CBOR Common Deterministic Encoding (CDE)

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Table of Contents

1. Introduction

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

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, they define 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.¶

[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 onto the tag contents may require additional attention to perform it in a deterministic way; see Section 3.2 of [I-D.bormann-cbor-det] for more explanation as well as examples. As the CDE would continually need to address additional issues raised by the registration of new tags, this specification recommends that new tag registrations address deterministic encoding in the context of CDE.¶

A particularly difficult field to obtain deterministic encoding for is floating point numbers, partially because they themselves are often obtained from processes that are not entirely deterministic between platforms. See Section 3.2.2 of [I-D.bormann-cbor-det] for more details. Section 4.2.2 of RFC 8949 [STD94] presents a number of choices, 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.¶

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 that the preferred serialization rules also apply to NaNs (with zero or non-zero payloads), using the canonical encoding of NaNs as defined in Section 6.2.1 of [IEEE754]. Specifically, this means that shorter forms of encodings for a NaN are used when that can be achieved by only removing trailing zeros in the NaN payload (example serializations are available in Appendix A.1.2 of [I-D.bormann-cbor-numbers]). Further clarifying a "should"-level statement in Section 6.2.1 of [IEEE754], the CBOR encoding always uses a leading bit of 1 in the significand to encode a quiet NaN; the use of signaling NaNs by application protocols is NOT RECOMMENDED but when presented by an application these are encoded by using a leading bit of 0.¶

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

5. There is no special handling of subnormal values.¶

6. CDE does not presume equivalence of basic floating point values with floating point values using other representations (e.g., tag 4/5). Such equivalences and related deterministic representation rules can be added at the ALDR level if desired, e.g., by stipulating additional equivalences and by restricting 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 ALDR Profiles, see Appendix A) can make their own decisions within that data model. E.g., an application's ALDR rules can decide that it only ever allows a single NaN value that would be encoded as 0xf97e00, so a CDE implementation focusing on this application would not need to provide processing for other NaN values. Basing the definition of both CDE and ALDR rules on the generic data model of CBOR also means that there is no effect on the Concise Data Definition Language (CDDL) [RFC8610], except where the data description is documenting specific encoding decisions for byte strings that carry embedded CBOR.¶

3. CDDL support

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

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

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

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

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

leaf = #6.24(bytes .cde any)

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

(Note that the .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, 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.¶

4. 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.¶

5. 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 the [IANA.cddl] registry group:¶

6. References

Appendix A. Application-level Deterministic Representation Rules

This appendix is informative.¶

While the CBOR Common Deterministic Encoding Profile (CDE) provides for commonality between different applications of CBOR, it can be 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.¶

For example, the dCBOR ALDR Profile [I-D.mcnally-deterministic-cbor] specifies the use of CDE together with some application-level rules, 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 those specified by an ALDR Profile) enable simply using the shared CBOR Common Deterministic Encoding Profile; they do not "fork" CBOR in the sense of requiring distinct generic encoder/decoder implementations.¶

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

Please note that the separation between standard CBOR processing and the processing required by the ALDR rules is a conceptual one: Instead of employing generic encoders/decoders, both ALDR rule processing and standard CBOR processing can be combined into an encoder/decoder specifically designed for a particular set of ALDR rules (such as those required by a particular application or set of applications, possibly specified as an ALDR Profile).¶

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

Appendix B. Implementers' Checklists

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

Notes:¶

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

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

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

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

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

Acknowledgments

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

Contributors

Laurence provided most of the text that became Appendix B.¶

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