CBOR C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Best Current Practice 12 May 2025
Expires: 13 November 2025
CBOR Common Deterministic Encoding (CDE)
draft-ietf-cbor-cde-latest
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) that can be shared by a
large set of applications with potentially diverging detailed
requirements. It also defines the term "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
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working documents as Internet-Drafts. The list of current Internet-
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This Internet-Draft will expire on 13 November 2025.
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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 . . . . . . . . . . . . . . . . . . 5
3. CBOR Common Deterministic Encoding (CDE) . . . . . . . . . . 6
3.1. CDE Constraints from Preferred Serialization . . . . . . 7
3.2. Additional CDE Constraint from Basic Serialization . . . 9
3.3. Additional CDE Constraint from CDE Itself . . . . . . . . 10
4. CDDL support . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Information Model, Data Model and Serialization . . 14
A.1. Data Model, Encoding Variants and Interoperability with
Partial Implementations . . . . . . . . . . . . . . . . . 15
Appendix B. Application-level Deterministic Representation . . . 17
Appendix C. Implementers' Checklists . . . . . . . . . . . . . . 20
C.1. Preferred Serialization . . . . . . . . . . . . . . . . . 21
C.1.1. Preferred Serialization Encoders . . . . . . . . . . 21
C.1.2. Decoders and Preferred Serialization . . . . . . . . 23
C.2. Basic Serialization . . . . . . . . . . . . . . . . . . . 23
C.2.1. Basic Serialization Encoders . . . . . . . . . . . . 24
C.2.2. Decoders and Basic Serialization . . . . . . . . . . 24
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C.3. CDE . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
C.3.1. CDE Encoders . . . . . . . . . . . . . . . . . . . . 24
C.3.2. CDE-checking Decoders . . . . . . . . . . . . . . . . 25
Appendix D. Encoding Examples . . . . . . . . . . . . . . . . . 25
D.1. Integer Value Examples . . . . . . . . . . . . . . . . . 25
D.2. Floating Point Value Examples . . . . . . . . . . . . . . 27
D.3. Failing Examples . . . . . . . . . . . . . . . . . . . . 29
Appendix E. Examples for Preferred Serialization of Integers . . 30
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . 31
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 32
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 32
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) that can be shared by a
large set of applications with potentially diverging detailed
requirements. It also defines the term "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 (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).
For use as background material, Appendix A introduces terminology for
the layering of models used to describe CBOR.
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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 B) 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.
The informative Appendix C provides brief checklists that
implementers can use to check their CDE implementations.
Appendix C.1 provides a checklist for implementing Preferred
Serialization. Appendix C.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 C.3 further restricts Basic Serialization to arrive at CDE.
Appendix D provides a few examples for CBOR data items in CDE
encoding, as well as a few failing examples.
1.2. Conventions and Definitions
The conventions and definitions of [STD94] apply. Appendix A
provides additional discussion of the terms information model, data
model, and serialization.
* 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.
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* "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.
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
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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
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 B). 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 (CDE)
This specification defines the _CBOR Common Deterministic Encoding_
(CDE) based on the _Core Deterministic Encoding Requirements_ defined
for CBOR in Section 4.2.1 of RFC 8949 [STD94].
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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, 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 Deterministic Encoding 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.
As discussed in Section 2, CDE combines the constraints of Preferred
Serialization with a constraint added by Basic Serialization and
another constraint added by CDE itself. While many CBOR
implementations do set out to provide Preferred Serialization, there
is less of a practical requirement to fully conform, as generic CBOR
decoders do not normally check for Preferred Serialization. However,
an application that relies on deterministic representation, during
ingestion of an encoded CBOR data item will often need to employ a
"CDE-checking decoder", i.e., a CBOR decoder configured to also check
that all CDE constraints are satisfied (see also Appendix C). Here,
small deviations from CDE, including deviations from Preferred
Serialization, turn into interoperability problems; hence the
additional attention of the present document on these constraints.
3.1. CDE Constraints from Preferred Serialization
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]; see also Appendix D.1 and
Appendix E).
1. By adopting the encoding constraints from Preferred
Serialization, 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. Note that not in all cases the tag's
deterministic encoding constraints will be confined to its definition
of Preferred Serialization.
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 that needed to be made to obtain the CBOR Common
Deterministic Encoding (CDE). Here, CDE entirely recurs to Preferred
Serialization and does _not_ itself define any additional
constraints.
