CBOR                                                          C. Bormann
Internet-Draft                                    Universität Bremen TZI
Intended status: Best Current Practice                  10 February 2025
Expires: 14 August 2025


                CBOR Common Deterministic Encoding (CDE)
                         draft-ietf-cbor-cde-08

Abstract

   CBOR (STD 94, RFC 8949) defines "Deterministically Encoded CBOR" in
   its Section 4.2, providing some flexibility for application specific
   decisions.  To facilitate Deterministic Encoding to be offered as a
   selectable feature of generic encoders, the present document defines
   a CBOR Common Deterministic Encoding (CDE) Profile that can be shared
   by a large set of applications with potentially diverging detailed
   requirements.  It also defines "Basic Serialization", which stops
   short of the potentially more onerous requirements that make CDE
   fully deterministic, while employing most of its reductions of the
   variability needing to be handled by decoders.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-cbor-cde/.

   Discussion of this document takes place on the Concise Binary Object
   Representation Maintenance and Extensions (CBOR) Working Group
   mailing list (mailto:cbor@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/cbor/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/cbor/.

   Source for this draft and an issue tracker can be found at
   https://github.com/cbor-wg/draft-ietf-cbor-cde.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Structure of This Document  . . . . . . . . . . . . . . .   3
     1.2.  Conventions and Definitions . . . . . . . . . . . . . . .   4
   2.  Encoding Choices in CBOR  . . . . . . . . . . . . . . . . . .   4
   3.  CBOR Common Deterministic Encoding Profile (CDE)  . . . . . .   6
   4.  CDDL support  . . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Application-level Deterministic Representation . . .  12
   Appendix B.  Implementers' Checklists . . . . . . . . . . . . . .  15
     B.1.  Preferred Serialization . . . . . . . . . . . . . . . . .  16
       B.1.1.  Preferred Serialization Encoders  . . . . . . . . . .  16
       B.1.2.  Preferred Serialization Decoders  . . . . . . . . . .  17
     B.2.  Basic Serialization . . . . . . . . . . . . . . . . . . .  18
       B.2.1.  Basic Serialization Encoders  . . . . . . . . . . . .  18
       B.2.2.  Basic Serialization Decoders  . . . . . . . . . . . .  18
     B.3.  CDE . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
       B.3.1.  CDE Encoders  . . . . . . . . . . . . . . . . . . . .  19
       B.3.2.  CDE Decoders  . . . . . . . . . . . . . . . . . . . .  19
   List of Tables  . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  20
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  20



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   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   CBOR (STD 94, RFC 8949) defines "Deterministically Encoded CBOR" in
   its Section 4.2, providing some flexibility for application specific
   decisions.  To facilitate Deterministic Encoding to be offered as a
   selectable feature of generic encoders, the present document defines
   a CBOR Common Deterministic Encoding (CDE) Profile that can be shared
   by a large set of applications with potentially diverging detailed
   requirements.  It also defines "Basic Serialization", which stops
   short of the potentially more onerous requirements that make CDE
   fully deterministic, while employing most of its reductions of the
   variability needing to be handled by decoders.

1.1.  Structure of This Document

   After introductory material (this introduction and Section 2),
   Section 3 defines the CBOR Common Deterministic Encoding Profile
   (CDE).  Section 4 defines Concise Data Definition Language (CDDL)
   support for indicating the use of CDE.  This is followed by the
   conventional sections for Security Considerations (5), IANA
   Considerations (6), and References (7).

   The informative Appendix B provides brief checklists that
   implementers can use to check their CDE implementations.
   Appendix B.1 provides a checklist for implementing Preferred
   Serialization.  Appendix B.2 introduces "Basic Serialization", a
   slightly more restricted form of Preferred Serialization that may be
   used by encoders to hit a sweet spot for maximizing interoperability
   with partial (e.g., constrained) CBOR decoder implementations.
   Appendix B.3 further restricts Basic Serialization to arrive at CDE.

   Instead of giving rise to the definition of application-specific,
   non-interoperable variants of CDE, this document identifies
   Application-level Deterministic Representation (ALDR) rules as a
   concept that is separate from CDE itself (Appendix A) and therefore
   out of scope for this document.  ALDR rules are situated at the
   application-level, i.e., on top of the CDE, and address requirements
   on deterministic representation of application data that are specific
   to an application or a set of applications.  ALDR rules are often
   provided as part of a specification for a CBOR-based protocol, or, if
   needed, can be provided by referencing a shared "ALDR ruleset" that
   is defined in a separate document.







