Internet Engineering Task Force A. Rundgren, Ed. Internet-Draft Independent Intended status: Informational 3 March 2025 Expires: 4 September 2025 Universal CBOR (U-CBOR) draft-rundgren-universal-cbor-02 Abstract This document defines Universal CBOR (U-CBOR), a strict subset of CBOR (RFC 8949) intended to serve as a viable replacement for JSON. To foster interoperability, deterministic encoding is mandated. Furthermore, the document outlines how deterministic encoding combined with enhanced CBOR tools, enables the support of cryptographic constructs that operate on "raw" (non-wrapped) CBOR data. This document mainly targets CBOR tool developers. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 4 September 2025. Copyright Notice Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved. Rundgren Expires 4 September 2025 [Page 1] Internet-Draft U-CBOR March 2025 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.2. Common Definitions . . . . . . . . . . . . . . . . . . . 3 2. Detailed Description . . . . . . . . . . . . . . . . . . . . 3 2.1. Supported CBOR Objects . . . . . . . . . . . . . . . . . 4 2.2. Deterministic Encoding Scheme . . . . . . . . . . . . . . 5 2.3. CBOR Tool Requirements . . . . . . . . . . . . . . . . . 6 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 4. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 5.1. Normative References . . . . . . . . . . . . . . . . . . 7 5.2. Informative References . . . . . . . . . . . . . . . . . 7 Appendix A. Deterministic Encoding Samples . . . . . . . . . . . 8 A.1. Integers . . . . . . . . . . . . . . . . . . . . . . . . 8 A.2. Floating Point Numbers . . . . . . . . . . . . . . . . . 10 A.3. Invalid Encodings . . . . . . . . . . . . . . . . . . . . 14 Appendix B. Enveloped Signatures . . . . . . . . . . . . . . . . 16 B.1. Unsigned Data . . . . . . . . . . . . . . . . . . . . . . 16 B.2. Signature Process . . . . . . . . . . . . . . . . . . . . 16 B.3. Validation Process . . . . . . . . . . . . . . . . . . . 17 B.4. Example Parameters . . . . . . . . . . . . . . . . . . . 18 B.5. Code Example . . . . . . . . . . . . . . . . . . . . . . 18 Appendix C. Supporting Existing Systems . . . . . . . . . . . . 19 Appendix D. Compatible Online Tools . . . . . . . . . . . . . . 19 Appendix E. Compatible Implementations . . . . . . . . . . . . . 20 Document History . . . . . . . . . . . . . . . . . . . . . . . . 20 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 20 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20 1. Introduction The Universal CBOR (U-CBOR) specification is based on CBOR [RFC8949]. While there are different ways you can encode certain CBOR objects, this is non-trivial to support in general purpose platform-based tools, not to mention the limited utility of such measures. To cope with this, U-CBOR defines a specific (non-variant) encoding scheme, aka "Deterministic Encoding". The selected encoding scheme is Rundgren Expires 4 September 2025 [Page 2] Internet-Draft U-CBOR March 2025 believed to be _compatible_ with most existing _systems_ using CBOR. See also Appendix C. U-CBOR is intended to be agnostic with respect to programming languages. By combining the compact binary representation and the rich set of data types offered by CBOR, with a deterministic encoding scheme, U-CBOR could for _new designs_, serve as viable alternative to JSON [RFC8259]. Although the mandated encoding scheme has proved to be deployable in constrained environments, the primary target is rather mainstream platforms like mobile phones and Web servers. However, for unleashing the full power of deterministic encoding, the ability to perform cryptographic operations on "raw" (non-wrapped) CBOR data, compliant U-CBOR tools need additional functionality. See also Appendix B. Section 2 contains the actual specification. 1.1. 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. 1.2. Common Definitions * This document uses the conventions defined in CDDL [RFC8610] for expressing the type of CBOR [RFC8949] data items. * Examples showing CBOR data, are expressed in "diagnostic notation" as defined in section 8 of [RFC8949]. * The term "CBOR object" is equivalent to "CBOR data item" used in [RFC8949]. * The term "Universal CBOR" is in this document abbreviated to "U-CBOR". 