However, this BCP responds to a perceived need to clarify some of the
Preferred Serialization constraints. 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).
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 a clarification 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
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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, non-negative 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 B) 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 even 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 (see Section 4).
3.2. Additional CDE Constraint from Basic Serialization
In addition to the encoding constraints from Preferred Serialization,
CDE adds the constraint of not using indefinite length encoding from
Basic Serialization. In many encoders, the use of indefinite length
encoding is controlled by its configuration and can simply be
switched off.
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Some encoders turn to indefinite length encoding for arrays and maps
with 256 or more elements/entries, to use the slightly smaller
serialization size indefinite length encoding offers for these cases.
Since leaving out support for indefinite length encoding is a common
form of partial implementation, this may reduce interoperability.
(Indefinite length encoding may also be used conditionally to avoid
having to compute the total size ahead of time if the platform uses
some form of chunking.) As CDE uses definite length encoding
exclusively, such behavior needs to be turned off for CDE.
3.3. Additional CDE Constraint from CDE Itself
In line with Section 4.2.1 of RFC 8949 [STD94], CDE adds the
constraint of map ordering to those from Basic Serialization. In
some implementations, where platform representations of maps preserve
ordering, this can be achieved using a generic CBOR encoder by pre-
ordering all maps to be encoded, as long as that generic encoder also
preserves the ordering in maps. In implementations without these
properties, a specialized CBOR encoder may need to be employed.
Map ordering is a deterministic encoding constraint specific to maps,
major type 5, that goes beyond maps' constraints for preferred
serialization. In the definition of a tag being employed, there may
also be some deterministic encoding constraints that are not covered
by the tag's constraints for Preferred Serialization (see
Section 3.1).
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).
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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, or the CBOR sequence could be constructed with .join
(Section 3.1 of [RFC9741]).)
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.
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:
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+---------+-----------+
| 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>.
[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:
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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-06, 2 March 2025,
<https://datatracker.ietf.org/doc/html/draft-bormann-
dispatch-modern-network-unicode-06>.
[I-D.ietf-cbor-edn-literals]
Bormann, C., "CBOR Extended Diagnostic Notation (EDN)",
Work in Progress, Internet-Draft, draft-ietf-cbor-edn-
literals-16, 8 January 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
edn-literals-16>.
[I-D.mcnally-deterministic-cbor]
McNally, W., Allen, C., Bormann, C., and L. Lundblade,
"dCBOR: A Deterministic CBOR Application Profile", Work in
Progress, Internet-Draft, draft-mcnally-deterministic-
cbor-12, 7 February 2025,
<https://datatracker.ietf.org/doc/html/draft-mcnally-
deterministic-cbor-12>.
[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015,
<https://www.rfc-editor.org/rfc/rfc7493>.
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[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>.
[RFC9741] Bormann, C., "Concise Data Definition Language (CDDL):
Additional Control Operators for the Conversion and
Processing of Text", RFC 9741, DOI 10.17487/RFC9741, March
2025, <https://www.rfc-editor.org/rfc/rfc9741>.
[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. Information Model, Data Model and Serialization
This appendix is informative.
For a good understanding of this document, it is helpful to
understand the difference between an information model, a data model
and serialization.
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+=============+============+==============+=========+==============+
| |Abstraction |Example |Standards|Implementation|
| |Level | | |Representation|
+=============+============+==============+=========+==============+
|Information |Top level; |The | | |
|Model |conceptual |temperature of| | |
| | |something | | |
+-------------+------------+--------------+---------+--------------+
|Data Model |Realization |A floating- |CDDL |API input to |
| |of |point number | |CBOR encoder |
| |information |representing | |library, |
| |in data |the | |output from |
| |structures |temperature | |CBOR decoder |
| |and data | | |library |
| |types | | | |
+-------------+------------+--------------+---------+--------------+
|Serialization|Actual bytes|Encoded CBOR |CBOR |Encoded CBOR |
| |encoded for |of a floating-| |in memory or |
| |transmission|point number | |for |
| | | | |transmission |
+-------------+------------+--------------+---------+--------------+
Table 3: A three-layer model of information representation
CBOR does not provide facilities for expressing information models.
They are mentioned here for completeness and to provide some context.