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1.2.  Conventions and Definitions

   The conventions and definitions of [STD94] apply.

   *  The term "CBOR Application" ("application" for short) is not
      explicitly defined in [STD94]; this document uses it in the same
      sense as it is used there, specifically for applications that use
      CBOR as an interchange format and use (often generic) CBOR
      encoders/decoders to serialize/ingest the CBOR form of their
      application data to be exchanged.

   *  Similarly, "CBOR Protocol" is used as in [STD94] for the protocol
      that governs the interchange of data in CBOR format for a specific
      application or set of applications.

   *  "Representation" stands for the process, and its result, of
      building the representation format out of (information-model
      level) application data.

   *  "Serialization" is used for the subset of this process, and its
      result, that represents ("serializes") data in CBOR generic data
      model form into encoded data items.  "Encoding" is often used as a
      synonym when the focus is on that.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [BCP14] (RFC2119) (RFC8174) when, and only when, they appear in all
   capitals, as shown here.

2.  Encoding Choices in CBOR

   In many cases, CBOR provides more than one way to encode a data item,
   i.e., to serialize it into a sequence of bytes.  This flexibility can
   provide convenience for the generator of the encoded data item, but
   handling the resulting variation can also put an onus on the decoder.
   In general, there is no single perfect encoding choice that is
   optimal for all applications.  Choosing the right constraints on
   these encoding choices is one element of application protocol design.
   Having predefined sets of such choices is a useful way to reduce
   variation between applications, enabling generic implementations.

   Section 4.1 of RFC 8949 [STD94] provides a recommendation for a
   _Preferred Serialization_. This recommendation is useful for most
   CBOR applications, and it is a good choice for most applications.
   Its main constraint is to choose the shortest _head_ (Section 3 of
   RFC 8949 [STD94]) that preserves the value of a data item.




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   Preferred Serialization allows indefinite length encoding
   (Section 3.2 of RFC 8949 [STD94]), which does not express the length
   of a string, an array, or a map in its head.  Supporting both
   definite length and indefinite length encoding is an additional onus
   on the decoder; many applications therefore choose not to use
   indefinite length encoding at all.  We call Preferred Serialization
   with this additional constraint _Basic Serialization_. Basic
   Serialization is a common choice for applications that need to
   further reduce the variability that needs to be handled by decoders,
   potentially maximizing interoperability with partial (e.g.,
   constrained) CBOR decoder implementations.

   These constraints still allow some variation.  In particular, there
   is more than one serialization for data items that contain maps: The
   order of serialization of map entries is ignored in CBOR (as it is in
   JSON), so maps with more than one entry have all permutations of
   these entries as valid Basic Serializations. _Deterministic
   Serialization_ builds on Basic Serialization by defining a common
   (namely, lexicographic) order for the entries in a map.  For many
   applications, ensuring this common order is an additional onus on the
   generator that is not actually needed, so they do not choose
   Deterministic Serialization.  However, if the objective is minimal
   effort for the consuming application, deterministic map ordering can
   be useful even outside the main use cases for Deterministic
   Serialization that are further discussed in Section 2 of
   [I-D.bormann-cbor-det].

   Table 1 summarizes the increasingly restrictive sets of encoding
   choices that have been given names in this section.

        +=========================+================+==============+
        | Set of Encoding Choices | Most Important | Applications |
        |                         | Constraint     |              |
        +=========================+================+==============+
        | preferred               | shortest       | most         |
        |                         | "head" variant |              |
        +-------------------------+----------------+--------------+
        | basic                   | + definite     | many         |
        |                         | lengths only   |              |
        +-------------------------+----------------+--------------+
        | _deterministic_ ("CDE") | + common map   | specific     |
        |                         | order          |              |
        +-------------------------+----------------+--------------+

             Table 1: Constraints on the Serialization of CBOR






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   Note that the objective to have a deterministic serialization for a
   specific application data item can only be fulfilled if the
   application itself does not generate multiple different CBOR data
   items that represent that same (equivalent) application data item.
   We speak of the need for Application-level Deterministic
   Representation (ALDR), and we may want to aid achieving this by the
   application defining rules for ALDR (see also Appendix A).  Where
   Deterministic Representation is not actually needed, application-
   level representation rules of course can still be useful to amplify
   the benefits of Preferred or Basic Serialization.

