S 5.67 Use of a time stamp service

Initiation responsibility: Head of IT Section, IT Security Management

Implementation responsibility: Administrators, users

It is relatively easy to manipulate the time information entered in the header of an e-mail. If it is necessary to know the precise time when an e-mail has been sent or received, a time stamp service must be used. A time stamp is a time entry which is made by a neutral body, and which cannot be corrupted. It is applied either fully automatically by a time stamp server, i.e. transparently for the user, or on request by the sender.

A time stamp consists of a time stamp certificate in which the current date and the current time are documented together with the identity of the time stamp service itself, and of a digital signature of e-mail and certificate. In this way the time stamp provides evidence and confirmation of the existence of a certain message with certain contents at a certain time. The assurance of the authenticity of an e-mail by a time stamp presupposes that the sender has digitally signed the e-mail.

A time stamp service can be provided and used both in an internal network and on the Internet. It receives signed files, or even only the signatures from those files, as a server on the Internet or in the intranet, and provides them with a synchronised time stamp. In turn, all of this together is signed by the time stamp service, and either forwarded to the recipient or alternatively also sent back to the sender.