diff --git a/client.go b/client.go
index 8120c7dd..7fcfec03 100644
--- a/client.go
+++ b/client.go
@@ -1105,6 +1105,12 @@ func (cli *Client) UploadCrossSigningKeys(keys *UploadCrossSigningKeysReq, uiAut
 	return err
 }
 
+func (cli *Client) UploadSignatures(req *ReqUploadSignatures) (resp *RespUploadSignatures, err error) {
+	urlPath := cli.BuildBaseURL("_matrix", "client", "unstable", "keys", "signatures", "upload")
+	_, err = cli.MakeRequest("POST", urlPath, req, &resp)
+	return
+}
+
 // GetPushRules returns the push notification rules for the global scope.
 func (cli *Client) GetPushRules() (*pushrules.PushRuleset, error) {
 	return cli.GetScopedPushRules("global")
diff --git a/crypto/cross_sign.go b/crypto/cross_sign.go
index 3d175d39..bcb60125 100644
--- a/crypto/cross_sign.go
+++ b/crypto/cross_sign.go
@@ -7,6 +7,7 @@
 package crypto
 
 import (
+	"github.com/pkg/errors"
 	"maunium.net/go/mautrix"
 	"maunium.net/go/mautrix/crypto/olm"
 	"maunium.net/go/mautrix/id"
@@ -200,3 +201,65 @@ func (mach *OlmMachine) uploadCrossSigningKeysToServer(keys *CrossSigningKeysCac
 		}
 	})
 }
+
+// SignUserAndUpload creates a cross-signing signature for a user, stores it and uploads it to the server.
+func (mach *OlmMachine) SignUserAndUpload(userID id.UserID) error {
+	if mach.crossSigningKeys == nil {
+		return errors.New("No cross-signing keys found")
+	}
+	if userID == mach.Client.UserID {
+		return nil
+	}
+
+	keys, err := mach.CryptoStore.GetCrossSigningKeys(userID)
+	if err != nil {
+		return err
+	}
+	masterKey, ok := keys[id.XSUsageMaster]
+	if !ok {
+		return errors.Errorf("No master key found for user %v", userID)
+	}
+
+	userSigningKey := mach.crossSigningKeys.UserSigningKey
+	masterKeyObj := mautrix.ReqKeysSignatures{
+		UserID: userID,
+		Usage:  []id.CrossSigningUsage{id.XSUsageMaster},
+		Keys: map[id.KeyID]id.Ed25519{
+			id.NewKeyID(id.KeyAlgorithmEd25519, masterKey.String()): masterKey,
+		},
+	}
+	signature, err := userSigningKey.SignJSON(masterKeyObj)
+	if err != nil {
+		return err
+	}
+	masterKeyObj.Signatures = mautrix.Signatures{
+		mach.Client.UserID: map[id.KeyID]string{
+			id.NewKeyID(id.KeyAlgorithmEd25519, userSigningKey.PublicKey.String()): signature,
+		},
+	}
+	mach.Log.Trace("Signed master key for user %v: `%v`", userID, signature)
+
+	resp, err := mach.Client.UploadSignatures(&mautrix.ReqUploadSignatures{
+		userID: map[string]mautrix.ReqKeysSignatures{
+			masterKey.String(): masterKeyObj,
+		},
+	})
+
+	if err != nil {
+		return err
+	}
+	if len(resp.Failures) > 0 {
+		return errors.Errorf("Key uploading failures: %v", resp.Failures)
+	}
+
+	if err := mach.CryptoStore.PutSignature(userID, masterKey, mach.Client.UserID, userSigningKey.PublicKey, signature); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// SignDeviceAndUpload creates a cross-signing signature for a device, stores it and uploads it to the server.
+func (mach *OlmMachine) SignDeviceAndUpload(device *DeviceIdentity) error {
+	return nil
+}
diff --git a/crypto/sql_store_upgrade/upgrade.go b/crypto/sql_store_upgrade/upgrade.go
index 6fb6ada5..a4bd2cdd 100644
--- a/crypto/sql_store_upgrade/upgrade.go
+++ b/crypto/sql_store_upgrade/upgrade.go
@@ -211,7 +211,7 @@ var Upgrades = [...]upgradeFunc{
 		if _, err := tx.Exec(
 			`CREATE TABLE IF NOT EXISTS crypto_cross_signing_keys (
 				user_id VARCHAR(255) NOT NULL,
-				usage   CHAR(20)     NOT NULL,
+				usage   VARCHAR(20)  NOT NULL,
 				key     CHAR(43)     NOT NULL,
 				PRIMARY KEY (user_id, usage)
 			)`,
diff --git a/requests.go b/requests.go
index 4cc5bd8a..520b301a 100644
--- a/requests.go
+++ b/requests.go
@@ -181,12 +181,12 @@ type ReqUploadKeys struct {
 }
 
 type ReqKeysSignatures struct {
-	UserID     id.UserID              `json:"user_id"`
-	DeviceID   id.DeviceID            `json:"device_id,omitempty"`
-	Algorithms []id.Algorithm         `json:"algorithms,omitempty"`
-	Usage      []id.CrossSigningUsage `json:"usage,omitempty"`
-	Keys       KeyMap                 `json:"keys"`
-	Signatures Signatures             `json:"signatures"`
+	UserID     id.UserID               `json:"user_id"`
+	DeviceID   id.DeviceID             `json:"device_id,omitempty"`
+	Algorithms []id.Algorithm          `json:"algorithms,omitempty"`
+	Usage      []id.CrossSigningUsage  `json:"usage,omitempty"`
+	Keys       map[id.KeyID]id.Ed25519 `json:"keys"`
+	Signatures Signatures              `json:"signatures"`
 }
 
 type ReqUploadSignatures map[id.UserID]map[string]ReqKeysSignatures