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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Felix,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> I had almost this exact conversation with Scott Burleigh and Kevin Fall yesterday.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> The approach to addressing this is to clarify what is meant by Canonical CBOR as part of AUTH48, with a statement of something like the following:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal">“the CBOR representations of the values of all fields in all blocks must conform to the Core Deterministic Encoding Requirements as specified in [RFC8949] except that indefinite-length items are not prohibited."<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> The intent of the BPv7 spec is clearly to allow some indefinite length items, as the spec uses indefinite-length items, and the wording in Section 4.1 is meant
to describe the encoding of fields within block otherwise. The exact wording will be figured out once we have the RFC-editor version of the text to review.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">-Ed<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Edward J. Birrane, III, Ph.D. (he/him/his)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Chief Engineer, Space Constellation Networking<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Space Exploration Sector<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Johns Hopkins Applied Physics Laboratory<br>
(W) 443-778-7423 / (C) 443-690-8272</span><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> SIS-DTN <sis-dtn-bounces@mailman.ccsds.org>
<b>On Behalf Of </b>Felix.Flentge@esa.int<br>
<b>Sent:</b> Thursday, November 18, 2021 7:02 AM<br>
<b>To:</b> sis-dtn@mailman.ccsds.org<br>
<b>Subject:</b> [EXT] [Sis-dtn] Deterministic CBOR encoding<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Dear All,</span>
<br>
<br>
<span style="font-size:10.0pt;font-family:"Arial",sans-serif">I have spend some time this morning to understand the requirements we get for the BPv7 CBOR encoding from draft-ietf-dtn-bpbis-31.txt, RFC 8949, and draft-ietf-dtn-bpsec-27 (actually, I have been
worried about a potential issue with the current BPv7 spec; now I think it is probably ok). Anyway, this is my understanding of the current situation. Please let me know whether you share it or not.</span>
<br>
<br>
<b><span style="font-size:10.0pt;font-family:"Courier New"">draft-ietf-dtn-bpbis-31.txt</span></b><span style="font-size:10.0pt;font-family:"Arial",sans-serif">:</span>
<br>
<br>
<span style="font-size:10.0pt;font-family:"Arial",sans-serif">We have:</span> <br>
<br>
<a href="https://datatracker.ietf.org/doc/html/draft-ietf-dtn-bpbis-31#section-4"><tt><b><span style="font-size:10.0pt">4</span></b></tt></a><tt><b><span style="font-size:10.0pt">. Bundle Format</span></b></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<br>
</span><a href="https://datatracker.ietf.org/doc/html/draft-ietf-dtn-bpbis-31#section-4.1"><tt><b><span style="font-size:10.0pt">4.1</span></b></tt></a><tt><b><span style="font-size:10.0pt">. Bundle Structure</span></b></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<br>
<tt> <b>The format of bundles SHALL conform to the Concise Binary Object</b></tt><b><br>
<tt> Representation (CBOR [</tt></b></span><a href="https://datatracker.ietf.org/doc/html/rfc8949"><tt><b><span style="font-size:10.0pt">RFC8949</span></b></tt></a><tt><b><span style="font-size:10.0pt">]).</span></b></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<br>
<tt> Cryptographic verification of a block is possible only if the</tt><br>
<tt> sequence of octets on which the verifying node computes its hash -</tt><br>
<tt> the canonicalized representation of the block - is identical to the</tt><br>
<tt> sequence of octets on which the hash declared for that block was</tt><br>
<tt> computed. <b>To ensure that blocks are always in canonical</b></tt><b><br>
<tt> representation when they are transmitted and received, the CBOR</tt><br>
<tt> representations of the values of all fields in all blocks must</tt><br>
<tt> conform to the rules for Canonical CBOR as specified in [</tt></b></span><a href="https://datatracker.ietf.org/doc/html/rfc8949"><tt><b><span style="font-size:10.0pt">RFC8949</span></b></tt></a><tt><b><span style="font-size:10.0pt">].</span></b></tt>
<br>
<br>
<span style="font-size:10.0pt;font-family:"Arial",sans-serif"> There are two small issues here:</span>
<br>