3.  CBOR Common Deterministic Encoding Profile (CDE)

   This specification defines the _CBOR Common Deterministic Encoding
   Profile_ (CDE) based on the _Core Deterministic Encoding
   Requirements_ defined for CBOR in Section 4.2.1 of RFC 8949 [STD94].

   Note that this specific set of requirements is elective — in
   principle, other variants of deterministic encoding can be defined
   (and have been, now being phased out slowly, as detailed in
   Section 4.2.3 of RFC 8949 [STD94]).  In many applications of CBOR
   today, deterministic encoding is not used at all, as its restriction
   of choices can create some additional performance cost and code
   complexity.

   [STD94]'s core requirements are designed to provide well-understood
   and easy-to-implement rules while maximizing coverage, i.e., the
   subset of CBOR data items that are fully specified by these rules,
   and also placing minimal burden on implementations.

   Section 4.2.2 of RFC 8949 [STD94] picks up on the interaction of
   extensibility (CBOR tags) and deterministic encoding.  CBOR itself
   uses some tags to increase the range of its basic generic data types,
   e.g., tags 2/3 extend the range of basic major types 0/1 in a
   seamless way.  Section 4.2.2 of RFC 8949 [STD94] recommends handling
   this transition the same way as with the transition between different
   integer representation lengths in the basic generic data model, i.e.,
   by mandating the preferred serialization for all integers
   (Section 3.4.3 of RFC 8949 [STD94]).

   1.  CDE turns this recommendation into a mandate: Integers that can
       be represented by basic major type 0 and 1 are encoded using the
       deterministic encoding defined for them, and integers outside
       this range are encoded using the preferred serialization
       (Section 3.4.3 of RFC 8949 [STD94]) of tag 2 and 3 (i.e., no
       leading zero bytes).





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   Most tags capture more specific application semantics and therefore
   may be harder to define a deterministic encoding for.  While the
   deterministic encoding of their tag internals is often covered by the
   _Core Deterministic Encoding Requirements_, the mapping of diverging
   platform application data types onto the tag contents may require
   additional attention to perform it in a deterministic way; see
   Section 3.2 of [I-D.bormann-cbor-det] for more explanation as well as
   examples.  As the CDE would continually need to address additional
   issues raised by the registration of new tags, this specification
   recommends that new tag registrations address deterministic encoding
   in the context of CDE.

   A particularly difficult field to obtain deterministic encoding for
   is floating point numbers, partially because they themselves are
   often obtained from processes that are not entirely deterministic
   between platforms.  See Section 3.2.2 of [I-D.bormann-cbor-det] for
   more details.  Section 4.2.2 of RFC 8949 [STD94] presents a number of
   choices; these need to be made to obtain the CBOR Common
   Deterministic Encoding Profile (CDE).  Specifically, CDE specifies
   (in the order of the bullet list at the end of Section 4.2.2 of RFC
   8949 [STD94]):

   2.  Besides the mandated use of preferred serialization, there is no
       further specific action for the two different zero values, e.g.,
       an encoder that is asked by an application to represent a
       negative floating point zero will generate 0xf98000.

   3.  There is no attempt to mix integers and floating point numbers,
       i.e., all floating point values are encoded as the preferred
       floating-point representation that accurately represents the
       value, independent of whether the floating point value is,
       mathematically, an integral value (choice 2 of the second
       bullet).


















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   4.  Apart from finite and infinite numbers, [IEEE754] floating point
       values include NaN (not a number) values
       [I-D.bormann-cbor-numbers].  In CDE, there is no special handling
       of NaN values, except that the preferred serialization rules also
       apply to NaNs (with zero or non-zero payloads), using the
       canonical encoding of NaNs as defined in Section 6.2.1 of
       [IEEE754].  Specifically, this means that shorter forms of
       encodings for a NaN are used when that can be achieved by only
       removing trailing zeros in the NaN payload (example
       serializations are available in Appendix A.1.2 of
       [I-D.bormann-cbor-numbers]).  Further clarifying a "should"-level
       statement in Section 6.2.1 of [IEEE754], the CBOR encoding always
       uses a leading bit of 1 in the significand to encode a quiet NaN;
       the use of signaling NaNs by application protocols is NOT
       RECOMMENDED but when presented by an application these are
       encoded by using a leading bit of 0.

