Common CBOR Deterministic Encoding and Application Profiles

Internet-Draft	CBOR Profiles	August 2023
Bormann	Expires 24 February 2024	[Page]

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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 discusses a Common CBOR Deterministic Encoding Profile that can be shared by a large set of applications with potentially diverging detailed requirements. The concept of Application Profiles is layered on top of the Common CBOR Deterministic Encoding Profile and can address those more application specific requirements. The document defines the application profile "Gordian dCBOR" as an example of an application profile built on the Common CBOR Deterministic Encoding Profile.¶

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. Common CBOR Deterministic Encoding Profile (CDE)

This specification defines the Common CBOR Deterministic Encoding Profile (CDE) based on the Core Deterministic Encoding Requirements defined for CBOR in Section 4.2.1 of [STD94].¶

In many cases, CBOR provides more than one way to encode a data item, but also provides a recommendation for a Preferred Encoding. The CoRE Deterministic Encoding Requirements generally pick the preferred encodings 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 [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.¶

The 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 [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., tag 2/3 extend the range of basic major types 0/1 in a seamless way. Section 4.2.2 of [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 Encoding (Section 3.4.3 of [STD94]).¶

The Common CBOR 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 [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 [STD94] presents a number of choices, which need to be made to obtain a Common CBOR Deterministic Encoding Profile (CDE). Specifically, CDE specifies (in the order of the bullet list at the end of Section 4.2.2 of [STD94]):¶

Besides the mandated use of preferred encoding, 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.¶
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).¶
There is no special handling of NaN values, except that the preferred encoding rules also apply to NaNs with payloads, using the canonical encoding of NaNs as defined in [IEEE754]. Typically, most applications that employ NaNs in their storage and communication interfaces will only use the NaN with payload 0, which encodes as 0xf97e00.¶
There is no special handling of subnormal values.¶
The Common CBOR Deterministic Encoding Profile does not presume equivalence of floating point values with other representation (e.g., tag 4/5) with basic floating point values.¶

The main intent here is to preserve the basic generic data model, so application profiles can make their own decisions. For an example of that, see Section 3.1.3.¶

While [I-D.rundgren-deterministic-cbor] is a relatively terse document that is not always easy to interpret, to this author its intent appears to be aligned with that of the Common CBOR Deterministic Encoding Profile defined here, except that it mandates reduction of all NaN values to the one encoded as 0xf97e00.¶

3. Application Profiles

While the Common CBOR 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.¶

The dCBOR Application Profile specifies the use of Deterministic Encoding as defined in Section 4.2 of [STD94] (see also [I-D.bormann-cbor-det] for more information) together with some application-level rules. To provide an example, but also to provide a normative definition, the rules for Gordian dCBOR [I-D.mcnally-deterministic-cbor] are specified in this section.¶

The application-level rules specified by an Application Profile are based on the same Common CBOR Deterministic Encoding Profile; they do not "fork" CBOR.¶

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: Both Application Profile processing and standard CBOR processing can be combined into a special dCBOR/CBOR encoder/decoder.¶

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, while the dCBOR application profile defines rules for the processing of floating point values, there is no requirement that dCBOR implementations 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.¶

3.1. Gordian dCBOR

Gordian dCBOR [I-D.mcnally-deterministic-cbor] provides an application profile that requires encoders to produce valid CBOR in deterministic encoding as defined in the Common CBOR Deterministic Encoding Profile. Gordian dCBOR also requires dCBOR decoders to reject CBOR data items that were not deterministically encoded.¶

Beyond the Common CBOR Deterministic Encoding Profile, dCBOR imposes certain limitations on the CBOR basic generic data model. Some items that can be represented in the CBOR basic generic data model are entirely outlawed by this application profile. Other items are represented by what are considered equivalent data items by the dCBOR equivalence model, so a recipient application might receive data that may not be the same data in the CBOR equivalence model as the ones the generating application produced.¶

These restrictions mainly are about numeric values, which are therefore the subject of the main subsection of this section.¶

3.1.1. Removing Simple Values

Only the three simple values false (0xf4), true (0xf5), and null (0xf6) are allowed at the application level; the remaining 253 values must be rejected.¶

3.1.2. Removing Integer Values

Only the integer values in range [-2⁶³, 2⁶⁴-1] can be expressed in dCBOR ("basic dCBOR integers"). Note that the range is asymmetric, with only 2⁶³ negative values, but 2⁶⁴ unsigned (non-negative) values, creating an (approximately) 64.6 bit integer.¶

This maps to a choice between a platform 64-bit two's complement signed integer (often called int64) and a 64-bit unsigned integer (uint64). (Specific applications will, of course, further restrict ranges of integers that are considered valid for the application, based on their position and semantics in the CBOR data item.)¶

