Internet DRAFT - draft-ietf-cbor-cde
draft-ietf-cbor-cde
CBOR C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Best Current Practice 3 March 2024
Expires: 4 September 2024
CBOR Common Deterministic Encoding (CDE)
draft-ietf-cbor-cde-02
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.
This document also introduces the concept of Application Profiles,
which are layered on top of the CBOR CDE Profile and can address more
application specific requirements. Application Profiles are defined
in separate documents.
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions and Definitions . . . . . . . . . . . . . . . 3
2. CBOR Common Deterministic Encoding Profile (CDE) . . . . . . 3
3. Application Profiles . . . . . . . . . . . . . . . . . . . . 5
4. CDDL support . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Implementers' Checklists . . . . . . . . . . . . . . 9
A.1. Preferred Serialization . . . . . . . . . . . . . . . . . 10
A.1.1. Preferred Serialization Encoders . . . . . . . . . . 10
A.1.2. Preferred Serialization Decoders . . . . . . . . . . 11
A.2. CDE . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
A.2.1. CDE Encoders . . . . . . . . . . . . . . . . . . . . 11
A.2.2. CDE Decoders . . . . . . . . . . . . . . . . . . . . 12
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
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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.
This document also introduces the concept of Application Profiles,
which are layered on top of the CBOR CDE Profile and can address more
application specific requirements. Application Profiles are defined
in separate documents. [I-D.mcnally-deterministic-cbor] is an
example for such a document.
1.1. Conventions and Definitions
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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. 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].
In many cases, CBOR provides more than one way to encode a data item,
but also provides a recommendation for a _Preferred Serialization_.
The _CoRE Deterministic Encoding Requirements_ generally pick the
preferred serializations as mandatory; they also pick additional
choices such as definite-length encoding. Finally, it defines a map
ordering based on lexicographic ordering of the (deterministically)
encoded map keys.
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.
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[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. The CBOR Common Deterministic Encoding Profile (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).
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 on the tag contents may be hard to do
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 this Profile.
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, which need to be made to obtain a 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.
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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. There is no special handling of NaN values, except that the
preferred serialization rules also apply to NaNs with payloads,
using the canonical encoding of NaNs as defined in [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 payload. Further clarifying [IEEE754], the CBOR
encoding uses a leading bit of 1 to encode a quiet NaN; encoding
of signaling NaN is NOT RECOMMENDED but is achieved by using a
leading bit of 0.
Typically, most applications that employ NaNs in their storage
and communication interfaces will only use the NaN with payload
0, which therefore deterministically encodes as 0xf97e00.
5. There is no special handling of subnormal values.
6. The CBOR Common Deterministic Encoding Profile does not presume
equivalence of basic floating point values with floating point
values using other representations (e.g., tag 4/5).
The main intent here is to preserve the basic generic data model, so
Application Profiles can make their own decisions within that data
model. E.g., an application profile can decide that it only ever
allows a single NaN value that would encoded as 0xf97e00, so a CDE
implementation focusing on this application profile would not need to
provide processing for other NaN values. Basing the definition of
both CDE and Application Profiles on the generic data model of CBOR
also means that there is no effect on CDDL [RFC8610], except where
the data description documents encoding decision for byte strings
carrying embedded CBOR.
3. Application Profiles
While the CBOR Common Deterministic Encoding Profile (CDE) provides
for commonality between different applications of CBOR, it is useful
to further constrain the set of data items handled in a group of
applications (_exclusions_) and to define further mappings
(_reductions_) that help the applications in such a group get by with
the exclusions.
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For example, the dCBOR Application Profile specifies the use of
Deterministic Encoding as defined in Section 4.2 of RFC 8949 [STD94]
(see also [I-D.bormann-cbor-det] for more information) together with
some application-level rules. See [I-D.mcnally-deterministic-cbor]
for a definition of the dCBOR Application Profile that makes use of
CDE.
In general, the application-level rules specified by an Application
Profile are based on the shared CBOR Common Deterministic Encoding
Profile; they do not "fork" CBOR in the sense of requiring distinct
generic encoder/decoder implementations.
An Application Profile 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. Similarly, generic CBOR
encoders will be able to produce valid CBOR that can be processed by
Application Profile implementations, if handed Application Profile
conforming data model level information from an application.
