Digital Signature 13 “subscriber” means a person who— (a) is the subject listed in a certificate; (b) accepts the certificate; and (c) holds a private key which corresponds to a public key listed in that certificate; “suspend a certificate” means to make a certificate ineffective temporarily for a specified time forward; “this Act” includes any regulations made under this Act; “time-stamp” means— (a) to append or attach to a message, digital signature or certificate a digitally signed notation indicating at least the date, time and identity of the person appending or attaching the notation; or (b) the notation so appended or attached; “transactional certificate” means a certificate, incorporating by reference one or more digital signatures, issued and valid for a specific transaction; “trustworthy system” means computer hardware and software which— (a) are reasonably secure from intrusion and misuse; (b) provide a reasonable level of availability, reliability and correct operation; and (c) are reasonably suited to performing their intended functions; “valid certificate” means a certificate which— (a) a licensed certification authority has issued; (b) has been accepted by the subscriber listed in it; (c) has not been revoked or suspended; and (d) has not expired: Provided that a transactional certificate is a valid certificate only in relation to the digital signature incorporated in it by reference;