There really isn't a workable way to store an encrypted version of a field in the repository. If you have a clear set of security requirements, you should be able to satisfy them with a combination of:

1. Disk encryption for the MSSQL data drive
2. TLS for all connections
3. Limiting access to the database
4. Security at the metadata field level - only grant read rights as necessary to sensitive fields
5. Security at the document level - only grant document read rights as necessary