]> Blockchain Commons

wolf@wolfmcnally.com

Blockchain Commons

christophera@lifewithalacrity.com

Universität Bremen TZI

cabo@tzi.org

Applications and Real-Time Network Working Group Internet-Draft The purpose of determinism is to ensure that semantically equivalent data items are encoded into identical byte streams. CBOR (RFC 8949) defines "Deterministically Encoded CBOR" in its Section 4.2, but leaves some important choices up to the application developer. The CBOR Common Deterministic Encoding (CDE) Internet Draft builds on this by specifying a baseline for application profiles that wish to implement deterministic encoding with CBOR. The present document provides an application profile "dCBOR" that can be used to help achieve interoperable deterministic encoding based on CDE for a variety of applications wishing an even narrower and clearly defined set of choices. About This Document Status information for this document may be found at . Source for this draft and an issue tracker can be found at .

Introduction CBOR has many advantages over other data serialization formats. One of its strengths is specifications and guidelines for serializing data deterministically, such that multiple agents serializing the same data automatically achieve consensus on the exact byte-level form of that serialized data. This is particularly useful when data must be compared for semantic equivalence by comparing the hash of its contents. Nonetheless, determinism is an opt-in feature of CBOR, and most existing CBOR codecs put the primary burden of correct deterministic serialization and validation of deterministic encoding during deserialization on the engineer. Furthermore, the specification leaves a number of important decisions around determinism up to the application developer. The CBOR Common Deterministic Encoding (CDE) Internet Draft builds on the basic CBOR specification by providing a baseline for application profiles that wish to implement deterministic encoding with CBOR. This document narrows CDE further into a set of requirements for the application profile "dCBOR". These requirements include but go beyond CDE, including requiring that dCBOR decoders validate that encoded CDE conforms to the requirements of this document.

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 when, and only when, they appear in all capitals, as shown here.

Application Profile The dCBOR Application Profile specifies the use of Deterministic Encoding as defined in and adds several exclusions and reductions specified in this section. Just as CDE does not "fork" CBOR, the rules specified here do not "fork" CDE: A dCBOR implementation produces well-formed, deterministically encoded CDE according to , and existing CBOR or CDE decoders will therefore be able to decode it. Similarly, CBOR or CDE encoders will be able to produce valid dCBOR if handed dCBOR conforming data model level information from an application. Note that the separation between standard CBOR or CDE processing and the processing required by the dCBOR application profile is a conceptual one: Both dCBOR processing and standard CDE/CBOR processing may be combined into a unified dCBOR/CDE/CBOR codec. The requirements in this document apply to encoding or decoding of dCBOR data, regardless of whether the codec is a unified dCBOR/CDE/CBOR codec operating in dCBOR-compliant modes, or a single-purpose dCBOR codec. Both of these are generically referred to as "dCBOR codecs" in this document. This application profile is intended to be used in conjunction with an application, which typically will use a subset of CDE/CBOR, which in turn influences which subset of the application profile is used. As a result, this application profile places no direct requirement on what subset of CDE/CBOR is implemented. For instance, there is no requirement that dCBOR implementations support floating point numbers (or any other kind of non-basic integer type, such as arbitrary precision integers or complex numbers) when they are used with applications that do not use them. However, this document does place requirements on dCBOR implementations that support negative 64-bit integers and 64-bit or smaller floating point numbers.

Common Deterministic Encoding Conformance dCBOR encoders:

1. MUST only emit CBOR conforming "CBOR Common Deterministic Encoding (CDE)" , including mandated preferred encoding of integers and floating point numbers and the lexicographic ordering of map keys.

dCBOR decoders:

2. MUST validate that encoded CBOR conforms to the requirements of .

Duplicate Map Keys CBOR defines maps with duplicate keys as invalid, but leaves how to handle such cases to the implementor (§2.2, §3.1, §5.4, §5.6). provides no additional mandates on this issue. dCBOR encoders:

1. MUST NOT emit CBOR maps that contain duplicate keys.

dCBOR decoders:

2. MUST reject encoded maps with duplicate keys.

65-bit Negative Integers dCBOR limits the valid encodings of integers to those that can be contained in a 64-bit machine register, either as a signed (int64) or unsigned (uint64) integer. This includes values in the range DCBOR_INT = [-2⁶³, 2⁶⁴-1]. As always with CBOR, whether the value is interpreted as non-negative or negative depends on whether it is encoded as a major type 0 or 1 value. CBOR integers in the range NEG_65 = [-2⁶⁴, -2⁶³ - 1] require 65 bits of precision, and are thus not representable in typical machine-sized integers. NEG_65 major type 1 values are invalid in dCBOR. dCBOR encoders:

1. MUST NOT encode major type 1 CBOR values in NEG_65.

dCBOR decoders:

2. MUST reject major type 1 CBOR values in NEG_65.

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.

