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 it   relates 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.