<span style="font-size:10.0pt;font-family:"Arial",sans-serif">- Only the first statement is a real requirement for BPv7, the 'must' in the last sentence is not capitalised meaning it is not normative (AFAIK).</span>
<br>
<span style="font-size:10.0pt;font-family:"Arial",sans-serif">- RFC8949 does not really specify 'Canonical CBOR':
</span><br>
<span style="font-size:10.0pt;font-family:"Arial",sans-serif">Section 4.2.3 contains the following note:</span>
<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt"><tt><span style="font-size:10.0pt"> | <b>Although [</b></span></tt><a href="https://datatracker.ietf.org/doc/html/rfc7049#section-3"><tt><b><span style="font-size:10.0pt">RFC7049</span></b></tt></a><tt><b><span style="font-size:10.0pt">]
used the term "Canonical CBOR" for its form</span></b></tt><b><span style="font-size:10.0pt;font-family:"Courier New""><br>
<tt> | of requirements on deterministic encoding, this document avoids</tt><br>
<tt> | this term because "canonicalization" is often associated with</tt><br>
<tt> | specific uses of deterministic encoding only. The terms are</tt><br>
<tt> | essentially interchangeable, however, and the set of core</tt><br>
<tt> | requirements in this document could also be called "Canonical</tt><br>
<tt> | CBOR",</tt></span></b><tt><span style="font-size:10.0pt"> while the length-first-ordered version of that could be</span></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<tt> | called "Old Canonical CBOR".</tt></span><o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">and G.3:</span><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p style="margin:0in;margin-bottom:.0001pt"><tt><span style="font-size:10.0pt"> For a single value in the data model, CBOR often provides multiple</span></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<tt> encoding options. A new section (</tt></span><a href="https://datatracker.ietf.org/doc/html/rfc8949#section-4"><tt><span style="font-size:10.0pt">Section 4</span></tt></a><tt><span style="font-size:10.0pt">) introduces the term</span></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<tt> "preferred serialization" (</tt></span><a href="https://datatracker.ietf.org/doc/html/rfc8949#section-4.1"><tt><span style="font-size:10.0pt">Section 4.1</span></tt></a><tt><span style="font-size:10.0pt">) and defines it for various</span></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<tt> kinds of data items. On the basis of this terminology, the section</tt><br>
<tt> then discusses how a CBOR-based protocol can define "deterministic</tt><br>
<tt> encoding" (</tt></span><a href="https://datatracker.ietf.org/doc/html/rfc8949#section-4.2"><tt><span style="font-size:10.0pt">Section 4.2</span></tt></a><tt><span style="font-size:10.0pt">), which avoids terms "canonical" and</span></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<tt> "canonicalization" from </tt></span><a href="https://datatracker.ietf.org/doc/html/rfc7049#section-3"><tt><span style="font-size:10.0pt">RFC 7049</span></tt></a><tt><span style="font-size:10.0pt">. The suggestion of "Core</span></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<tt> Deterministic Encoding Requirements" (</tt></span><a href="https://datatracker.ietf.org/doc/html/rfc8949#section-4.2.1"><tt><span style="font-size:10.0pt">Section 4.2.1</span></tt></a><tt><span style="font-size:10.0pt">) enables generic</span></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<tt> support for such protocol-defined encoding requirements. This</tt><br>
<tt> document further eases the implementation of deterministic encoding</tt><br>
<tt> by simplifying the map ordering suggested in </tt></span><a href="https://datatracker.ietf.org/doc/html/rfc7049#section-3"><tt><span style="font-size:10.0pt">RFC 7049</span></tt></a><tt><span style="font-size:10.0pt"> to a simple</span></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<tt> lexicographic ordering of encoded keys. A description of the older</tt><br>
<tt> suggestion is kept as an alternative, now termed "length-first map</tt><br>
<tt> key ordering" (</tt></span><a href="https://datatracker.ietf.org/doc/html/rfc8949#section-4.2.3"><tt><span style="font-size:10.0pt">Section 4.2.3</span></tt></a><tt><span style="font-size:10.0pt">).</span></tt><o:p></o:p></p>
<p class="MsoNormal"><br>
<span style="font-size:10.0pt;font-family:"Arial",sans-serif"> Anyway, for our purposes we probably can understand 'rules for Canonical CBOR' as the Core Deterministic Requirements in Section 4.2.1 (at least as long as we don't use maps, see 4.2.3). This view