CBOR defines a palette of basic data items that can be grouped into
data types such as the usual integer or floating-point numbers, text
or byte strings, arrays and maps, and certain special "simple values"
such as Booleans and null. Extended data types may be constructed
from these basic types. These basic and extended types are used to
construct the data model of a CBOR protocol. One notation that is
often used for describing the data model of a CBOR protocol is CDDL
[RFC8610]. The various types of data items in the data model are
serialized per RFC 8949 [STD94] to create encoded CBOR data items.
A.1. Data Model, Encoding Variants and Interoperability with Partial
Implementations
In contrast to JSON, CBOR-related documents explicitly discuss the
data model separately from its serialization. Both JSON and CBOR
allow variation in the way some data items can be serialized:
* In JSON, the number 1 can be serialized in several different ways
(1, 0.1e1, 1.0, 100e-2) — while it may seem obvious to use 1 for
this case, this is less clear for 1000000000000000000000000000000
vs. 1e+30 or 1e30. (As its serialization also doubles as a human-
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readable interface, JSON also allows the introduction of blank
space for readability.) The lack of an agreed data model for JSON
led to the need for a complementary specification documenting an
interoperable subset [RFC7493].
* The CBOR standard addresses constrained environments, both by
being concise and by limiting variation, but also by conversely
allowing certain data items in the data model to be serialized in
multiple ways, which may ease implementation on low-resource
platforms. On the other hand, constrained environments may
further save resources by only partially implementing the decoder
functionality, e.g., by not implementing all those variations.
To deal with this encoding variation provided for certain data items,
CBOR defines a _Preferred Serialization_ (Section 4.1 of RFC 8949
[STD94]). _Partial CBOR implementations_ are more likely to
interoperate if their encoder uses Preferred Serialization and the
decoder implements decoding at least the Preferred Serialization as
well. A specific protocol for a constrained application may specify
restrictions that allow, e.g., some fields to be of fixed length,
guaranteeing interoperability even with partial implementations
optimized for this application.
Another encoding variation is provided by indefinite-length encoding
for strings, arrays, and maps, which enables these to be streamed
without knowing their length upfront (Section 3.2 of RFC 8949
[STD94]). For applications that do not perform streaming of this
kind, variation can be reduced (and often performance improved) by
only allowing definite-length encoding. The present document coins
the term _Basic Serialization_ for combining definite-length-only
with preferred encoding, further reducing the variation that a
decoder needs to deal with. The Common Deterministic Encoding, CDE,
finally combines basic serialization with a deterministic ordering of
entries in a map (Table 1).
Partial implementations of a representation format are quite common
in embedded applications. Protocols for embedded applications often
reduce the footprint of an embedded JSON implementation by explicitly
restricting the breadth of the data model, e.g., by not using
floating point numbers with 64 bits of precision or by not using
floating point numbers at all. These data-model-level restrictions
do not get in the way of using complete implementations ("generic
encoders/decoders", Section 5.2 of RFC 8949 [STD94]). (Note that
applications may need to complement deterministic encoding with
decisions on the deterministic representation of application data
into CBOR data items, see Appendix B.)
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The increasing constraints on encoding (unconstrained, preferred,
basic, CDE) are orthogonal to data-model-level data definitions as
provided by [RFC8610]. To be useful in all applications, these
constraints have been defined for all possible data items, covering
the full range of values offered by CBOR's data types. This ensures
that these serialization constraints can be applied to any CBOR
protocol, without requiring protocol-specific modifications to
generic encoder/decoder implementations.
Appendix B. 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.
For a CBOR protocol to provide deterministic representation, both the
encoding and application layer must be deterministic. While CDE
ensures determinism at the encoding layer, requirements at the
application layer may also be necessary.
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.
| For example, an application protocol that needs to represent
| birthdate/times could specify:
|
| * At the sender’s convenience, the birthdate/time MAY be
| sent either in epoch date format (as in tag 1) or string
| date format (as in tag 0).
|
| * The receiver MUST decode both formats.
|
| While this specification is interoperable, it lacks
| determinism. There is variability in the application layer
| akin to variability in the CBOR encoding layer when CDE is not
| required.
|
| To make this example application layer specification
| deterministic, allow only one date format (or at least be
| deterministic when there is a choice, e.g., by specifying
| string format for leap seconds only).
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Application protocols that need to represent a timestamp 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).