Numeric Reduction The purpose of determinism is to ensure that semantically equivalent data items are encoded into identical byte streams. Numeric reduction ensures that semantically equal numeric values (e.g. 2 and 2.0) are encoded into identical byte streams (e.g. 0x02) by encoding "Integral floating point values" (floating point values with a zero fractional part) as integers when possible. dCBOR implementations that support floating point numbers:

1. MUST check whether floating point values to be encoded have the numerically equal value in DCBOR_INT as defined above. If that is the case, it MUST be converted to that numerically equal integer value before encoding it. (Preferred encoding will then ensure the shortest length encoding is used.) If a floating point value has a non-zero fractional part, or an exponent that takes it out of DCBOR_INT, the original floating point value is used for encoding. (Specifically, conversion to a CBOR 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 means some valid CDE/CBOR maps are not valid dCBOR maps, as numeric reduction can result in duplicate keys, for example, this is an invalid dCBOR map:

2. MUST reduce all encoded NaN values to the quiet NaN value having the half-width CBOR representation 0xf97e00.

dCBOR decoders that support floating point numbers:

3. MUST reject any encoded floating point values that are not encoded according to the above rules.

Simple Values Only the three "simple" (major type 7) values false (0xf4), true (0xf5), and null (0xf6) and the floating point values are valid in dCBOR. dCBOR encoders:

1. MUST NOT encode major type 7 values other than false, true, null, and the floating point values.

dCBOR decoders:

2. MUST reject any encoded major type 7 values other than false, true, null, and the floating point values.

CDDL support Similar to the CDDL support in CDE , this specification adds two CDDL control operators that can be used to specify that the data items should be encoded in CBOR Common Deterministic Encoding (CDE), with the dCBOR application profile applied as well. 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 : ...can now be formalized as:

Implementation Status {::boilerplate rfc7942info}

Swift

• Description: Single-purpose dCBOR reference implementation for Swift.
• Organization: Blockchain Commons
• Implementation Location:
• Primary Maintainer: Wolf McNally
• Languages: Swift
• Coverage: Complete
• Testing: Unit tests
• Licensing: BSD-2-Clause-Patent

Rust

• Description: Single-purpose dCBOR reference implementation for Rust.
• Organization: Blockchain Commons
• Implementation Location:
• Primary Maintainer: Wolf McNally
• Languages: Rust
• Coverage: Complete
• Testing: Unit tests
• Licensing: BSD-2-Clause-Patent

TypeScript

• Description: Single-purpose dCBOR reference implementation for TypeScript.
• Organization: Blockchain Commons
• Implementation Location:
• Primary Maintainer: Wolf McNally
• Languages: TypeScript (transpiles to JavaScript)
• Coverage: Complete
• Testing: Unit tests
• Licensing: BSD-2-Clause-Patent

Ruby

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

Security Considerations This document inherits the security considerations of CBOR . Vulnerabilities regarding dCBOR will revolve around whether an attacker can find value in producing semantically equivalent documents that are nonetheless serialized into non-identical byte streams. Such documents could be used to contain malicious payloads or exfiltrate sensitive data. The ability to create such documents could indicate the failure of a dCBOR decoder to correctly validate according to this document, or the failure of the developer to properly specify or implement application protocol requirements using dCBOR. Whether these possibilities present an identifiable attack surface is a question that developers should consider.

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 : CDDL Control Operators for dCBOR

Name	Reference
.dcbor	[RFCXXXX]
.dcborseq	[RFCXXXX]

References Normative References The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. Universität Bremen TZI 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. To demonstrate how Application Profiles can be built on the CDE, a companion document defines the application profile "dCBOR". IANA In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References n.d. n.d. n.d. Blockchain Commons Blockchain Commons Gordian Envelope specifies a structured format for hierarchical binary data focused on the ability to transmit it in a privacy- focused way, offering support for privacy as described in RFC 6973 and human rights as described in RFC 8280. Envelopes are designed to facilitate "smart documents" and have a number of unique features including: easy representation of a variety of semantic structures, a built-in Merkle-like digest tree, deterministic representation using CBOR, and the ability for the holder of a document to selectively elide specific parts of a document without invalidating the digest tree structure. This document specifies the base Envelope format, which is designed to be extensible. n.d. n.d. n.d.

Acknowledgments The authors are grateful for the contributions of Joe Hildebrand, Laurence Lundblade, and Anders Rundgren in the CBOR working group.