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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