2. Detailed Description This section describes the three pillars that U-CBOR relies on. Rundgren Expires 4 September 2025 [Page 3] Internet-Draft U-CBOR March 2025 2.1. Supported CBOR Objects The following table shows the CBOR subset supported by U-CBOR: +==============+========================================+ | CDDL | Note | +==============+========================================+ | int | Integer | +--------------+----------------------------------------+ | bigint | Big integer | +--------------+----------------------------------------+ | float | 16-, 32-, and 64-bit [IEEE754] numbers | +--------------+----------------------------------------+ | tstr | Text string encoded as UTF-8 [RFC3629] | +--------------+----------------------------------------+ | bstr | Byte string | +--------------+----------------------------------------+ | bool | Boolean true and false | +--------------+----------------------------------------+ | null | Represents a null object | +--------------+----------------------------------------+ | [] | Array | +--------------+----------------------------------------+ | {} | Map | +--------------+----------------------------------------+ | #6.nnn(type) | Tagged data | +--------------+----------------------------------------+ Table 1: Supported CBOR Objects Conforming implementations (of course) only have to implement the U-CBOR types required by the targeted application(s). Although extensions are imaginable (like supporting all "simple" types), extensions will most likely cause _interoperability issues_ and are thus NOT RECOMMENDED. In addition, the mandated CBOR subset is compatible with most computer languages and platforms. Compared to the current state-of-the-art, JSON [RFC8259], the availability of bigint, bstr, and "tagged data" represent major improvements. However, nothing prevents developers from at the application (API) level, through mapping concepts, support additional, "virtual" data types, analogous to how you map an application's data model to the set of data types available, be it a data interchange format, a database, or a programming language. Rundgren Expires 4 September 2025 [Page 4] Internet-Draft U-CBOR March 2025 2.2. Deterministic Encoding Scheme The U-CBOR encoding scheme adheres to section 4.2 of [RFC8949], but adds a few constraints (denoted by RFC+), where the RFC offers choices. The deterministic encoding rules are as follows: * RFC+: Floating point and integer objects MUST be treated as _distinct types_ regardless of their numeric value. This is compliant with Rule 2 in section 4.2.2 of [RFC8949]. * RFC: Integers, represented by the int and bigint types, MUST use the int type if the value is between -2^64 and 2^64-1, otherwise the bigint type MUST be used. Appendix A.1 features a list of compliant integer sample values. * RFC+: Floating point numbers MUST always use the shortest [IEEE754] variant that preserves the precision of the original value. Appendix A.2 features a list of compliant floating point sample values. Note that NaN "signaling" (like f97e01), MUST be _rejected_. * RFC: Map keys MUST be sorted in the bytewise lexicographic order of their deterministic encoding. Duplicate keys MUST be _rejected_. Somewhat surprisingly the following represents a properly sorted map: { "a": ... , "b": ... , "aa": ... } * RFC+: Since CBOR encoding according to this specification maintains type and data uniqueness, there are no specific restrictions or tests needed in order to determine map key equivalence. As an example, the floating-point numbers 0.0 and -0.0, and the integer number 0 represent the distinct keys f90000, f98000, and 00 respectively. * RFC+: Indefinite length objects MUST be _rejected_. Rundgren Expires 4 September 2025 [Page 5] Internet-Draft U-CBOR March 2025 2.3. CBOR Tool Requirements The primary feature that deterministic encoding brings to the table is that wrapping CBOR data to be signed in bstr objects, like specified by COSE [RFC9052] (Section 2), no longer is a prerequisite. That is, cryptographic operations can _optionally_ be performed on "raw" CBOR data. Turn to Appendix B for an example of an application depending on such features. However, to make this a reality, the following functionality MUST be provided by CBOR tools compliant with this specification: * It MUST be possible to _add_, _delete_, and _update_ the contents of CBOR map and array objects, of received and decoded CBOR data. Note: CBOR primitives MUST remain _immutable_. * It MUST be possible to _reserialize_ received CBOR data, be it updated or not. * Irrespective of if CBOR data was received, updated, or created programmatically, deterministic encoding MUST be maintained. * Invalid or unsupported CBOR constructs, as well as CBOR data not adhering to the deterministic encoding scheme MUST be _rejected_. See