From: Tim Dierks <timd@consensus.com>
Date: Mon, 20 Jan 1997 11:13:00 -0800 (PST)
To: Amanda Walker <amanda@intercon.com>
Subject: Re: Question on Diffie-Hellman patent
Message-ID: <199701201913.LAA15649@toad.com>
MIME-Version: 1.0
Content-Type: text/plain


At 11:24 AM -0500 1/20/97, Amanda Walker wrote:
>Please excuse if this is a FAQ.
>
>I'd been operating under the assumption that the Diffie-Hellman patent
>(4200770) will expire on April 29, 1997 (17 years after the date of issue).
>Several people have recently told me it doesn't expire until September,
>evidently confusing the date of issue with the filing date.  However,
>does anyone know of another patent (Hellman-Merkle, perhaps?) that would hve
>an impact on software that does D-H key exchange?  We've got some planned
>uses
>of DH; while it doesn't seem cost effective to pay to license DH at this
>point, that six-month difference is something I need to plan around if
>necessary.

As a result of the GATT treaty, patent expiration times for some US patents
changed. The international standard is 20 years from filing, while old US
law was 17 years from issue. On the day which GATT took force for the US
(June 8, 1995), all outstanding patents' expiration dates were changed to
20 years from filing or 17 years from issue, whichever is later. This
affects some cryptography patents as follows:

Here's a summary of some relevant crypto patents:

 Name             Number     Filed          Issued          Expires

Diffie-Hellman   4,200,770  Sept. 6, 1977  Apr. 29, 1980   Sept. 6, 1997
Hellman-Merkle   4,218,582  Oct. 6, 1977   Aug. 19, 1980   Oct. 6, 1997
RSA              4,405,829  Dec. 14, 1977  Sept. 20, 1983  Sept. 20, 2000

Data from <http://patents.cnidr.org/patbib_index.html>

References:
	<http://www.lightlink.com/bbm/gatt.html>
	<http://www.cushman.com/articles/artpat10.htm>

 - Tim

Tim Dierks - timd@consensus.com - www.consensus.com
     Software Haruspex - Consensus Development
  Developer of SSL Plus: SSL 3.0 Integration Suite