Internet-Draft dCBOR May 2023
McNally & Allen Expires 5 November 2023 [Page]
Network Working Group
Intended Status:
W. McNally
Blockchain Commons
C. Allen
Blockchain Commons

Gordian dCBOR: Deterministic CBOR Implementation Practices


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. Nonetheless, determinism is an opt-in feature of the specification, and most existing CBOR codecs put the primary burden of correct deterministic serialization and validation of deterministic encoding during deserialization on the engineer. This document specifies a set of norms and practices for CBOR codec implementors intended to support deterministic CBOR ("dCBOR") at the codec API level.

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1. Introduction

The goal of determinism in data encoding is that multiple agents serializing the same data will automatically achieve consensus on the byte-level form of that serialized data. Many data serialization formats give developers wide latitude on the serialized form, for example:

  • The use of whitespace in JSON, which may be omitted or used to taste.
  • The key-value pairs of map/dictionary structures are usually considered unordered. Therefore their order of serialization is taken to be semantically insignificant and so varies depending on the implementation.
  • Standards for the binary encoding of floating point numeric values often include bit patterns that are functionally equivalent, such as 0.0 and -0.0 or NaN and signalling NaN.
  • The number of bytes used to encode an integer or floating point value; e.g., in well-formed CBOR there are four valid ways to encode the integer 1 and three valid ways to encode the floating point value 1.0 giving a total of seven valid ways to encode the semantic concept 1.0. In JSON the problem is even worse, given that 1, 1., 1.0, 1.00, 1.000, etc. are equivalent representations of the same value.

Each of these choices made differently by separate agents yield different binary serializations that cannot be compared based on their hash values, and which therefore must be separately parsed and validated semantically field-by-field to decide whether they are identical. Such fast comparison for identicality using hashes is important in certain classes of application, where the hash is published or incorporated into other documents, hence "freezing" the form of the document. Where the hash is known or fixed, it is impossible to substitute a different document for the original that differs by even a single bit.

The CBOR standard addresses this problem in [RFC8949] §4.2, by narrowing the scope of choices available for encoding various values, but does not specify a set of norms and practices for CBOR codec implementors who value the benefits of deterministic CBOR, hereinafter called "dCBOR".

This document's goal is to specify such a set of norms and practices for dCBOR codec implementors.

It is important to stress that dCBOR is not a new dialect of CBOR, and that all dCBOR is well-formed CBOR that can be read by existing CBOR codecs.

This document is segmented into four sections. They include norms and practices that:

  • MUST be implemented in the codec (Serialization level),
  • MUST be implemented by developers of specifications dependent on dCBOR (Application level).
  • are acknowledged to fall under the purview of this document, but which are not yet specified (Future work).
  • are RECOMMENDED for dCBOR codec implementors (Recommendations).

2. Terminology

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.

This specification makes use of the following terminology:


Used in its now-customary sense as a synonym for "octet".


"coder-decoder", a software suite that both encodes (serializes) and decodes (deserializes) a data format.


"deterministic CBOR" encoded in conformance with the CBOR specification in this document.


To convert platform-native or application-centric data structures to/from an in-memory symbolic representation of CBOR.


To convert an in-memory symbolic representation of CBOR to/from a byte stream.

3. Serialization Level

This section defines requirements and practices falling in the purview of the dCBOR codec.

3.1. Base Requirements

dCBOR encoders MUST only emit CBOR conforming to the requirements of [RFC8949] §4.2.1. To summarize:

  • Variable-length integers MUST be as short as possible.
  • Floating-point values MUST use the shortest form that preseves the value.
  • Indefinite-length arrays and maps MUST NOT be used.
  • Map keys MUST be sorted in bytewise lexicographic order of their deterministic encodings.

dCBOR codecs MUST validate and return errors for any CBOR that is not conformant.

