[Name] Sign Key Reference Mechanism [Default Value] direct [Description] 
Mechanism used when signing the request. Valid values include: direct—X.509 Token is included in the request. ski—Subject Key Identifier (SKI) extension value of the X.509 certificate used to reference the certificate. (Some certificates may not have this extension.) The recipient of the message looks up its keystore for a certificate corresponding to the SKI and validates the signature against it. issuerserial—Composite key of issuer name and serial number attributes used to reference the X.509 certificate. The recipient of the message looks up its keystore for a certificate corresponding to Issuer name and Serial Number and validates the signature using it.