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+% $Id$
+%
+\documentclass[11pt]{article}
+\usepackage[pdftex]{graphicx}
+\usepackage{palatino}
+\usepackage{color}
+\DeclareGraphicsExtensions{.pdf}
+\setlength{\topmargin}{0in}
+\setlength{\headheight}{0in}
+\setlength{\headsep}{0in}
+\setlength{\oddsidemargin}{0in}
+\setlength{\evensidemargin}{0in}
+\setlength{\textheight}{9in}
+\setlength{\textwidth}{6.5in}
+\setlength{\parskip}{2ex}
+\setlength{\parindent}{0in}
+\hyphenpenalty=5000
+\tolerance=1000
+\hbadness=9999
+\sloppy
+\pagestyle{plain}
+%
+\begin{document}
+
+\section*{Implementation experience with the two BPKI models}
+
+Having recently converted my code base to the single-trust-anchor
+model Russ recommended, I thought it might be useful to share what
+I've learned.  This may not apply to all implementations, but it does
+apply to mine, and given what I understand of RIPE's business model,
+it will probably apply to RIPE's implementation as well.
+
+In spite of a strong desire to do so, I was not able to use exactly
+the same BPKI keys and certificates for HTTPS and CMS.  The reason for
+this is simple: each hosted entity in my engine has its own BPKI, as
+does the hosting entity, but the HTTPS listener is shared.  The only
+ways I know of to avoid this would be to use separate listeners for
+each hosted entity, which scales poorly, or to rely on the TLS
+``Server Name Indication'' extension (RFC 4366 3.1) which is not yet
+widely implemented.
+
+\begin{figure}[hbp]
+\includegraphics[width = 6.5in]{bpki-symmetric}
+\caption{Symmetric BPKI model}
+\label{bpki-symmetric}
+\end{figure}
+
+Figure \ref{bpki-symmetric} shows my engine's view of the BPKI tree in
+the symmetric model.  Black objects belong to the hosting entity, blue
+objects belong to the hosted entities, red objects are cross-certified
+objects from peers.  The arrows indicate certificate issuance: solid
+arrows are the ones that my own RPKI engine will care about during
+certificate validation, dotted arrows show the origin of EE
+certificates my engine uses to sign things.  ``BSC'' stands for
+``business signing context,'' which is a database object in my
+implementation representing the context needed to sign a CMS message
+or TLS session.
+
+Other than the above-mentioned annoyance with the HTTPS server
+certificate, the ``symmetric'' BPKI model worked out pretty much as
+expected here.  The certificate tree looks complicated, but the set of
+certificates needed to build a particular validation chain is obvious,
+again excepting the HTTPS server case, where client certificate is the
+first hint that the engine has of the client's identity, so the server
+must be prepared to accept any current client certificate.
+
+\begin{figure}[hbp]
+\includegraphics[width = 6.5in]{bpki-asymmetric}
+\caption{Asymmetric BPKI model}
+\label{bpki-asymmetric}
+\end{figure}
+
+Figure \ref{bpki-asymmetric} shows my engine's view of the BPKI tree
+in the asymmetric model.  Note that not much has changed here from the
+symmetric case.  As far as I can tell, the asymmetric model is just as
+complex for my engine as the symmetric model; the only real difference
+is that the engine has to keep track of a larger number of BSC EE
+certificates in the asymmetric case.
+
+\end{document}