diff --git a/lean/commands/library/add.py b/lean/commands/library/add.py
index 7e9dd280..71ecaf21 100644
--- a/lean/commands/library/add.py
+++ b/lean/commands/library/add.py
@@ -303,6 +303,19 @@ def add(project: Path, name: str, version: Optional[str], no_local: bool) -> Non
     library_dir = Path(name).expanduser().resolve()
 
     if library_manager.is_lean_library(library_dir):
+        # check encryption conditions
+        if project_config.get('encrypted', False) and project_config.get('encryption-key-path', None):
+            library_project_config = container.project_config_manager.get_project_config(library_dir)
+            is_library_encrypted = library_project_config.get('encrypted', False)
+            library_encryption_key_path = library_project_config.get('encryption-key-path', None)
+            if library_encryption_key_path:
+                library_encryption_key_path = Path(library_encryption_key_path)
+            project_encryption_key_path = project_config.get('encryption-key-path', None)
+            if project_encryption_key_path:
+                project_encryption_key_path = Path(project_encryption_key_path)
+            if is_library_encrypted and library_encryption_key_path != project_encryption_key_path:
+                raise RuntimeError(f"Library is encrypted with a different key {library_encryption_key_path}. "
+                                   f"Please use the same key as project {project_encryption_key_path}.")
         logger.info(f"Adding Lean CLI library {library_dir} to project {project}")
         if project_language == "CSharp":
             library_manager.add_lean_library_to_csharp_project(project, library_dir, no_local)