Another source of application layer variability comes from the
variety of number types CBOR offers. For instance, the number 2 can
be represented as an integer, float, big number, decimal fraction and
other. Most protocols designs will just specify one number type to
use, and that will give determinism, but here’s an example
specification that doesn’t:
| For instance, CWT [RFC8392] 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]):
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|
| | 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.
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-checking decoders", see also
Appendix C). 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.
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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).
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 C. 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 as well as CDE Encoders and CDE-
checking 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.
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* 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-checking 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
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 its input for proper
CDE encoding is to re-encode the decoded data with CDE and check
for bit-to-bit equality with the original input.
C.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.
C.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).
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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.
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 means that a double or single
quiet NaN that has a zero NaN payload will always be
represented 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.)
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C.1.2. Decoders and Preferred Serialization
There are no special requirements that CBOR decoders need to meet to
be what could be called a "Preferred Serialization Decoder".
Partial decoder implementations that want to accept at least
Preferred Serialization 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, both definite-length and
indefinite-length arrays or maps MUST be accepted.
3. If text or byte strings are supported, both definite-length and
indefinite-length text or byte strings MUST be accepted.
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.
C.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.
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C.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).
C.2.2. Decoders and Basic Serialization
There are no special requirements that CBOR decoders need to meet to
be what could be called a "Basic Serialization Decoder".
Partial decoder implementations that want to accept at least Basic
Serialization need to pay attention to the requirements for partial
decoder implementations that accept Preferred Serialization, with the
following relaxations from the items 2 and 3 of Appendix C.1.2:
1. If arrays or maps are supported, definite-length arrays or maps
MUST be accepted.
2. If text or byte strings are supported, definite-length text or
byte strings MUST be accepted.
C.3. CDE
C.3.1. CDE Encoders
1. CDE encoders MUST only emit CBOR that fulfills the basic
serialization rules (Appendix C.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.
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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.
C.3.2. CDE-checking Decoders
The term "CDE-checking Decoder" is a shorthand for a CBOR decoder
that advertises _supporting_ CDE (see the start of this appendix).
1. CDE-checking decoders MUST follow the rules for decoders that
accept Basic Serialization (Appendix C.2.2) and MUST check the
input for keeping the Basic Serialization constraints.
2. CDE-checking 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-checking 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).
Appendix D. Encoding Examples
The following three tables provide examples of CDE-encoded CBOR data
items, each giving Diagnostic Notation (EDN
[I-D.ietf-cbor-edn-literals]), the encoded data item in hexadecimal,
and a comment.
Implementers that want to use these examples as test input may be
interested in the file example-table-input.csv in the github
repository cbor-wg/draft-ietf-cbor-cde.
D.1. Integer Value Examples
+-----------------------+------------------------+----------------+
| EDN | CBOR (hex) | Comment |
+-----------------------+------------------------+----------------+
| 0 | 00 | Smallest |
| | | unsigned |
| | | immediate int |
| -1 | 20 | Largest |
| | | negative |
