add helper for local user subject

idp/dex/provider.go

@@ -41,6 +41,8 @@ type Config struct {
 	GRPCAddr string
 }
 
+const localConnectorID = "local"
+
 // Provider wraps a Dex server
 type Provider struct {
 	config       *Config
@@ -544,7 +546,7 @@ func (p *Provider) CreateUser(ctx context.Context, email, username, password str
 
 	// Encode the user ID in Dex's format: base64(protobuf{user_id, connector_id})
 	// This matches the format Dex uses in JWT tokens
-	encodedID := EncodeDexUserID(userID, "local")
+	encodedID := EncodeDexUserID(userID, localConnectorID)
 	return encodedID, nil
 }
 
@@ -619,6 +621,13 @@ func DecodeDexUserID(encodedID string) (userID, connectorID string, err error) {
 	return userID, connectorID, nil
 }
 
+// IsLocalUserID reports whether encodedID is a Dex subject for the built-in
+// local password connector.
+func IsLocalUserID(encodedID string) bool {
+	_, connectorID, err := DecodeDexUserID(encodedID)
+	return err == nil && connectorID == localConnectorID
+}
+
 // GetUser returns a user by email
 func (p *Provider) GetUser(ctx context.Context, email string) (storage.Password, error) {
 	return p.storage.GetPassword(ctx, email)

idp/dex/provider_test.go

@@ -115,6 +115,26 @@ func TestDecodeDexUserID(t *testing.T) {
 	}
 }
 
+func TestIsLocalUserID(t *testing.T) {
+	tests := []struct {
+		name      string
+		encodedID string
+		want      bool
+	}{
+		{name: "local connector", encodedID: EncodeDexUserID("7aad8c05-3287-473f-b42a-365504bf25e7", "local"), want: true},
+		{name: "federated connector", encodedID: EncodeDexUserID("entra-user", "entra"), want: false},
+		{name: "non-dex external IdP id", encodedID: "google-oauth2|1234567890", want: false},
+		{name: "invalid base64", encodedID: "not-valid-base64!!!", want: false},
+		{name: "empty", encodedID: "", want: false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.want, IsLocalUserID(tt.encodedID))
+		})
+	}
+}
+
 func TestEncodeDexUserID(t *testing.T) {
 	userID := "7aad8c05-3287-473f-b42a-365504bf25e7"
 	connectorID := "local"