       Typically, most applications that employ NaNs in their storage
       and communication interfaces will only use a single NaN value,
       quiet NaN with payload 0, which therefore deterministically
       encodes as 0xf97e00.

   5.  There is no special handling of subnormal values.

   6.  CDE does not presume equivalence of basic floating point values
       with floating point values using other representations (e.g., tag
       4/5).  Such equivalences and related deterministic representation
       rules can be added at the ALDR level if desired, e.g., by
       stipulating additional equivalences and deterministically
       choosing exactly one representation for each such equivalence,
       and by restricting in general the set of data item values
       actually used by an application.

   The main intent here is to preserve the basic generic data model, so
   applications (in their ALDR rules or by referencing a separate ALDR
   ruleset document, see Appendix A) can make their own decisions within
   that data model.  E.g., an application's ALDR rules can decide that
   it only ever allows a single NaN value that would be encoded as
   0xf97e00, so a CDE implementation focusing on this application would
   not need to provide processing for other NaN values.  Basing the
   definition of both CDE and ALDR rules on the generic data model of
   CBOR also means that there is no effect on the Concise Data
   Definition Language (CDDL) [RFC8610], except where the data
   description is documenting specific encoding decisions for byte
   strings that carry embedded CBOR.






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4.  CDDL support

   CDDL defines the structure of CBOR data items at the data model
   level; it enables being specific about the data items allowed in a
   particular place.  It does not specify encoding, but CBOR protocols
   can specify the use of CDE (or simply Basic Serialization).  For
   instance, it allows the specification of a floating point data item
   as "float16"; this means the application data model only foresees
   data that can be encoded as [IEEE754] binary16.  Note that specifying
   "float32" for a floating point data item enables all floating point
   values that can be represented as binary32; this includes values that
   can also be represented as binary16 and that will be so represented
   in Basic Serialization.

   [RFC8610] defines control operators to indicate that the contents of
   a byte string carries a CBOR-encoded data item (.cbor) or a sequence
   of CBOR-encoded data items (.cborseq).

   CDDL specifications may want to specify that the data items should be
   encoded in Common CBOR Deterministic Encoding.  The present
   specification adds two CDDL control operators that can be used for
   this.

   The control operators .cde and .cdeseq are exactly like .cbor and
   .cborseq except that they also require the encoded data item(s) to be
   encoded according to CDE.

   For example, a byte string of embedded CBOR that is to be encoded
   according to CDE can be formalized as:

   leaf = #6.24(bytes .cde any)

   More importantly, if the encoded data item also needs to have a
   specific structure, this can be expressed by the right-hand side
   (instead of using the most general CDDL type any here).

   (Note that the .cdeseq control operator does not enable specifying
   different deterministic encoding requirements for the elements of the
   sequence.  If a use case for such a feature becomes known, it could
   be added.)

   Obviously, specifications that document ALDR rules can define related
   control operators that also embody the processing required by those
   ALDR rules, and are encouraged to do so.







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5.  Security Considerations

   The security considerations in Section 10 of RFC 8949 [STD94] apply.
   The use of deterministic encoding can mitigate issues arising out of
   the use of non-preferred serializations specially crafted by an
   attacker.  However, this effect only accrues if the decoder actually
   checks that deterministic encoding was applied correctly.  More
   generally, additional security properties of deterministic encoding
   can rely on this check being performed properly.

6.  IANA Considerations


   // RFC Editor: please replace RFCXXXX with the RFC number of this RFC
   // and remove this note.

   This document requests IANA to register the contents of Table 2 into
   the registry "CDDL Control Operators" of the [IANA.cddl] registry
   group:

                          +=========+===========+
                          | Name    | Reference |
                          +=========+===========+
                          | .cde    | [RFCXXXX] |
                          +---------+-----------+
                          | .cdeseq | [RFCXXXX] |
                          +---------+-----------+

                            Table 2: New control
                              operators to be
                                 registered

7.  References

7.1.  Normative References

   [BCP14]    Best Current Practice 14,
              <https://www.rfc-editor.org/info/bcp14>.
              At the time of writing, this BCP comprises the following:

              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

              Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.



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   [IANA.cddl]
              IANA, "Concise Data Definition Language (CDDL)",
              <https://www.iana.org/assignments/cddl>.

