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+#+COLUMNS: %40ITEM %10BEAMER_env(Env) %9BEAMER_envargs(Env Args) %10BEAMER_act(Act) %4BEAMER_col(Col) %10BEAMER_extra(Extra) %8BEAMER_opt(Opt)
+#
+# Software Heritage PIDs: here we present our rationale for introducing a new identifier schema, and the identifier schema itself
+#
+#+INCLUDE: "prelude.org" :minlevel 1
+#
+# We need tcolorbox here: add the following lines to your main .org document!
+#
+#+LATEX_HEADER: \usepackage{tcolorbox}
+#+BEAMER_HEADER: \usepackage{tcolorbox}
+
+* The quest for a PID
+  :PROPERTIES:
+  :CUSTOM_ID: main
+  :END:
+** Systems of identifiers
+  :PROPERTIES:
+  :CUSTOM_ID: definition
+  :END:
+*** A /system of identifiers/ is
+    - a set of labels (the identifiers)
+    - mechanisms to perform :
+   |------------------------+---------------------------|
+   | /Generation (minting)/ | create a new label        |
+   | /Assignment/           | associate label to object |
+   | /Retrieval/            | get object from a label   |
+   |------------------------+---------------------------|
+    - optionally, mechanisms to perform:
+   |------------------+---------------------------|
+   | /Verification/   | check label and object    |
+   | /Reverse Lookup/ | get label from an object  |
+   | /Description/    | get metadata of an object |
+   |------------------+---------------------------|
+** Mechanisms offered in some systems of identifiers
+  :PROPERTIES:
+  :CUSTOM_ID: survey
+  :END:
+   |--------------------+----------+-------+-------+--------|
+   | *Mech.* / *System* | *Handle* | *DOI* | *Ark* | *PURL* |
+   |--------------------+----------+-------+-------+--------|
+   | Generation         | Yes      | Yes   | Yes   | Yes    |
+   | Assignment         | Yes      | Yes   | Yes   | Yes    |
+   | Retrieval          | Yes      | Yes   | Yes   | Yes    |
+   | Verification       | N.A.     | N.A.  | N.A.  | N.A.   |
+   | Reverse Lookup     | N.A.     | N.A.  | N.A.  | N.A.   |
+   | Description        | Yes      | Yes   | Yes   | N.A.   |
+   |--------------------+----------+-------+-------+--------|
+** Our challenges in the PID landscape
+  :PROPERTIES:
+  :CUSTOM_ID: challenges
+  :END:
+*** Typical properties of systems of identifiers
+   \hfill uniqueness, non ambiguity, persistence, abstraction (opacity)
+*** Key needed properties from our use cases
+   - gratis :: identifiers are free (billions of objects)
+   - integrity :: the associated object cannot been changed (sw dev, reproducibility)
+   - no middle man :: no central authority is needed  (sw dev, reproducibility)
+*** 
+   \hfill we could not find systems with both *integrity* and *no middle man* !
+** An important distinction: DIOs vs. IDOs
+  :PROPERTIES:
+  :CUSTOM_ID: diovsido
+  :END:
+#+BEGIN_EXPORT latex
+  \begin{quote}
+    The term “Digital Object Identifier” is construed as “digital identifier of an object," rather than “identifier of a digital object” \hfill Norman Paskin. 2010
+  \end{quote}
+#+END_EXPORT
+*** DIO (Digital Identifier of an Object)
+    digital identifiers for (potentially) *non digital objects*
+      - epistemic complexity (manifestations, versions, locations, etc.)
+      - need an authority to ensure persistence and uniqueness
+#+BEAMER: \pause
+*** IDO (Identifier of a Digital Object)
+    digital identifiers (only) for *digital objects*
+      - can provide both *integrity* and *no middle man*
+      - broadly used in modern software development (git, etc.)