| | | immediate int |
| 23 | 17 | Largest |
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| | | unsigned |
| | | immediate int |
| -24 | 37 | Smallest |
| | | negative |
| | | immediate int |
| 24 | 1818 | Smallest |
| | | unsigned one- |
| | | byte int |
| -25 | 3818 | Largest |
| | | negative one- |
| | | byte int |
| 255 | 18ff | Largest |
| | | unsigned one- |
| | | byte int |
| -256 | 38ff | Smallest |
| | | negative one- |
| | | byte int |
| 256 | 190100 | Smallest |
| | | unsigned two- |
| | | byte int |
| -257 | 390100 | Largest |
| | | negative two- |
| | | byte int |
| 65535 | 19ffff | Largest |
| | | unsigned two- |
| | | byte int |
| -65536 | 39ffff | Smallest |
| | | negative two- |
| | | byte int |
| 65536 | 1a00010000 | Smallest |
| | | unsigned four- |
| | | byte int |
| -65537 | 3a00010000 | Largest |
| | | negative four- |
| | | byte int |
| 4294967295 | 1affffffff | Largest |
| | | unsigned four- |
| | | byte int |
| -4294967296 | 3affffffff | Smallest |
| | | negative four- |
| | | byte int |
| 4294967296 | 1b0000000100000000 | Smallest |
| | | unsigned |
| | | eight-byte int |
| -4294967297 | 3b0000000100000000 | Largest |
| | | negative |
| | | eight-byte int |
| 18446744073709551615 | 1bffffffffffffffff | Largest |
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| | | unsigned |
| | | eight-byte int |
| -18446744073709551616 | 3bffffffffffffffff | Smallest |
| | | negative |
| | | eight-byte int |
| 18446744073709551616 | c249010000000000000000 | Smallest |
| | | unsigned |
| | | bigint |
| -18446744073709551617 | c349010000000000000000 | Largest |
| | | negative |
| | | bigint |
+-----------------------+------------------------+----------------+
Table 4: Integer Value Examples
D.2. Floating Point Value Examples
+---------------------------+--------------------+------------------+
| EDN | CBOR (hex) | Comment |
+---------------------------+--------------------+------------------+
| 0.0 | f90000 | Zero |
| -0.0 | f98000 | Negative zero |
| Infinity | f97c00 | Infinity |
| -Infinity | f9fc00 | -Infinity |
| NaN | f97e00 | NaN |
| NaN | f97e01 | NaN with non- |
| | | zero payload |
| 5.960464477539063e-8 | f90001 | Smallest |
| | | positive |
| | | 16-bit float |
| | | (subnormal) |
| 0.00006097555160522461 | f903ff | Largest |
| | | positive |
| | | subnormal |
| | | 16-bit float |
| 0.00006103515625 | f90400 | Smallest non- |
| | | subnormal |
| | | positive |
| | | 16-bit float |
| 65504.0 | f97bff | Largest |
| | | positive |
| | | 16-bit float |
| 1.401298464324817e-45 | fa00000001 | Smallest |
| | | positive |
| | | 32-bit float |
| | | (subnormal) |
| 1.1754942106924411e-38 | fa007fffff | Largest |
| | | positive |
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| | | subnormal |
| | | 32-bit float |
| 1.1754943508222875e-38 | fa00800000 | Smallest non- |
| | | subnormal |
| | | positive |
| | | 32-bit float |
| 3.4028234663852886e+38 | fa7f7fffff | Largest |
| | | positive |
| | | 32-bit float |
| 5.0e-324 | fb0000000000000001 | Smallest |
| | | positive |
| | | 64-bit float |
| | | (subnormal) |
| 2.225073858507201e-308 | fb000fffffffffffff | Largest |
| | | positive |
| | | subnormal |
| | | 64-bit float |
| 2.2250738585072014e-308 | fb0010000000000000 | Smallest non- |
| | | subnormal |
| | | positive |
| | | 64-bit float |
| 1.7976931348623157e+308 | fb7fefffffffffffff | Largest |
| | | positive |
| | | 64-bit float |
| -0.0000033333333333333333 | fbbecbf647612f3696 | Arbitrarily |
| | | selected |
| | | number |
| 10.559998512268066 | fa4128f5c1 | -"- |
| 10.559998512268068 | fb40251eb820000001 | Next in |
| | | succession |
| 295147905179352830000.0 | fa61800000 | 2^68 |
| | | (diagnostic |
| | | notation |
| | | truncates |
| | | precision) |
| 2.0 | f94000 | Number |
| | | without a |
| | | fractional |
| | | part |
| -5.960464477539063e-8 | f98001 | Largest |
| | | negative |
| | | subnormal |
| | | 16-bit float |
| -5.960464477539062e-8 | fbbe6fffffffffffff | Adjacent to |
| | | largest |
| | | negative |
| | | subnormal |
| | | 16-bit float |
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| -5.960464477539064e-8 | fbbe70000000000001 | -"- |
| -5.960465188081798e-8 | fab3800001 | -"- |
| 0.0000609755516052246 | fb3f0ff7ffffffffff | Adjacent to |
| | | largest |
| | | subnormal |
| | | 16-bit float |
| 0.000060975551605224616 | fb3f0ff80000000001 | -"- |
| 0.000060975555243203416 | fa387fc001 | -"- |
| 0.00006103515624999999 | fb3f0fffffffffffff | Adjacent to |
| | | smallest |
| | | 16-bit float |
| 0.00006103515625000001 | fb3f10000000000001 | -"- |
| 0.00006103516352595761 | fa38800001 | -"- |
| 65503.99999999999 | fb40effbffffffffff | Adjacent to |