   [IEEE754]  IEEE, "IEEE Standard for Floating-Point Arithmetic", IEEE
              Std 754-2019, DOI 10.1109/IEEESTD.2019.8766229,
              <https://ieeexplore.ieee.org/document/8766229>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.

   [STD94]    Internet Standard 94,
              <https://www.rfc-editor.org/info/std94>.
              At the time of writing, this STD comprises the following:

              Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

7.2.  Informative References

   [I-D.bormann-cbor-det]
              Bormann, C., "CBOR: On Deterministic Encoding and
              Representation", Work in Progress, Internet-Draft, draft-
              bormann-cbor-det-04, 21 January 2025,
              <https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
              det-04>.

   [I-D.bormann-cbor-numbers]
              Bormann, C., "On Numbers in CBOR", Work in Progress,
              Internet-Draft, draft-bormann-cbor-numbers-01, 8 January
              2025, <https://datatracker.ietf.org/doc/html/draft-
              bormann-cbor-numbers-01>.

   [I-D.bormann-dispatch-modern-network-unicode]
              Bormann, C., "Modern Network Unicode", Work in Progress,
              Internet-Draft, draft-bormann-dispatch-modern-network-
              unicode-05, 30 August 2024,
              <https://datatracker.ietf.org/doc/html/draft-bormann-
              dispatch-modern-network-unicode-05>.

   [I-D.mcnally-deterministic-cbor]
              McNally, W., Allen, C., Bormann, C., and L. Lundblade,
              "dCBOR: A Deterministic CBOR Application Profile", Work in



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              Progress, Internet-Draft, draft-mcnally-deterministic-
              cbor-12, 7 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-mcnally-
              deterministic-cbor-12>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/rfc/rfc8392>.

   [RFC9581]  Bormann, C., Gamari, B., and H. Birkholz, "Concise Binary
              Object Representation (CBOR) Tags for Time, Duration, and
              Period", RFC 9581, DOI 10.17487/RFC9581, August 2024,
              <https://www.rfc-editor.org/rfc/rfc9581>.

   [RFC9679]  Isobe, K., Tschofenig, H., and O. Steele, "CBOR Object
              Signing and Encryption (COSE) Key Thumbprint", RFC 9679,
              DOI 10.17487/RFC9679, December 2024,
              <https://www.rfc-editor.org/rfc/rfc9679>.

   [STD96]    Internet Standard 96,
              <https://www.rfc-editor.org/info/std96>.
              At the time of writing, this STD comprises the following:

              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", STD 96, RFC 9052,
              DOI 10.17487/RFC9052, August 2022,
              <https://www.rfc-editor.org/info/rfc9052>.

              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Countersignatures", STD 96, RFC 9338,
              DOI 10.17487/RFC9338, December 2022,
              <https://www.rfc-editor.org/info/rfc9338>.

   [UAX-15]   "Unicode Normalization Forms", Unicode Standard Annex,
              <https://unicode.org/reports/tr15/>.

Appendix A.  Application-level Deterministic Representation

   This appendix is informative.

   CBOR application protocols are agreements about how to use CBOR for a
   specific application or set of applications.  Application protocols
   make representation decisions in order to constrain the variety of
   ways in which some aspect of the information model could be
   represented in the CBOR data model for the application.  For
   instance, there are several CBOR tags that can be used to represent a
   time stamp (such as tag 0, 1, 1001), each with some specific
   properties.  Application protocols that need to represent a timestamp



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   typically choose a specific tag and further constrain its use where
   necessary (e.g., tag 1001 was designed to cover a wide variety of
   applications [RFC9581]).  Where no tag is available, the application
   protocol can design its own format for some application data.  Even
   where a tag is available, the application data can choose to use its
   definitions without actually encoding the tag (e.g., by using its
   content in specific places in an "unwrapped" form).

   For instance, CWT defines an application data type "NumericDate"
   which (as an application-level rule) is formed by "unwrapping" tag 1
   (see Sections 2 and 5 of [RFC8392]).  CWT does stop short of using
   deterministic encoding.  A hypothetical deterministic variant of CWT
   would need to make an additional ALDR rule for NumericDate, as the
   definition of tag 1 allows both integer and floating point numbers
   (Section 3.4.2 of RFC 8949 [STD94]), which allows multiple
   application-level representations of integral numbers.  These
   application rules may choose to only ever use integers, or to always
   use integers when the numeric value can be represented as such
   without loss of information, or to always use floating point numbers,
   or some of these for some application data and different ones for
   other application data.