would be supported by draft-ietf-dtn-bpsec-27: </span><br>
<a href="https://datatracker.ietf.org/doc/html/draft-ietf-dtn-bpsec-27#section-4"><tt><b><span style="font-size:10.0pt">4</span></b></tt></a><tt><b><span style="font-size:10.0pt">. Canonical Forms</span></b></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<br>
<tt> [...]</tt><br>
<br>
<tt> The canonical form of the primary block is as specified in</tt><br>
<tt> [</tt></span><a href="https://datatracker.ietf.org/doc/html/draft-ietf-dtn-bpsec-27#ref-I-D.ietf-dtn-bpbis"><tt><span style="font-size:10.0pt">I-D.ietf-dtn-bpbis</span></tt></a><tt><span style="font-size:10.0pt">] with the following constraint.</span></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<br>
<tt> o <b>CBOR values from the primary block MUST be canonicalized using the</b></tt><b><br>
<tt> rules for Deterministically Encoded CBOR, as specified in</tt><br>
<tt> [</tt></b></span><a href="https://datatracker.ietf.org/doc/html/rfc8949"><tt><b><span style="font-size:10.0pt">RFC8949</span></b></tt></a><tt><b><span style="font-size:10.0pt">].</span></b></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<br>
<tt> All non-primary blocks share the same block structure and are</tt><br>
<tt> canonicalized as specified in [</tt></span><a href="https://datatracker.ietf.org/doc/html/draft-ietf-dtn-bpsec-27#ref-I-D.ietf-dtn-bpbis"><tt><span style="font-size:10.0pt">I-D.ietf-dtn-bpbis</span></tt></a><tt><span style="font-size:10.0pt">] with the
following</span></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<tt> constraints.</tt><br>
<br>
<tt> o <b>CBOR values from the non-primary block MUST be canonicalized using</b></tt><b><br>
<tt> the rules for Deterministically Encoded CBOR, as specified in</tt><br>
<tt> [</tt></b></span><a href="https://datatracker.ietf.org/doc/html/rfc8949"><tt><b><span style="font-size:10.0pt">RFC8949</span></b></tt></a><tt><b><span style="font-size:10.0pt">].</span></b></tt>
<br>
<br>
<span style="font-size:10.0pt;font-family:"Arial",sans-serif">(It would be even better if there would be an explicit reference to Section 4.2.1.)
</span><br>
<br>
<span style="font-size:10.0pt;font-family:"Arial",sans-serif">So, for CCSDS the main questions seems to be whether we are happy enough with the core deterministic requirements in RFC8949, Section 4.2.1, basically:</span>
<o:p></o:p></p>
<p style="margin:0in;margin-bottom:.0001pt"><tt><span style="font-size:10.0pt">* Preferred serialization MUST be used. In particular, this means</span></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<tt> that arguments (see </tt></span><a href="https://datatracker.ietf.org/doc/html/rfc8949#section-3"><tt><span style="font-size:10.0pt">Section 3</span></tt></a><tt><span style="font-size:10.0pt">)
<b>for integers, lengths in major</b></span></tt><b><span style="font-size:10.0pt;font-family:"Courier New""><br>
<tt> types 2 through 5, and tags MUST be as short as possible</tt></span></b><tt><span style="font-size:10.0pt">, for</span></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<tt> instance:</tt><br>
<tt>[...]</tt><br>
<br>
<tt> * <b>Indefinite-length items MUST NOT appear</b>. They can be encoded as</tt><br>
<tt> definite-length items instead.</tt><br>
<br>
<tt> * The keys in every map MUST be sorted in the bytewise lexicographic</tt><br>
<tt> order of their deterministic encodings. [...]</tt></span><o:p></o:p></p>
<p class="MsoNormal"><br>
<br>
<span style="font-size:10.0pt;font-family:"Arial",sans-serif">We are not using maps (currently; and may want to avoid in the future) and shall not use indefinite-length items in (extension) blocks (the whole indefinite length bundle array should be ok). I would
suggest that we consult with more hardware-oriented people whether the '... as short as possible ...' requirements would be a problem. If not, I would suggest that we explicitly require Deterministically Encoded CBOR according to RFC8949, 4.2.1. Otherwise,
we might have to define something more CCSDS-specific (and would risk loosing interoperability with other implementations, in particular, regarding bundle security).</span>
<br>
<br>
<span style="font-size:10.0pt;font-family:"Arial",sans-serif">Regards,</span> <br>
<span style="font-size:10.0pt;font-family:"Arial",sans-serif">Felix </span><b><span style="font-size:10.0pt;font-family:"Courier New""><br>
</span></b><br>
<br>
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