3.2. Reduction of Floating Point Values to Integers

While there is no requirement that dCBOR codecs implement support for floating point numbers, dCBOR codecs that do support them MUST reduce floating point values with no fractional part to the smallest integer value that can accurately represent it. If a numeric value has a fractional part or an exponent that takes it out of the range of representable integers, then it SHALL be encoded as a floating point value.

This practice still produces well-formed CBOR according to the standard, and all existing implementations will be able to read it. It does exclude a map such as the following from being validated as dCBOR, as it would have a duplicate key:

   10: "ten",
   10.0: "floating ten"

3.3. Reduction of NaNs and Infinities.

[IEEE754] defines the NaN (Not a Number) value [NAN]. This is usually divided into two types: quiet NaNs and signalling NaNs, and the sign bit is used to distinguish between these two types. However, the specification also includes a range of "payload" bits. These bit fields have no definite purpose and could be used to break CBOR determinism.

dCBOR encoders that support floating point MUST reduce all NaN values to the half-width quiet NaN value having the canonical bit pattern 0x7e00.

Similarly, encoders that support floating point MUST reduce all +INF values to the half-width +INF having the canonical bit pattern 0x7c00 and likewise with -INF to 0xfc00.

3.4. Reduction of BigNums to Integers

While there is no requirement that dCBOR codecs implement support for BigNums ≥ 2^64 (tags 2 and 3), codecs that do support them MUST use regular integer encodings where integers can represent the value.

3.5. CBOR_NEGATIVE_INT_MAX disallowed

The largest negative integer that can be represented in 64 bits two's complement (STANDARD_NEGATIVE_INT_MAX) is -2^63 (0x8000000000000000).

However, the largest negative integer that can be represented in CBOR (CBOR_NEGATIVE_INT_MAX) is -2^64 (0x10000000000000000), which requires 65 bits. The CBOR encoding for CBOR_NEGATIVE_INT_MAX is 0x3BFFFFFFFFFFFFFFFF.

Because of this incompatibility between the CBOR and standard representations, dCBOR disallows CBOR_NEGATIVE_INT_MAX: conformant encoders MUST never encode this sequence and conformant decoders MUST reject CBOR_NEGATIVE_INT_MAX as not well-formed.

Implementations that support BIGNUM are able to encode and decode this value as BIGNUM.

4. Application Level

4.1. Optional/Default Values

Protocols that depend on dCBOR MUST specify the optionality and semantics of field values. In key-value paired structures like CBOR maps, protocols MUST specify whether the field:

  • REQUIRED and the value MUST NOT be null.
  • OPTIONAL but if present the value MUST NOT be null.
  • REQUIRED and the value MAY be null.
  • OPTIONAL and the value MAY be null.

In the last case, the protocol specifier MUST state the semantic difference between the field being not present at all, and being present but having a null value. For example, in a map representing user preferences:

  • The absence of the field means the user needs to be asked for their preference,
  • The presence of the field with a null value means the user has been asked, but specified that they accept the current default.
  • If the field is present and the value is non-null, the user would have affirmatively specified a preference.

The rationale for this specificity is to remove semantic ambiguity and eliminate the choice over whether to encode a key-value pair where the value is null or omit it entirely.

4.2. Tagging Items

Protocols that depend on dCBOR MUST specify the circumstances under which a data item MUST or MUST NOT be tagged.

The codec API SHOULD afford conveniences such as protocol conformances that allow the association of a tag with a particular data type. The encoder MUST use such an associated tag when serializing, and the decoder MUST expect the associated tag when extracting a structure of that type.

5. Future Work

The following issues are currently left for future work:

  • How to deal with subnormal floating point values [SUBNORMAL].

6. API-Level Recommendations

This section is informative.

Many existing CBOR implementations give little or no guidance at the API level as to whether the CBOR being read conforms to the CBOR specification for deterministic encoding [RFC8949] §4.2, for example by emitting errors or warnings at deserialization time. Conversely, many existing implementations do not carry any burden of ensuring that CBOR is serialized in conformance with the CBOR determinstic encoding specification, again putting that burden on developers.

