To may handle the REST API of Themis, the mechanism of authentication offers to use tokens of access that you may generate in the "Administration" part of Themis, then by selecting the "Key of API" menu.
You arrive then on a page allowing you, by clicking on the icon in the form of a key, to generate a key and to copy/paste it in the clipboard. You may generate as much keys as you wish, but keep well your keys once those generated. Indeed, it is not possible to list the set of keys.
It is not possible to revoke keys, those having a limited duration.