with an average price of US$6. Public-domain software with source code was also shared on BBS networks. Public-domain software was commercialized sometimes by a donationware model, asking the users for a financial donation to be sent by mail. The public-domain "free sharing" and donationware commercialization models evolved in the following years to the (non-voluntary) shareware model, and software free of charge, called freeware. Additionally, due to other changes in the computer industry, the sharing of source code became less common. With the Berne Convention Implementation Act of 1988 (and the earlier Copyright Act of 1976), the legal basis for public-domain software changed drastically. Before the act, releasing software without a copyright notice was enough to dedicate it to the public domain. With the new act, software was by default copyright-protected and needed an explicit waiver statement or license from the author. Reference implementations of algorithms, often cryptographic meant or applied for standardization are still often released into the public domain; examples include CERN httpd in 1993 and Serpent cipher in 1999. The Openwall Project maintains a list of several algorithms and their source code in the public domain. Free and open-source software as successor As a response of the academic software ecosystem to the change in the copyright system in the late 1980s, permissive license texts were developed, like the BSD license and its derivatives. Permissive-licensed software, which is a kind of free and open-source software, shares most characteristics of earlier public-domain software but stands on the legal basis of copyright law. In the 1980s Richard Stallman, who for long worked in an academic environment of "public-domain"-like software sharing, noticed the emergence of proprietary software and the decline of the public-domain software ecosystem. In an effort to preserve this ecosystem he created a software license, the GPL, which encodes the public-domain rights and enforces them irrevocably on software. Paradoxically, his copyleft approach relies on the enforceability of the copyright to be effective. Copyleft free software, therefore, shares many properties with public-domain software, but does not allow relicensing or sublicensing. Unlike real public-domain software or permissive-licensed software, Stallman's copyleft license tries to enforce the free shareability of software also for the future by not allowing license changes. To refer to free software (which is under a free software license) or to software distributed and usable free of charge (freeware) as "public-domain" is therefore incorrect. While public domain gives up the author's