I believe what you're looking for is the ability to Obfuscate Values so that the users just see a masked field of gibberish, unless they have the explicit password to unlock it. There are a couple pages that review the process, which is slightly different depending on the version of Workbench you're running.
Encrypting, Decrypting, and Hashing Workbench 5 Data
Encrypting and Decrypting Workbench 4 Data
It's a nice feature, but its biggest shortcoming is that a user with the permission to unlock/unmask the protected data can only view that unmasked data in Sumo cards. You can't use the unmasked data in visualizations.