Internet Engineering Task Force                         A. Rundgren, Ed.
Internet-Draft                                               Independent
Intended status: Informational                          29 November 2025
Expires: 2 June 2026

                       CBOR Simple Value for CSF
                  draft-rundgren-cbor-simple-4-csf-01

Abstract

   This document defines two CBOR "simple" values to be used as unique
   labels in a CBOR map holding an embedded signature.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   3.  Description and Rationale . . . . . . . . . . . . . . . . . .   2
     3.1.  Current Solution  . . . . . . . . . . . . . . . . . . . .   2
     3.2.  Enhanced Solution . . . . . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Document History  . . . . . . . . . . . . . . . . . . . . . . . .   5
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   This document defines two CBOR [RFC8949] "simple" values to be used
   as unique labels in conjunction with an embedded signature [CSF]
   design.  The purpose of the unique labels is to securely decouple
   application-specific labels from the signature container respectively
   data that should be excluded from the signature.  In addition to
   eliminating the need for application-specific labels for embedded
   signatures, the net result includes simplified signature APIs as
   well.

2.  Requirements Language

   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.

3.  Description and Rationale

   This section describes the problem and its possible solution.

   The CBOR examples are provided in "Extended Diagnostic Notation
   (EDN)" [I-D.ietf-cbor-edn-literals].

3.1.  Current Solution

   The [CSF] embedded signature scheme currently depends on an
   application-specific label holding the embedded signature container.

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   The following CBOR code shows a very simple example using an HMAC
   signature:

   {
     1: "data",         # Application data
     2: "more data",    # Application data
     -1: {              # Embedded signature (CSF container)
       1: 5,
       6: h'4853d7730cc1340682b1748dc346cf627a5e91ce62c67fff15c40257ed2a37a1'
     }
   }

   Having to define an application-specific ("custom") label for the
   embedded signature container is certainly not a showstopper, but it
   lacks "finesse".  In addition, signature APIs need to deal with such
   labels like the following:

     sign(_signatureLabel_, _applicationMap_).

3.2.  Enhanced Solution

   Replacing the application-specific label with a CBOR simple value,
   yields the following:

   {
     1: "data",         # Application data
     2: "more data",    # Application data
     simple(99): {      # Embedded signature (CSF container)
       1: 5,
       6: h'237e674c7be1818ddd7eaacf40ca80415b9ad816880751d2136c45385207420c'
     },
     simple(100): [     # Optional: data that should NOT be signed
       "just passing through"
     ]
   }

   The advantages with using simple(99) include:

   *  Eliminates the need for application-specific labels for signature
      containers.

   *  Simplifies signature APIs: sign(_applicationMap_).

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   *  Using deterministic encoding (a [CSF] prerequisite), CBOR simple
      types lexicographically follow after other CBOR elements (of the
      type normally used as labels).  This makes sense for embedded
      signatures, since they usually "attest" the application data that
      is (list-wise), situated above the signature container, like in
      the example.

4.  IANA Considerations

   In the registry [IANA.cbor-simple-values], IANA is requested to
   allocate the simple value defined in Table 1.

      +=======+==============+=====================================+
      | Value |  Semantics   |              Reference              |
      +=======+==============+=====================================+
      |   99  | Unique label | draft-rundgren-cbor-simple-4-csf-XX |
      +-------+--------------+-------------------------------------+
      |  100  | Unique label | draft-rundgren-cbor-simple-4-csf-XX |
      +-------+--------------+-------------------------------------+

                          Table 1: Simple Values

5.  Security Considerations

   The proposed enhanced solution does not reduce security compared to
   the current solution because duplicate labels SHOULD in both cases be
   rejected by conforming CBOR encoders and decoders.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

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   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [IANA.cbor-simple-values]
              IANA, "Concise Binary Object Representation (CBOR) Simple
              Values",
              <https://www.iana.org/assignments/cbor-simple-values>.

6.2.  Informative References

   [I-D.ietf-cbor-edn-literals]
              Bormann, C., "CBOR Extended Diagnostic Notation (EDN)",
              Work in Progress, Internet-Draft, draft-ietf-cbor-edn-
              literals-16, 8 January 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
              edn-literals-16>.

   [CSF]      Rundgren, A., "CBOR Signature Format (CSF)",
              <https://cyberphone.github.io/javaapi/org/webpki/cbor/doc-
              files/signatures.html>.

Document History

   *  00.  First cut.

   *  01. simple(100) added.

Acknowledgements

   TBD

Author's Address

   Anders Rundgren (editor)
   Independent
   Montpellier
   France
   Email: anders.rundgren.net@gmail.com
   URI:   https://www.linkedin.com/in/andersrundgren/

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