3.1.3. Numeric Reduction of Floating-Point Values

dCBOR implementations that do support floating point numbers MUST perform the following two reductions of numeric values when constructing CBOR data items:¶

When representing integral floating point values (floating point values with a zero fractional part), check whether the mathematically identical value can be represented as a dCBOR integer value, i.e., is in the range [-2⁶³, 2⁶⁴-1] given above. If that is the case, convert the integral floating point to that mathematically identical integer value before encoding it. (Deterministic Encoding will then ensure the shortest length encoding is used.) This means that if a floating point value has a non-zero fractional part, or an exponent that takes it out of the given range of basic dCBOR integers, the original floating point value is used for encoding. (Specifically, conversion to a bignum is never considered.)¶

This also means that the three representations of a zero number in CBOR (0, 0.0, -0.0 in diagnostic notation) are all reduced to the basic integer 0 (with preferred encoding 0x00).¶

Note that this reduction can turn valid maps into invalid ones, as it can create duplicate keys, e.g., for:¶
```
{
   10: "integer ten",
   10.0: "floating ten"
}
```
¶
This means that, at the application level, the application MUST prevent the creation of maps that would turn invalid in dCBOR processing.¶
In addition, before encoding, represent all NaN values by using the quiet NaN value having the half-width CBOR representation 0xf97e00.¶

dCBOR-based applications MUST accept these "reduced" numbers in place of the original value, e.g., a dCBOR-based application that expects a floating point value needs to accept a basic dCBOR integer in its place (and, if needed, convert it to a floating point value for its own further processing).¶

dCBOR-based applications MUST NOT accept numbers that have not been reduced as specified in this section, except maybe by making the unreduced numbers available for their diagnostic value when there has been an explicit request to do so. This is similar to a checking flag mentioned in Section 5.1 (API Considerations) of [I-D.bormann-cbor-det] being set by default.¶

3.2. Extensibility

[I-D.mcnally-deterministic-cbor] does not discuss extensibility. A meaningful way to handle extensibility in this application profile would be to lift value range restrictions, keeping the profile-specific equivalence rules shown here intact and possibly adding equivalences as needed for newly allowed values.¶

This subsection presents two speculative extensions of dCBOR, called dCBOR-wide1 and dCBOR-wide2, to point out different objectives that can lead the development of an extension.¶

3.2.1. dCBOR-wide1

This speculative extension of dCBOR attempts to meet two objectives:¶

All instances that meet dCBOR are also instances of dCBOR-wide1; due to the nature of deterministic serialization this also means that dCBOR-wide1 instances that only use application data model values that are allowed by dCBOR are also dCBOR instances.¶
The range of integers that can be provided by an application and can be interchanged as exact numbers is expanded to [-2¹²⁷, 2¹²⁸-1], now also covering the types i128 and u128 in Rust [i128][u128].¶

This extension is achieved by simply removing the integers in the extended range from the exclusion range of dCBOR. The numeric reduction rule is not changed, so it still applies only to integral-valued floating-point numbers in the range [-2⁶³, 2⁶⁴-1].¶

Examples for the application-to-CDE mapping of dCBOR-wide1 are shown in Table 1. In the dCBOR column, items that are not excluded in dCBOR are marked ✓, items that are excluded in dCBOR and therefore are new in dCBOR-wide1 are marked 👎.¶

Table 1: Speculative "dCBOR-wide1" application profile
Application data Numeric reduction (if any) Encoding via CDE	dCBOR?
`0` `—` `00`	✓
`0.0` `0` `00`	✓
`-0.0` `0` `00`	✓
`4.0` `4` `04`	✓
`-4.0` `-4` `23`	✓
`1.0e+19` `10000000000000000000` `1B8AC7230489E80000`	✓
`-1.0e+19` `—` `FBC3E158E460913D00`	✓
`10000000000000000000` `—` `1B8AC7230489E80000`	✓
`-10000000000000000000` `—` `3B8AC7230489E7FFFF`	👎
`1.0e+38` `—` `FB47D2CED32A16A1B1`	✓
`-1.0e+38` `—` `FBC7D2CED32A16A1B1`	✓
`100000000000000000000000000000000000000` `—` `C2504B3B4CA85A86C47A098A224000000000`	👎
`-100000000000000000000000000000000000000` `—` `C3504B3B4CA85A86C47A098A223FFFFFFFFF`	👎

This speculative extended profile does not meet a potential objective number 3 that unextended dCBOR does meet:¶

All integral-valued floating point numbers coming from an application that fit into an integer representation allowed by the application profile are represented as such.¶

