OpenStack Identity supports customizable token providers. This is
specified in the [token] section of the configuration file.
The token provider controls the token construction, validation, and
revocation operations.

You can register your own token provider by configuring the following
property:

• provider – token provider driver. Defaults to
  fernet. Implemented by keystone.token.providers.fernet.Provider. This is
  the entry point for the token provider in the
  keystone.token.provider namespace.

Below is the detailed list of the token formats supported by
keystone.:

Fernet

fernet tokens do not need to be persisted at all, but
require that you run keystone-manage fernet_setup (also see
the keystone-manage fernet_rotate command).

JWS

jws tokens do not need to be persisted at all, but
require that you configure an asymmetric key pair to sign and validate
tokens. The key pair can be generated using
keystone-manage create_jws_keypair or it can be generated
out-of-band manually so long as it is compatible with the JWT
ES256 Elliptic Curve Digital Signature Algorithm (ECDSA)
using a P-256 curve and a SHA-256 hash algorithm.