I do not believe their is a way to grant field permissions on a granular basis with SharePoint or InfoPath without digital signatures. However digital signatures are a bit overkill and probably would be tough to manage.
You can use the Database Accelerator Web Services Suite to have this granular control. Qdabra Database Accelerator Suite Version 2.0 - Developer
You can use some active directory integration to identify the user and their group membership and then have rules which allows edit of fields or sections. AD integration is included with Database Accelerator or you can build your own.