The authors of this document believe that for applications where dCBOR correctness as specified in this document is important, the codec itself should carry as much of this burden as possible. This is important both to minimize cognitive load during development, and help ensure interoperability between implementations.

6.1. General Practices for dCBOR Codecs

It is RECOMMENDED that dCBOR codecs:

  • Make it easy to emit compliant dCBOR.
  • Make it hard to emit non-compliant dCBOR.
  • Make it an error to read non-compliant dCBOR.

6.2. API Handling of Maps

It is RECOMMENDED that dCBOR APIs provide a dCBOR Map structure or similar that models the dCBOR canonical key encoding and order:

  • Supports insertion of unencoded key-value pairs.
  • Supports iteration through entries in dCBOR canonical key order.
  • Supports treating keys as duplicate that have identical dCBOR encodings, e.g., 10 and 10.0.

The dCBOR decoder SHOULD return an error if it encounters misordered or duplicate map keys.

6.3. API Handling of Numeric Values

The authors do make the following recommendations:

  • The encoder API SHOULD accept any supported numeric type for insertion into the CBOR stream and decide the dCBOR-conformant form for its encoding.
  • The API SHOULD allow any supported numeric type to be extracted, and return errors when the actual type encountered is not representable in the requested type. For example,

    • If the encoded value is "1.5" then requesting extraction of the value as floating point will succeed but requesting extraction as an integer will fail.
    • Similarly, if the value has a large exponent and therefore can be represented as either a floating point value or a BigNum, then attempting to extract it as a machine integer will fail.

6.4. Validation Errors

It is RECOMMENDED that a dCBOR decoder return errors when it encounters any of these conditions in the input stream:

  • underrun: early end of stream
  • badHeaderValue: unsupported CBOR major/minor item header
  • nonCanonicalNumeric: An integer, floating-point value, or BigNum was encoded in non-canonical form
  • invalidString: An invalid UTF-8 string was encountered
  • unusedData: Unused data encountered past the expected end of the input stream
  • misorderedMapKey: A map has keys not in canonical order
  • duplicateMapKey: A map has a duplicate key

7. Reference Implementations

This section is informative.

The current reference implementations that conform to these specifications are:

8. Security Considerations

This document inherits the security considerations of CBOR [RFC8949].

Vulnerabilities regarding dCBOR will revolve around whether an attacker can find value in either:

  • producing semantically different documents that are serialized using identical byte streams, or
  • producing semantically equivalent documents that are nonetheless serialized into non-identical byte streams

The first consideration is unlikely due to the Law of Identity (A is A). The second consideration 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-level requirements for dCBOR. Whether these possibilities present an identifiable attack surface is an open question that developers should consider.

9. IANA Considerations

This document makes no requests of IANA.

We considered requesting a new media type [RFC6838] for deterministic CBOR, e.g., application/d+cbor, but chose not to pursue this as all dCBOR is well-formed CBOR. Therefore, existing CBOR codecs can read dCBOR, and many existing codecs can also write dCBOR if the encoding rules are observed. Protocols that adopt dCBOR will simply have more stringent requirments for the CBOR they emit and ingest.

10. References

10.1. Normative References

"IEEE, "IEEE Standard for Floating-Point Arithmetic", IEEE Std 754-2019, DOI 10.1109/IEEESTD.2019.8766229", n.d., <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, , <>.

10.2. Informative References

"NaN", n.d., <>.
"Deterministic CBOR ("dCBOR") for Rust.", n.d., <>.
"Subnormal number", n.d., <>.
"Deterministic CBOR ("dCBOR") for Swift.", n.d., <>.


TODO acknowledge.

Authors' Addresses

Wolf McNally
Blockchain Commons
Christopher Allen
Blockchain Commons