These features are not yet implemented. While the Apex Developer Guide doesn't mention these, you can find them in the SOAP documentation:
encrypted - boolean - This page is about Shield Platform Encryption, not Classic Encryption. ... Indicates whether this field is encrypted. This value only appears in the results of a describeSObjects() call when it is true; otherwise, it is omitted from the results. This field is available in API version 31.0 and later.
mask - string - Reserved for future use.
maskType - string - Reserved for future use.
As you can see, these features are not yet available in the current API (v43.0). It will likely be announced in a future release. You should not depend on these values, because they may not work correctly.