| Internet-Draft | dCBOR | May 2023 |
| McNally & Allen | Expires 5 November 2023 | [Page] |
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.¶
This note is to be removed before publishing as an RFC.¶
Source for this draft and an issue tracker can be found at https://github.com/BlockchainCommons/WIPs-IETF-draft-deterministic-cbor.¶
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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:¶
0.0 and -0.0 or NaN and signalling NaN.¶
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:¶
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.¶
This section defines requirements and practices falling in the purview of the dCBOR codec.¶
dCBOR encoders MUST only emit CBOR conforming to the requirements of [RFC8949] §4.2.1. To summarize:¶
dCBOR codecs MUST validate and return errors for any CBOR that is not conformant.¶
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"
}
¶
[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.¶
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.¶
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.¶
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:¶
null.¶
null.¶
null.¶
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:¶
null value means the user has been asked, but specified that they accept the current default.¶
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.¶
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.¶
The following issues are currently left for future work:¶
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.¶
It is RECOMMENDED that dCBOR codecs:¶
It is RECOMMENDED that dCBOR APIs provide a dCBOR Map structure or similar that models the dCBOR canonical key encoding and order:¶
10 and 10.0.¶
The dCBOR decoder SHOULD return an error if it encounters misordered or duplicate map keys.¶
The authors do make the following recommendations:¶
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,¶
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¶
This section is informative.¶
The current reference implementations that conform to these specifications are:¶
This document inherits the security considerations of CBOR [RFC8949].¶
Vulnerabilities regarding dCBOR will revolve around whether an attacker can find value in either:¶
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.¶
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.¶
TODO acknowledge.¶