Please note that the separation between standard CBOR processing and
the processing required by the Application Profile is a conceptual
one: Instead of employing generic encoders/decoders, both Application
Profile processing and standard CBOR processing can be combined into
a encoder/decoder specifically designed for the Application Profile.
An Application Profile is intended to be used in conjunction with an
application, which typically will use a subset of the CBOR generic
data model, which in turn influences which subset of the application
profile is used. As a result, an Application Profile itself places
no direct requirement on what minimum subset of CBOR is implemented.
For instance, an application profile might define rules for the
processing of floating point values, but there is no requirement that
implementations of that Application Profile 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.
4. CDDL support
[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. This specification
adds two CDDL control operators that can be used for this.
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The control operators .cde and .cdeseq are exactly like .cbor and
.cborseq except that they also require the encoded data item(s) to be
in Common CBOR Deterministic Encoding.
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 .cborseq 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, Application Profiles can define similar control operators
that also embody the processing required by the Application Profile,
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 1 into
the registry "CDDL Control Operators" of [IANA.cddl]:
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+=========+===========+
| Name | Reference |
+=========+===========+
| .cde | [RFCXXXX] |
+---------+-----------+
| .cdeseq | [RFCXXXX] |
+---------+-----------+
Table 1: New control
operators to be
registered
7. References
7.1. Normative References
[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>.
[RFC2119] 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/rfc/rfc2119>.
[RFC8174] 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/rfc/rfc8174>.
[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>.
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7.2. Informative References
[I-D.bormann-cbor-det]
Bormann, C., "CBOR: On Deterministic Encoding", Work in
Progress, Internet-Draft, draft-bormann-cbor-det-02, 3
March 2024, <https://datatracker.ietf.org/doc/html/draft-
bormann-cbor-det-02>.
[I-D.mcnally-deterministic-cbor]
McNally, W., Allen, C., and C. Bormann, "dCBOR: A
Deterministic CBOR Application Profile", Work in Progress,
Internet-Draft, draft-mcnally-deterministic-cbor-07, 9
January 2024, <https://datatracker.ietf.org/doc/html/
draft-mcnally-deterministic-cbor-07>.
Appendix A. 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.
* 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. Application profiles such as dCBOR are
different as they can affect the data model by restricting some
values and ranges.
* CBOR decoders in general are not required to check for preferred
serialization or CDE and reject inputs that do not do not fulfill
their requirements.. However, in an environment that employs
deterministic encoding, this negates many of its benefits.
Decoder implementations that advertise "support" for preferred
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serialization or CDE need to check the encoding and reject input
that is not encoded to the encoding specification in use. Again,
application profiles such as 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.
A.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.
A.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 maps or arrays are emitted, they MUST use definite-length
encoding (never indefinite-length).
3. If text or byte strings are emitted, they MUST use definite-
length encoding (never indefinite-length).
4. 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
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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. Positive and negative infinity and zero MUST be
represented in half-precision floating point.
* NaNs, and thus NaN payloads MUST be supported.
As with all floating point numbers, NaNs with payloads MUST be
reduced to the shortest of double, single or half precision
that preserves the NaN payload. The reduction is performed by
removing the rightmost N bits of the payload, where N is the
difference in the number of bits in the significand (mantissa)
between the original format and the reduced format. The
reduction is performed only (preserves the value only) if all
the rightmost bits removed are zero. (This will always reduce
a double or single quiet NaN with a zero NaN payload to a
half-precision quiet NaN.)
A.1.2. Preferred Serialization Decoders
1. Decoders MUST accept shortest-form encoded arguments.
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.
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.
* NaNs, and thus NaN payloads, MUST be accepted.
A.2. CDE
A.2.1. CDE Encoders
1. CDE encoders MUST only emit CBOR fulfilling the preferred
serialization rules (Appendix A.1.1).
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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.
A.2.2. CDE Decoders
1. CDE decoders MUST follow the rules for preferred serialization
decoders (Appendix A.1.2).
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]; more recent revisions of that
document now make use of the present document and the concept of
Application Profile. We would like to explicitly acknowledge that
this work has contributed greatly to shaping the concept of a CBOR
Common Deterministic Encoding and Application Profiles on top of
that.
Contributors
Laurence Lundblade
Security Theory LLC
Email: lgl@securitytheory.com
Laurence provided the text that became Appendix A.
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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