Objective 1 prevents numeric reduction from being applied to values that are not excluded in dCBOR but do to receive numeric reduction there.¶

3.2.2. dCBOR-wide2

The speculative dCBOR-wide2 extension of dCBOR attempts to meet objectives 2 and 3 mentioned in Section 3.2.1. It cannot meet objective 1: items in Table 2 marked with a 💣 character are allows in dCBOR but have different serializations.¶

Table 2: Speculative "dCBOR-wide2" application profile
Application data Numeric reduction (if any) Encoding via CDE	dCBOR?
`0` `—` `00`	✓
`0.0` `0` `00`	✓
`-0.0` `0` `00`	✓
`4.0` `4` `04`	✓
`-4.0` `-4` `23`	✓
`1.0e+19` `10000000000000000000` `1B8AC7230489E80000`	✓
`-1.0e+19` `-10000000000000000000` `3B8AC7230489E7FFFF`	✓ 💣
`10000000000000000000` `—` `1B8AC7230489E80000`	✓
`-10000000000000000000` `—` `3B8AC7230489E7FFFF`	👎
`1.0e+38` `99999999999999997748809823456034029568` `C2504B3B4CA85A86C4000000000000000000`	✓ 💣
`-1.0e+38` `-99999999999999997748809823456034029568` `C3504B3B4CA85A86C3FFFFFFFFFFFFFFFFFF`	✓ 💣
`100000000000000000000000000000000000000` `—` `C2504B3B4CA85A86C47A098A224000000000`	👎
`-100000000000000000000000000000000000000` `—` `C3504B3B4CA85A86C47A098A223FFFFFFFFF`	👎

This extension is achieved by removing the integers in the extended range from the exclusion range of dCBOR, and by adding the extended range to the target range of numeric reduction.¶

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, or with the dCBOR application profile applied as well. This specification adds four 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 in Common CBOR Deterministic Encoding.¶

The control operators .dcbor and .dcborseq are exactly like .cde and .cdeseq except that they also require the encoded data item(s) to conform to the dCBOR application profile.¶

For example, the normative comment in Section 3 of [I-D.draft-mcnally-envelope-03]:¶

leaf = #6.24(bytes)  ; MUST be dCBOR

...can now be formalized as:¶

leaf = #6.24(bytes .dcbor 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 ...seq control operators do 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.)¶

5. Implementation Status

This section is to be removed before publishing as an RFC.¶

(Boilerplate as per Section 2.1 of [RFC7942]:)¶

This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.¶

According to [RFC7942], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".¶

5.1. Gordian dCBOR Application Profile

5.1.1. TypeScript

Implementation Location: [bc-dcbor-ts]¶
Primary Maintainer:¶
Languages: TypeScript (transpiles to JavaScript)¶
Coverage:¶
Testing:¶
Licensing:¶

5.1.2. Swift

Implementation Location: [BCSwiftDCBOR]¶
Primary Maintainer:¶
Languages: Swift¶
Coverage:¶
Testing:¶
Licensing: BSD-2-Clause-Patent¶

5.1.3. Rust

Implementation Location: [bc-dcbor-rust]¶
Primary Maintainer:¶
Languages: Rust¶
Coverage:¶
Testing:¶
Licensing: Custom¶

5.1.4. Ruby

Implementation Location: [cbor-dcbor]¶
Primary Maintainer: Carsten Bormann¶
Languages: Ruby¶
Coverage: Complete specification; complemented by CBOR encoder/decoder and command line interface from [cbor-diag] and deterministic encoding from [cbor-deterministic]. Checking of dCBOR exclusions not yet implemented.¶
Testing: Also available at https://cbor.me ¶
Licensing: Apache-2.0¶

7. 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 3 into the registry "CDDL Control Operators" of [IANA.cddl]:¶

Table 3: New control operators to be registered
Name	Reference
.cde	[RFCXXXX]
.cdeseq	[RFCXXXX]
.dcbor	[RFCXXXX]
.dcborseq	[RFCXXXX]

Acknowledgments

Section 3.1 of this document is based on the work of Wolf McNally and Christopher Allen as documented in [I-D.mcnally-deterministic-cbor] and discussed in 2023 in the CBOR working group.¶

Contributors

Wolf McNally

Blockchain Commons

Email: wolf@wolfmcnally.com

Christopher Allen

Blockchain Commons

Email: christophera@lifewithalacrity.com

Anders Rundgren

Independent

Montpellier

France

Email: anders.rundgren.net@gmail.com

URI: https://www.linkedin.com/in/andersrundgren/

Author's Address

Carsten Bormann

Universität Bremen TZI

Postfach 330440

D-28359 Bremen

Germany

Phone: +49-421-218-63921

Email: cabo@tzi.org