Digital Time-stamping to Support Non Repudiation in Electronic Communications

Fernando Pinto, Vasco Freitas

Universidade do Minho
Departamento de Informática
P-4700-320 Braga, Portugal

Tel.: +351 253 604475
Fax.: +351 253 604471
Email: {fernando,vf} (at)


Public key digital signatures may constitute a basis for non repudiation of electronic transactions if it can be guaranteed that they were created at an epoch of time when the credentials were valid. In fact, a private key may be compromised and therefore the digital signature for itself is not sufficient to warrant non repudiation. In order to repudiate a signature, a malicious signer may even claim that his credentials were already compromised when the signature was issued. Additionally, the lifetime of credentials is limited for other reasons, such as, certificates having a certain period of validity, the eventual change of the signer affiliation, etc.

At some instants in time, signatures must be checked for authenticity. At those moments the verifier tries to validate the credentials used for signing a piece of information by verifying certificates and revocation lists related to the public key of the signer. It is then and not at a latter time that non repudiation is guaranteed.

In this paper a proposal for a digital time-stamping service is presented together with a model for its integration in a secure communications environment with special incidence on its application in the provision of non repudiation services for electronic mail.

A protocol has been specified which supports both anonymous and authenticated requests for digital time-stamps for documents and also requests for information that can be used to solve disputes on the ordering of such documents and provides assurance of the well behavior of the service. The anonymous service is used for the automatic request of time-stamps. In this case, where there is no human intervention, it may not be desirable that the applications have access to the user credentials.

The mechanisms used to guarantee the correct operation of the time-stamping service are emphasized and some results obtained from the operation of a prototype server which was developed for this service are presented. Some considerations concerning the possible distribution of the time-stamping service are also made.