   Applications that require Deterministic Representation, and that
   derive CBOR data items from application data without maintaining a
   record of which choices are to be made when representing these
   application data, generally make rules for these choices as part of
   the application protocol.  In this document, we speak about these
   choices as Application-level Deterministic Representation Rules (ALDR
   rules for short).

   As an example, [RFC9679] is intended to derive a (deterministic)
   thumbprint from a COSE key [STD96].  Section 4 of [RFC9679] provides
   the rules that are used to construct a deterministic application-
   level representation (ALDR rules).  Only certain data from a COSE key
   are selected to be included in that ALDR, and, where the COSE can
   choose multiple representations of semantically equivalent
   application data, the ALDR rules choose one of them, potentially
   requiring a conversion (Section 4.2 of [RFC9679]):

   |  Note: [RFC9052] supports both compressed and uncompressed point
   |  representations.  For interoperability, implementations adhering
   |  to this specification MUST use the uncompressed point
   |  representation.  Therefore, the y-coordinate is expressed as a
   |  bstr.  If an implementation uses the compressed point
   |  representation, it MUST first convert it to the uncompressed form
   |  for the purpose of thumbprint calculation.





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   CDE provides for encoding commonality between different applications
   of CBOR once these application-level choices have been made.  It can
   be useful for an application or a group of applications to document
   their choices aimed at deterministic representation of application
   data in a general way, constraining the set of data items handled
   (_exclusions_, e.g., no compressed point representations) and
   defining further mappings (_reductions_, e.g., conversions to
   uncompressed form) that help the application(s) get by with the
   exclusions.  This can be done in the application protocol
   specification (as in [RFC9679]) or as a separate document.

   An early example of a separate document is the dCBOR specification
   [I-D.mcnally-deterministic-cbor]. dCBOR specifies the use of CDE
   together with some application-level rules, i.e., an ALDR ruleset,
   such as a requirement for all text strings to be in Unicode
   Normalization Form C (NFC) [UAX-15] — this specific requirement is an
   example for an _exclusion_ of non-NFC data at the application level,
   and it invites implementing a _reduction_ by routine normalization of
   text strings.

   ALDR rules (including rules specified in a ALDR ruleset document)
   enable simply using implementations of the common CDE; they do not
   "fork" CBOR in the sense of requiring distinct generic encoder/
   decoder implementations for each application.

   An implementation of specific ALDR rules combined with a CDE
   implementation produces well-formed, deterministically encoded CBOR
   according to [STD94], and existing generic CBOR decoders will
   therefore be able to decode it, including those that check for
   Deterministic Encoding ("CDE decoders", see also Appendix B).
   Similarly, generic CBOR encoders will be able to produce valid CBOR
   that can be ingested by an implementation that enforces an
   application's ALDR rules if the encoder was handed data model level
   information from an application that simply conformed to those ALDR
   rules.

   Please note that the separation between standard CBOR processing and
   the processing required by the ALDR rules is a conceptual one:
   Instead of employing generic encoders/decoders, both ALDR rule
   processing and standard CBOR processing can be combined into a
   specialized encoder/decoder specifically designed for a particular
   set of ALDR rules.

   ALDR rules are intended to be used in conjunction with an
   application, which typically will naturally use a subset of the CBOR
   generic data model, which in turn influences which subset of the ALDR
   rules is used by the specific application (in particular if the
   application simply references a more general ALDR ruleset document).



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   As a result, ALDR rules themselves place no direct requirement on
   what minimum subset of CBOR is implemented.  For instance, a set of
   ALDR rules might include rules for the processing of floating point
   values, but there is no requirement that implementations of that set
   of ALDR rules support floating point numbers (or any other kind of
   number, such as arbitrary precision integers or 64-bit negative
   integers) when they are used with applications that do not use them.

Appendix B.  Implementers' Checklists

   This appendix is informative.  It provides brief checklists that
   implementers can use to check their implementations.  It uses RFC2119
   language, specifically the keyword MUST, to highlight the specific
   items that implementers may want to check.  It does not contain any
   normative mandates.  This appendix is informative.

   Notes:

   *  This is largely a restatement of parts of Section 4 of RFC 8949
      [STD94].  The purpose of the restatement is to aid the work of
      implementers, not to redefine anything.

