) error
 
-	CrossSigningKeysForUser(ctx context.Context, userID string) (types.CrossSigningKeyMap, error)
+	CrossSigningKeysForUser(ctx context.Context, userID string) (map[gomatrixserverlib.CrossSigningKeyPurpose]gomatrixserverlib.CrossSigningKey, error)
+	CrossSigningKeysDataForUser(ctx context.Context, userID string) (types.CrossSigningKeyMap, error)
 	CrossSigningSigsForTarget(ctx context.Context, targetUserID string, targetKeyID gomatrixserverlib.KeyID) (types.CrossSigningSigMap, error)
 
 	StoreCrossSigningKeysForUser(ctx context.Context, userID string, keyMap types.CrossSigningKeyMap) error
diff --git a/keyserver/storage/shared/storage.go b/keyserver/storage/shared/storage.go
index 76724295..64ce53ef 100644
--- a/keyserver/storage/shared/storage.go
+++ b/keyserver/storage/shared/storage.go
@@ -159,7 +159,46 @@ func (d *Database) MarkDeviceListStale(ctx context.Context, userID string, isSta
 }
 
 // CrossSigningKeysForUser returns the latest known cross-signing keys for a user, if any.
-func (d *Database) CrossSigningKeysForUser(ctx context.Context, userID string) (types.CrossSigningKeyMap, error) {
+func (d *Database) CrossSigningKeysForUser(ctx context.Context, userID string) (map[gomatrixserverlib.CrossSigningKeyPurpose]gomatrixserverlib.CrossSigningKey, error) {
+	keyMap, err := d.CrossSigningKeysTable.SelectCrossSigningKeysForUser(ctx, nil, userID)
+	if err != nil {
+		return nil, fmt.Errorf("d.CrossSigningKeysTable.SelectCrossSigningKeysForUser: %w", err)
+	}
+	results := map[gomatrixserverlib.CrossSigningKeyPurpose]gomatrixserverlib.CrossSigningKey{}
+	for purpose, key := range keyMap {
+		keyID := gomatrixserverlib.KeyID("ed25519:" + key.Encode())
+		result := gomatrixserverlib.CrossSigningKey{
+			UserID: userID,
+			Usage:  []gomatrixserverlib.CrossSigningKeyPurpose{purpose},
+			Keys: map[gomatrixserverlib.KeyID]gomatrixserverlib.Base64Bytes{
+				keyID: key,
+			},
+		}
+		sigMap, err := d.CrossSigningSigsTable.SelectCrossSigningSigsForTarget(ctx, nil, userID, keyID)
+		if err != nil {
+			continue
+		}
+		for sigUserID, forSigUserID := range sigMap {
+			if userID != sigUserID {
+				continue
+			}
+			if result.Signatures == nil {
+				result.Signatures = map[string]map[gomatrixserverlib.KeyID]gomatrixserverlib.Base64Bytes{}
+			}
+			if _, ok := result.Signatures[sigUserID]; !ok {
+				result.Signatures[sigUserID] = map[gomatrixserverlib.KeyID]gomatrixserverlib.Base64Bytes{}
+			}
+			for sigKeyID, sigBytes := range forSigUserID {
+				result.Signatures[sigUserID][sigKeyID] = sigBytes
+			}
+		}
+		results[purpose] = result
+	}
+	return results, nil
+}
+
+// CrossSigningKeysForUser returns the latest known cross-signing keys for a user, if any.
+func (d *Database) CrossSigningKeysDataForUser(ctx context.Context, userID string) (types.CrossSigningKeyMap, error) {
 	return d.CrossSigningKeysTable.SelectCrossSigningKeysForUser(ctx, nil, userID)
 }
 
diff --git a/syncapi/internal/keychange_test.go b/syncapi/internal/keychange_test.go
index 38be34f6..0c567a96 100644
--- a/syncapi/internal/keychange_test.go
+++ b/syncapi/internal/keychange_test.go
@@ -49,6 +49,8 @@ func (k *mockKeyAPI) QueryDeviceMessages(ctx context.Context, req *keyapi.QueryD
 }
 func (k *mockKeyAPI) InputDeviceListUpdate(ctx context.Context, req *keyapi.InputDeviceListUpdateRequest, res *keyapi.InputDeviceListUpdateResponse) {
 
+}
+func (k *mockKeyAPI) QuerySignatures(ctx context.Context, req *keyapi.QuerySignaturesRequest, res *keyapi.QuerySignaturesResponse) {
 }
 
 type mockRoomserverAPI struct {