| | | largest |
| | | 16-bit float |
| 65504.00000000001 | fb40effc0000000001 | -"- |
| 65504.00390625 | fa477fe001 | -"- |
| 1.4012984643248169e-45 | fb369fffffffffffff | Adjacent to |
| | | smallest |
| | | subnormal |
| | | 32-bit float |
| 1.4012984643248174e-45 | fb36a0000000000001 | -"- |
| 1.175494210692441e-38 | fb380fffffbfffffff | Adjacent to |
| | | largest |
| | | subnormal |
| | | 32-bit float |
| 1.1754942106924412e-38 | fb380fffffc0000001 | -"- |
| 1.1754943508222874e-38 | fb380fffffffffffff | Adjacent to |
| | | smallest |
| | | 32-bit float |
| 1.1754943508222878e-38 | fb3810000000000001 | -"- |
| 3.4028234663852882e+38 | fb47efffffdfffffff | Adjacent to |
| | | largest |
| | | 32-bit float |
| 3.402823466385289e+38 | fb47efffffe0000001 | -"- |
+---------------------------+--------------------+------------------+
Table 5: Floating Point Value Examples
D.3. Failing Examples
+-----------------------+--------------------------+--------------+
| EDN | CBOR (hex) | Comment |
+-----------------------+--------------------------+--------------+
| {"b":0,"a":1} | a2616200616101 | Incorrect |
| | | map key |
| | | ordering |
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| [4, 5] | 98020405 | Array length |
| | | not in |
| | | preferred |
| | | encoding |
| 255 | 1900ff | Integer not |
| | | in preferred |
| | | encoding |
| -18446744073709551617 | c34a00010000000000000000 | Bigint with |
| | | leading zero |
| | | bytes |
| 10.5 | fa41280000 | Not in |
| | | preferred |
| | | encoding |
| NaN | fa7fc00000 | Not in |
| | | preferred |
| | | encoding |
| 65536 | c243010000 | Integer |
| | | value too |
| | | small for |
| | | bigint |
| (_ h'01', h'0203') | 5f4101420203ff | Indefinite |
| | | length |
| | | encoding |
| (Not CBOR) | f818 | Simple |
| | | values |
| | | 24..31 not |
| | | in use |
| (Not CBOR) | fc | Reserved (ai |
| | | = 28..30) |
+-----------------------+--------------------------+--------------+
Table 6: Failing Examples
Appendix E. Examples for Preferred Serialization of Integers
This appendix looks at the set of encoded CBOR data items that
represent the integer number 1. Preferred Serialization chooses one
of them (0x01), which is then always used to encode the number. The
CDE encoding constraints include those of preferred serialization. A
CDE-checking decoder checks that no other serialization is being used
in the encoded data item being decoded.
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+===================================+=====================+
| Serialization of integer number 1 | Preferred? |
+===================================+=====================+
| 0x01 | yes (shortest mt0) |
+-----------------------------------+---------------------+
| 0x1801, 0x190001, 0x1a00000001, | no (mt0, but not |
| 0x1b0000000000000001 | shortest argument) |
+-----------------------------------+---------------------+
| 0xc24101 | no (could use mt0) |
+-----------------------------------+---------------------+
| 0xc2420001, 0xc243000001, etc. | no (could use mt0, |
| | uses leading zeros) |
+-----------------------------------+---------------------+
| 0xc25f41004101ff, and similar | no (could use mt0, |
| | uses leading zeros) |
+-----------------------------------+---------------------+
Table 7: Serializations of integer number 1
For the integer number 100000000000000000000 (1 with 20 decimal
zeros), the only _basic_ serialization is:
C2 # tag(2)
49 # bytes(9)
056BC75E2D63100000 #
(Note that, in addition to this serialization, there are multiple
serializations that would also count as _preferred_ serializations,
as the preferred serialization constraint by itself does not exclude
indefinite length encoding of the byte string that is the content of
tag 2.)
List of Tables
1. Constraints on the Serialization of CBOR (Table 1)
2. New control operators to be registered (Table 2)
3. A three-layer model of information representation (Table 3)
4. Integer Value Examples (Table 4)
5. Floating Point Value Examples (Table 5)
6. Failing Examples (Table 6)
7. Serializations of integer number 1 (Table 7)
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Acknowledgments
An early 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 the use of ALDR rules/rulesets on
top of that. Mikolai Gütschow proposed adding Section 2. Anders
Rundgren provided most of the initial text that turned into
Appendix D.
Contributors
Laurence Lundblade
Security Theory LLC
Email: lgl@securitytheory.com
Laurence provided most of the text that became Appendix A and
Appendix C.
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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