return a RecipientKeyIdentifier object from a tagged object.
Parameters: _ato - the tagged object holding the object we want. Parameters: _explicit - true if the object is meant to be explicitlytagged false otherwise. exception: IllegalArgumentException - if the object held by thetagged object cannot be converted.
return a RecipientKeyIdentifier object from the given object.
Parameters: _obj - the object we want converted. exception: IllegalArgumentException - if the object cannot be converted.