CBOR Object Type Extension (COTX)

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Table of Contents

1. Introduction

This specification introduces a method for augmenting data expressed in the CBOR [RFC8949] notation, with a universal type identifier mechanism.¶

The primary purpose is to enable developers defining application specific type identifiers without having to go through an external registration process. Although the described scheme imposes no restrictions on type identifiers (beyond being valid CBOR data items), using URLs [URL] should due to their ubiquity be a candidate for CBOR based standards. See also Appendix A.¶

This specification is also intended to provide a path for ISO using CBOR as a possible alternative to XML by supporting their current URN [RFC8141] based identifier naming scheme. See also Appendix B.¶

Since the type identifier scheme is supposed to be an integral part of CBOR data items, objects compliant with this specification may also be embedded in other CBOR and non-CBOR constructs, as well as stored in databases without any additional information.¶

If applied to top level items, the type identifier scheme may also reduce the need for application specific media types. In many cases "application/cbor" should suffice.¶

2. Specification

This specification builds on the CBOR [RFC8949] tag feature (major type 6), by defining a fixed tag with the preliminary decimal value of 1010. See also Section 3.¶

This tag MUST in turn enclose a CBOR array with two elements, where the first element is assumed to contain an object type identifier, while the second element holds the object (instance) data itself. Both arguments MUST be valid (but arbitrary) CBOR objects.¶

The syntax expressed in CBOR diagnostic notation (section 8 of [RFC8949]) would read as:¶

    1010([Object Identifier, Object Data])¶

Note that real-world usages will typically impose constraints like requiring object identifiers to be expressed as HTTPS URLs etc.¶

1010(["https://example.com/myobject", {
  1: "data",
  2: "more data"
}])

D9 03F2                             # tag(1010)
   82                               # array(2)
      78 1C                         # text(28)
         68747470733A2F2F6578616D706C652E636F6D2F6D796F626A656374
                                    # "https://example.com/myobject"
      A2                            # map(2)
         01                         # unsigned(1)
         64                         # text(4)
            64617461                # "data"
         02                         # unsigned(2)
         69                         # text(9)
            6D6F72652064617461      # "more data"

3. IANA Considerations

In the registry [IANA.cbor-tags], IANA is requested to allocate the tag defined in Table 1.¶

4. Security Considerations

This specification inherits all the security considerations of CBOR [RFC8949].¶

URL-based type identifiers MUST NOT be used for automatically downloading CBOR schema data like CDDL [RFC8610] to CBOR processors, since this introduces potential vulnerabilities.¶

The availability of type information does in no way limit the need for input data validation.¶

For signed CBOR objects, it is RECOMMENDED to include the type identifier extension in the signature calculation as well. The same considerations apply to encryption using AEAD algorithms.¶

Appendix A. URI and URL Identifiers

The primary reason for using URI or URL [URL] based identifiers is for maintaining a single name-space for the entire specification of a system. Note that the referenced URL specification does not distinguish between URIs and URLs.¶

A core issue with identifiers depending on host (DNS) names is that host names may not necessarily remain valid during the anticipated life time of an identifier. The originator of a host name may due to organizational changes, neglect, lack of interest, or even death, lose control over its use, effectively leaving associated identifiers orphaned.¶

This non-normative section describes different methods for dealing with identifiers expressed as URIs or URLs.¶

Appendix B. URN Identifiers

ISO currently use URN [RFC8141] [RFC5141] based identifiers like "urn:iso:std:iso:20022:tech:xsd:pain.001.001.10" for data definitions using XML schema [XSD]. This method could be applied to CBOR and CDDL [RFC8610] as well.¶

Acknowledgements

People who have contributed with valuable feedback to this specification include Christian Amsüss, Carsten Bormann, and Joe Hildebrand.¶

Document History

[[ This section to be removed by the RFC Editor before publication as an RFC ]]¶

Version 00:¶

• Initial publication.¶

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