[Sis-dtn] [EXT] Deterministic CBOR encoding

Thu Nov 18 14:17:55 UTC 2021

Felix,

  I had almost this exact conversation with Scott Burleigh and Kevin Fall yesterday.

  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:

"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."

  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.

-Ed

Edward J. Birrane, III, Ph.D. (he/him/his)
Chief Engineer, Space Constellation Networking
Space Exploration Sector
Johns Hopkins Applied Physics Laboratory
(W) 443-778-7423 / (C) 443-690-8272

From: SIS-DTN <sis-dtn-bounces at mailman.ccsds.org> On Behalf Of Felix.Flentge at esa.int
Sent: Thursday, November 18, 2021 7:02 AM
To: sis-dtn at mailman.ccsds.org
Subject: [EXT] [Sis-dtn] Deterministic CBOR encoding

Dear All,

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.

draft-ietf-dtn-bpbis-31.txt:

We have:

4<https://datatracker.ietf.org/doc/html/draft-ietf-dtn-bpbis-31#section-4>. Bundle Format

4.1<https://datatracker.ietf.org/doc/html/draft-ietf-dtn-bpbis-31#section-4.1>. Bundle Structure

  The format of bundles SHALL conform to the Concise Binary Object
  Representation (CBOR [RFC8949<https://datatracker.ietf.org/doc/html/rfc8949>]).

  Cryptographic verification of a block is possible only if the
  sequence of octets on which the verifying node computes its hash -
  the canonicalized representation of the block - is identical to the
  sequence of octets on which the hash declared for that block was
  computed.  To ensure that blocks are always in canonical
  representation when they are transmitted and received, the CBOR
  representations of the values of all fields in all blocks must
  conform to the rules for Canonical CBOR as specified in [RFC8949<https://datatracker.ietf.org/doc/html/rfc8949>].

 There are two small issues here:
- 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).
- RFC8949 does not really specify 'Canonical CBOR':
Section 4.2.3 contains the following note:

      |  Although [RFC7049<https://datatracker.ietf.org/doc/html/rfc7049#section-3>] used the term "Canonical CBOR" for its form
     |  of requirements on deterministic encoding, this document avoids
     |  this term because "canonicalization" is often associated with
     |  specific uses of deterministic encoding only.  The terms are
     |  essentially interchangeable, however, and the set of core
     |  requirements in this document could also be called "Canonical
     |  CBOR", while the length-first-ordered version of that could be
     |  called "Old Canonical CBOR".

and G.3:

   For a single value in the data model, CBOR often provides multiple
  encoding options.  A new section (Section 4<https://datatracker.ietf.org/doc/html/rfc8949#section-4>) introduces the term
  "preferred serialization" (Section 4.1<https://datatracker.ietf.org/doc/html/rfc8949#section-4.1>) and defines it for various
  kinds of data items.  On the basis of this terminology, the section
  then discusses how a CBOR-based protocol can define "deterministic
  encoding" (Section 4.2<https://datatracker.ietf.org/doc/html/rfc8949#section-4.2>), which avoids terms "canonical" and
  "canonicalization" from RFC 7049<https://datatracker.ietf.org/doc/html/rfc7049#section-3>.  The suggestion of "Core
  Deterministic Encoding Requirements" (Section 4.2.1<https://datatracker.ietf.org/doc/html/rfc8949#section-4.2.1>) enables generic
  support for such protocol-defined encoding requirements.  This
  document further eases the implementation of deterministic encoding
  by simplifying the map ordering suggested in RFC 7049<https://datatracker.ietf.org/doc/html/rfc7049#section-3> to a simple
  lexicographic ordering of encoded keys.  A description of the older
  suggestion is kept as an alternative, now termed "length-first map
  key ordering" (Section 4.2.3<https://datatracker.ietf.org/doc/html/rfc8949#section-4.2.3>).

 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:
4<https://datatracker.ietf.org/doc/html/draft-ietf-dtn-bpsec-27#section-4>.  Canonical Forms

 [...]

  The canonical form of the primary block is as specified in
  [I-D.ietf-dtn-bpbis<https://datatracker.ietf.org/doc/html/draft-ietf-dtn-bpsec-27#ref-I-D.ietf-dtn-bpbis>] with the following constraint.

  o  CBOR values from the primary block MUST be canonicalized using the
     rules for Deterministically Encoded CBOR, as specified in
     [RFC8949<https://datatracker.ietf.org/doc/html/rfc8949>].

  All non-primary blocks share the same block structure and are
  canonicalized as specified in [I-D.ietf-dtn-bpbis<https://datatracker.ietf.org/doc/html/draft-ietf-dtn-bpsec-27#ref-I-D.ietf-dtn-bpbis>] with the following
  constraints.

  o  CBOR values from the non-primary block MUST be canonicalized using
     the rules for Deterministically Encoded CBOR, as specified in
     [RFC8949<https://datatracker.ietf.org/doc/html/rfc8949>].

(It would be even better if there would be an explicit reference to Section 4.2.1.)

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:

*  Preferred serialization MUST be used.  In particular, this means
     that arguments (see Section 3<https://datatracker.ietf.org/doc/html/rfc8949#section-3>) for integers, lengths in major
     types 2 through 5, and tags MUST be as short as possible, for
     instance:
[...]

  *  Indefinite-length items MUST NOT appear.  They can be encoded as
     definite-length items instead.

  *  The keys in every map MUST be sorted in the bytewise lexicographic
     order of their deterministic encodings. [...]

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).

Regards,
Felix

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