      Preferred Serialization Encoders and Decoders as well as CDE
      Encoders and Decoders have certain properties that are expressed
      using RFC2119 keywords in this appendix.

   *  Duplicate map keys are never valid in CBOR at all (see list item
      "Major type 5" in Section 3.1 of RFC 8949 [STD94]) no matter what
      sort of serialization is used.  Of the various strategies listed
      in Section 5.6 of RFC 8949 [STD94], detecting duplicates and
      handling them as an error instead of passing invalid data to the
      application is the most robust one; achieving this level of
      robustness is a mark of quality of implementation.

   *  Preferred serialization and CDE only affect serialization.  They
      do not place any requirements, exclusions, mappings or such on the
      data model level.  ALDR rules such as the ALDR ruleset defined by
      dCBOR are different as they can affect the data model by
      restricting some values and ranges.

   *  CBOR decoders in general (as opposed to "CDE decoders"
      specifically advertised as supporting CDE) are not required to
      check for preferred serialization or CDE and reject inputs that do
      not fulfill their requirements.  However, in an environment that
      employs deterministic encoding, employing non-checking CBOR
      decoders negates many of its benefits.  Decoder implementations
      that advertise "support" for preferred serialization or CDE need
      to check the encoding and reject input that is not encoded to the



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      encoding specification in use.  Again, ALDR rules such as those in
      dCBOR may pose additional requirements, such as requiring
      rejection of non-conforming inputs.

      If a generic decoder needs to be used that does not "support" CDE,
      a simple (but somewhat clumsy) way to check for proper CDE
      encoding is to re-encode the decoded data and check for bit-to-bit
      equality with the original input.

B.1.  Preferred Serialization

   In the following, the abbreviation "ai" will be used for the 5-bit
   additional information field in the first byte of an encoded CBOR
   data item, which follows the 3-bit field for the major type.

B.1.1.  Preferred Serialization Encoders

   1.  Shortest-form encoding of the argument MUST be used for all major
       types.  Major type 7 is used for floating-point and simple
       values; floating point values have its specific rules for how the
       shortest form is derived for the argument.  The shortest form
       encoding for any argument that is not a floating point value is:

       *  0 to 23 and -1 to -24 MUST be encoded in the same byte as the
          major type.

       *  24 to 255 and -25 to -256 MUST be encoded only with an
          additional byte (ai = 0x18).

       *  256 to 65535 and -257 to -65536 MUST be encoded only with an
          additional two bytes (ai = 0x19).

       *  65536 to 4294967295 and -65537 to -4294967296 MUST be encoded
          only with an additional four bytes (ai = 0x1a).

   2.  If floating-point numbers are emitted, the following apply:

       *  The length of the argument indicates half (binary16, ai =
          0x19), single (binary32, ai = 0x1a) and double (binary64, ai =
          0x1b) precision encoding.  If multiple of these encodings
          preserve the precision of the value to be encoded, only the
          shortest form of these MUST be emitted.  That is, encoders
          MUST support half-precision and single-precision floating
          point.

       *  [IEEE754] Infinites and NaNs, and thus NaN payloads, MUST be
          supported, to the extent possible on the platform.




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          As with all floating point numbers, Infinites and NaNs MUST be
          encoded in the shortest of double, single or half precision
          that preserves the value:

          -  Positive and negative infinity and zero MUST be represented
             in half-precision floating point.

          -  For NaNs, the value to be preserved includes the sign bit,
             the quiet bit, and the NaN payload (whether zero or non-
             zero).  The shortest form is obtained by removing the
             rightmost N bits of the payload, where N is the difference
             in the number of bits in the significand (mantissa
             representation) between the original format and the
             shortest format.  This trimming is performed only
             (preserves the value only) if all the rightmost bits
             removed are zero.  (This will always represent a double or
             single quiet NaN with a zero NaN payload in a half-
             precision quiet NaN.)

   3.  If tags 2 and 3 are supported, the following apply:

       *  Positive integers from 0 to 2^64 - 1 MUST be encoded as a type
          0 integer.

       *  Negative integers from -(2^64) to -1 MUST be encoded as a type
          1 integer.

       *  Leading zeros MUST NOT be present in the byte string content
          of tag 2 and 3.

       (This also applies to the use of tags 2 and 3 within other tags,
       such as 4 or 5.)

