The Cryptographic Implementations group follows the mathematical tradition of alphabetizing author lists. The order of authors says nothing about who was "lead" author. In particular, the fact that Bernstein is almost always first author does not mean that he is "lead" author; it simply means that his name is very early in the alphabet.
Most good multi-author papers in mathematics and in computer science are truly joint work: in the words of the American Mathematical Society, "the ideas grow from complex discussions among all partners". (See the 2004 statement "The Culture of Research and Scholarship in Mathematics: Joint Research and Its Publication".) For these papers the concept of a "lead" author does not even make sense.
Listing someone as an author is stating that the author made an important contribution to the work. Honorary authorship is prohibited. This does not mean that all listed authors participated in writing the paper; writing the paper is only one part of the work.
On occasion it is clear that an individual author is responsible for an exceptionally important contribution to a joint paper. There is no prohibition on making this fact public, either in the paper itself or in other venues; and senior authors are expected to report junior authors' contributions in detailed recommendation letters. However, reordering an author list is a low-bandwidth, noisy, superficial, error-prone method of attempting to communicate this information.
Cryptographic Implementations personnel are not prohibited from entering into collaborations with other groups that require authors to be listed in decreasing order of contribution. In such cases the order should be settled as a first step in paper-writing. However, honorary authorship is prohibited even in such cases.
In recognition of the fact that author lists are often alphabetized, Cryptographic Implementations personnel are required to cite the full author lists for all papers; e.g., citing "Bernstein et al." is prohibited. This requirement applies to all citations (bibliography entries, textual credits, parenthetical notes, etc.), whether in papers or in slides. This requirement also applies to joint papers with other groups.
Version: This is version 2014.06.01 of the Authorship policies web page.