Automatic renewal system extends life for electronic signatures

A new mechanism for re-signing electronic signatures promises to increase the number of years data and images can be safely stored, according to German researchers.

Current legislation in most European Union countries requires that digital images be stored securely for up to 30 years for liability purposes, necessitating the development of procedures for re-signing electronic signatures.

Currently, electronic signatures normally expire after only five years.

"This is the challenge archive providers face," said Peter Pharow, Ph.D., of the department of health informatics and health telematics at University Hospital Magdeburg in Germany.

Pharow proposes a re-signing mechanism that addresses this issue, in the process guaranteeing accessibility, integrity, accountability, and availability of the data over long periods (Int J Med Inform 2005;74(2-4):279-287).

The five-year electronic signature lifetime is not related so much to the cryptographic algorithm itself as it is to the likelihood that new methods of attack will have been developed.

Before the official five-year expiration date, data items stored in electronic archives must therefore be re-signed in order to ensure security, Pharow said.

Two options are available to perform re-signing:

• File content must be unwrapped, or decrypted, raising confidentiality issues. The content is then rewrapped and re-signed using a new encryption key.

• File content is used in its wrapped (encrypted) form, and a new signature is added to existing signatures. Both the new and old signature keys are archived in this case.

Generally, the second method is preferable since it means that the content remains unchanged, confidentiality is not compromised, and another valid signature is merely added as a new shell.

"The advantage of this procedure is obvious," Pharow said. "The electronic signature of the originator remains unchanged, so origination of medical content can be proved even decades later."

The only problem with this technique is the need to keep all signature certificates in a special part of the directory tree even if they are eligible for revocation after expiration. A mechanism to address this issue has been defined in the standardized multipart, multipurpose Internet mail extensions approach that allows files to carry multiple types of data as attachments, Pharow said.