B.1.2.  Preferred Serialization Decoders

   There are no special requirements that CBOR decoders need to meet to
   be a Preferred Serialization Decoder.  Partial decoder
   implementations need to pay attention to at least the following
   requirements:

   1.  Decoders MUST accept shortest-form encoded arguments (see
       Section 3 of RFC 8949 [STD94]).

   2.  If arrays or maps are supported, definite-length arrays or maps
       MUST be accepted.

   3.  If text or byte strings are supported, definite-length text or
       byte strings MUST be accepted.



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   4.  If floating-point numbers are supported, the following apply:

       *  Half-precision values MUST be accepted.

       *  Double- and single-precision values SHOULD be accepted;
          leaving these out is only foreseen for decoders that need to
          work in exceptionally constrained environments.

       *  If double-precision values are accepted, single-precision
          values MUST be accepted.

       *  Infinites and NaNs, and thus NaN payloads, MUST be accepted
          and presented to the application (not necessarily in the
          platform number format, if that doesn't support those values).

   5.  If big numbers (tags 2 and 3) are supported, type 0 and type 1
       integers MUST be accepted where a tag 2 or 3 would be accepted.
       Leading zero bytes in the tag content of a tag 2 or 3 MUST be
       ignored.

B.2.  Basic Serialization

   Basic Serialization further restricts Preferred Serialization by not
   using indefinite length encoding.  A CBOR encoder can choose to
   employ Basic Serialization in order to reduce the variability that
   needs to be handled by decoders, potentially maximizing
   interoperability with partial (e.g., constrained) CBOR decoder
   implementations.

B.2.1.  Basic Serialization Encoders

   The Basic Serialization Encoder requirements are identical to the
   Preferred Serialization Encoder requirements, with the following
   additions:

   1.  If maps or arrays are emitted, they MUST use definite-length
       encoding (never indefinite-length).

   2.  If text or byte strings are emitted, they MUST use definite-
       length encoding (never indefinite-length).

B.2.2.  Basic Serialization Decoders

   The Basic Serialization Decoder requirements are identical to the
   Preferred Serialization Decoder requirements.

B.3.  CDE




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B.3.1.  CDE Encoders

   1.  CDE encoders MUST only emit CBOR fulfilling the basic
       serialization rules (Appendix B.2.1).

   2.  CDE encoders MUST sort maps by the CBOR representation of the map
       key.  The sorting is byte-wise lexicographic order of the encoded
       map key data items.

   3.  CDE encoders MUST generate CBOR that fulfills basic validity
       (Section 5.3.1 of RFC 8949 [STD94]).  Note that this includes not
       emitting duplicate keys in a major type 5 map as well as emitting
       only valid UTF-8 in major type 3 text strings.

       Note also that CDE does NOT include a requirement for Unicode
       normalization [UAX-15]; Appendix C of
       [I-D.bormann-dispatch-modern-network-unicode] contains some
       rationale that went into not requiring routine use of Unicode
       normalization processes.

B.3.2.  CDE Decoders

   The term "CDE Decoder" is a shorthand for a CBOR decoder that
   advertises _supporting_ CDE (see the start of this appendix).

   1.  CDE decoders MUST follow the rules for preferred (and thus basic)
       serialization decoders (Appendix B.1.2).

   2.  CDE decoders MUST check for ordering map keys and for basic
       validity of the CBOR encoding (see Section 5.3.1 of RFC 8949
       [STD94], which includes a check against duplicate map keys and
       invalid UTF-8).

       To be called a CDE decoder, it MUST NOT present to the
       application a decoded data item that fails one of these checks
       (except maybe via special diagnostic channels with no potential
       for confusion with a correctly CDE-decoded data item).

List of Tables

   1.  Constraints on the Serialization of CBOR (Table 1)
   2.  New control operators to be registered (Table 2)









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Acknowledgments

   An earlier version of this document was based on the work of Wolf
   McNally and Christopher Allen as documented in
   [I-D.mcnally-deterministic-cbor], which serves as an example for an
   ALDR ruleset document.  We would like to explicitly acknowledge that
   this work has contributed greatly to shaping the concept of a CBOR
   Common Deterministic Encoding and ALDR rules/rulesets on top of that.

Contributors

   Laurence Lundblade
   Security Theory LLC
   Email: lgl@securitytheory.com


   Laurence provided most of the text that became Appendix B.

Author's Address

   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany
   Phone: +49-421-218-63921
   Email: cabo@tzi.org
























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