+*** 
+   \hfill for the core Software Heritage archive, IDOs are enough
+** IDOs in Software Development: the origins
+   # R. C. Merkle, A digital signature based on a conventional encryption
+   # function, Crypto '87
+   #+BEAMER: \vspace{-3mm}
+***** Merkle tree (R. C. Merkle, Crypto 1979)                    :B_picblock:
+      :PROPERTIES:
+      :BEAMER_opt: pic=merkle, leftpic=true, width=.5\linewidth
+      :BEAMER_env: picblock
+      :BEAMER_act:
+      :END:
+      Combination of 
+       - tree
+       - hash function
+***** Classical cryptographic construction
+      fast, parallel signature of large data structures, built-in deduplication
+#+BEAMER: \pause
+      - satisfies all three criteria
+      - widely used in industry (e.g., Git, nix, blockchains, IPFS, ...)
+
+** IDOs in Software Heritage: a worked example
+     #+LATEX: \centering
+     #+LATEX: \only<1>{\colorbox{white}{\includegraphics[width=.75\linewidth]{git-merkle/merkle_1}}}
+     #+LATEX: \only<2>{\colorbox{white}{\includegraphics[width=.75\linewidth]{git-merkle/contents}}}
+     #+LATEX: \only<3>{\colorbox{white}{\includegraphics[width=.75\linewidth]{git-merkle/merkle_2_contents}}}
+     #+LATEX: \only<4>{\colorbox{white}{\includegraphics[width=.75\linewidth]{git-merkle/directories}}}
+     #+LATEX: \only<5>{\colorbox{white}{\includegraphics[width=.75\linewidth]{git-merkle/merkle_3_directories}}}
+     #+LATEX: \only<6>{\colorbox{white}{\includegraphics[width=.75\linewidth]{git-merkle/revisions}}}
+     #+LATEX: \only<7>{\colorbox{white}{\includegraphics[width=.75\linewidth]{git-merkle/merkle_4_revisions}}}
+     #+LATEX: \only<8>{\colorbox{white}{\includegraphics[width=.75\linewidth]{git-merkle/releases}}}
+     #+LATEX: \only<9>{\colorbox{white}{\includegraphics[width=.75\linewidth]{git-merkle/merkle_5_releases}}}
+** The Software Heritage IDO schema \hfill (see *\url{http://bit.ly/swhpids}*)
+#+BEGIN_EXPORT latex
+\small
+\begin{tcolorbox}
+\href{https://archive.softwareheritage.org/swh:1:cnt:94a9ed024d3859793618152ea559a168bbcbb5e2}
+{swh:1:{\bf cnt}:94a9ed024d3859793618152ea559a168bbcbb5e2} \hfill full text of the GPL3 license
+\end{tcolorbox}
+\pause
+\begin{tcolorbox}
+\href{https://archive.softwareheritage.org/swh:1:dir:d198bc9d7a6bcf6db04f476d29314f157507d505}
+{swh:1:{\bf dir}:d198bc9d7a6bcf6db04f476d29314f157507d505} \hfill Darktable source code
+\end{tcolorbox}
+\pause
+\begin{tcolorbox}
+\href{https://archive.softwareheritage.org/swh:1:rev:309cf2674ee7a0749978cf8265ab91a60aea0f7d}
+{swh:1:{\bf rev}:309cf2674ee7a0749978cf8265ab91a60aea0f7d}
+\end{tcolorbox}
+\hfill a {\bf revision} in the development history of Darktable\\\pause
+\begin{tcolorbox}
+\href{https://archive.softwareheritage.org/swh:1:rel:22ece559cc7cc2364edc5e5593d63ae8bd229f9f}
+{swh:1:{\bf rel}:22ece559cc7cc2364edc5e5593d63ae8bd229f9f}
+\end{tcolorbox}
+\hfill {\bf release} 2.3.0 of Darktable, dated 24 December 2016\\\pause
+\begin{tcolorbox}
+\href{https://archive.softwareheritage.org/swh:1:snp:c7c108084bc0bf3d81436bf980b46e98bd338453}
+{swh:1:{\bf snp}:c7c108084bc0bf3d81436bf980b46e98bd338453}
+\end{tcolorbox}
+\hfill a {\bf snapshot} of the entire Darktable repository (4 May 2017, GitHub)
+#+END_EXPORT
+#+LATEX: \pause
+*** 
+   *Current resolvers:* \url{archive.softwareheritage.org} and \url{n2t.org}