I have written a draft proposal on S-expressions, and some C-code that supports the proposed canonical, transport, and advanced formats for S-expressions. This material is available at http://theory.lcs.mit.edu/~rivest/sexp.html This is just on S-expressions, independent of its application to SPKI/SDSI. (No promises on the portability of the code; let me know if you have problems with it.) Ron Rivest