15. Secure digital signature.- If, by application of a security procedure agreed to by the parties concerned, it can be verified that a digital signature, at the time it was affixed, was (a) unique to the subscriber affixing it;

(b) capable of identifying such subscriber;

(c) created in a manner or using a means under the exclusive control of the subscriber and is linked to the electronic record to which related in such a manner that if the electronic record was altered the digital signature would be invalidated, then such digital signature shall be deemed to be a secure digital signature.