A method and apparatus in a public crypto system, control the use of a public key, based on the level of import integrity for the public key. The method and apparatus generate a control vector associated with the public key, having a history field. The public key and the control vector are transmitted from the location of generation over a communications link to a receiving location, using the selected one of a plurality of levels of import integrity for the transmission. At the receiving location, the public key and the control vector are tested to determine the actual level of import integrity for the transmission. Then, a value is written into the history field of the control vector which characterizes the actual level of import integrity. Thereafter, cryptographic applications for the public key are limited by control vector checking, to only those applications which have a required level of integrity which is not greater than the actual level of import integrity characterized by the history field in the control vector.