Enabling per column-qualifier access controls should be fairly straightforward to add to the current state of HBASE-3025.

Currently we store per column-family ACLs in the .META.:acl column family, using keys in the form principal,family. We could extend this for column-qualifier specific assignments to: principal,family,qualifier.

The next step is then enabling authorization checks per column-qualifier, which only becomes tricky in the case of wildcard gets or scans. However, we already have a good user-exposed facility for matching columns to return via filters. So we would create a new AccessControlFilter and inject it (when security is enabled) into user requests via preGet() and preScannerOpen() requests.

AccessControlFilter would then consult TableAuthManager for each KeyValue and filter out those for which access was denied.

In the process, the current logic for rejecting requests on a column-family level would need to change to allow requests from users with specific column qualifier access to continue.

Currently we store per column-family ACLs in the .META.:acl column family, using keys in the form principal,family. We could extend this for column-qualifier specific assignments to: principal,family,qualifier.
The next step is then enabling authorization checks per column-qualifier, which only becomes tricky in the case of wildcard gets or scans. However, we already have a good user-exposed facility for matching columns to return via filters. So we would create a new AccessControlFilter and inject it (when security is enabled) into user requests via preGet() and preScannerOpen() requests.
AccessControlFilter would then consult TableAuthManager for each KeyValue and filter out those for which access was denied.
In the process, the current logic for rejecting requests on a column-family level would need to change to allow requests from users with specific column qualifier access to continue.

I've done all of this in the context of work for HBASE-6222.