also Appendix C and Appendix A.3. It is RECOMMENDED _separating_ CBOR data and application / platform- level data, since the latter may not always _reserialize_ as expected, like in this Chrome browser console example: > let date = new Date('2025-03-02T13:08:55.0001Z'); > date.toISOString() '2025-03-02T13:08:55.000Z' How this separation actually is accomplished is out of scope for this specification. However, _encapsulation_ of CBOR data in _high- level_, and _self-rendering objects_, represents a usable method. Similar approaches are used by most ASN.1 tools. The code in Appendix B.5 shows an example that _updates_ and _reserializes_ decoded CBOR data. 3. IANA Considerations This memo includes no request to IANA. Rundgren Expires 4 September 2025 [Page 6] Internet-Draft U-CBOR March 2025 4. Security Considerations All is good 😸 5. References 5.1. 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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, December 2020, . [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, . [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, . [IEEE754] IEEE, "IEEE Standard for Floating-Point Arithmetic", IEEE Std 754-2019, DOI 10.1109/IEEESTD.2019.8766229, . 5.2. Informative References [RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, August 2022, . [RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE): Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053, August 2022, . Rundgren Expires 4 September 2025 [Page 7] Internet-Draft U-CBOR March 2025 [RFC8785] Rundgren, A., Jordan, B., and S. Erdtman, "JSON Canonicalization Scheme (JCS)", RFC 8785, DOI 10.17487/RFC8785, June 2020, . [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, December 2017, . [CSF] Rundgren, A., "CBOR Signature Format (CSF)", . [CEF] Rundgren, A., "CBOR Encryption Format (CEF)", . [CREDENTIALS] Sporny (et al), M., "Verifiable Credential Data Integrity 1.0", 2025, . [ECMASCRIPT] Ecma International, "ECMAScript 2020 Language Specification", Standard ECMA-262, 11th Edition, June 2020, . [GordianEnvelope] McNally, W. and C. Allen, "The Gordian Envelope Structured Data Format", . Appendix A. Deterministic Encoding Samples A.1. Integers This _normative_ section holds a selection of CBOR integer values, with an emphasize on edge cases. +=======================+========================+================+ | Value | CBOR Encoding | Note | +=======================+========================+================+ | 0 | 00 | Smallest | | | | positive | | | | implicit int | +-----------------------+------------------------+----------------+ | -1 | 20 | Smallest | Rundgren Expires 4 September 2025 [Page 8] Internet-Draft U-CBOR March 2025 | | | negative | | | | implicit int | +-----------------------+------------------------+----------------+ | 23 | 17 | Largest | | | | positive | | | | implicit int | +-----------------------+------------------------+----------------+ | -24 | 37 | Largest | | | | negative | | | | implicit int | +-----------------------+------------------------+----------------+ | 24 | 1818 | Smallest | | | | positive one- | | | | byte int | +-----------------------+------------------------+----------------+ | -25 | 3818 | Smallest | | | | negative one- | | | | byte int | +-----------------------+------------------------+----------------+ | 255 | 18ff | Largest | | | | positive one- | | | | byte int | +-----------------------+------------------------+----------------+ | -256 | 38ff | Largest | | | | negative one- | | | | byte int | +-----------------------+------------------------+----------------+ | 256 | 190100 | Smallest | | | | positive two- | | | | byte int | +-----------------------+------------------------+----------------+ | -257 | 390100 | Smallest | | | | negative two- | | | | byte int | +-----------------------+------------------------+----------------+ | 65535 | 19ffff | Largest | | | | positive two- | | | | byte int | +-----------------------+------------------------+----------------+ | -65536 | 39ffff | Largest | | | | negative two- | | | | byte int | +-----------------------+------------------------+----------------+ | 65536 | 1a00010000 | Smallest | | | | positive four- | | | | byte int | +-----------------------+------------------------+----------------+ | -65537 | 3a00010000 | Smallest | Rundgren Expires 4 September 2025 [Page 9] Internet-Draft U-CBOR March 2025 | | | negative four- | | | | byte int | +-----------------------+------------------------+----------------+ | 4294967295 | 1affffffff | Largest | | | | positive four- | | | | byte int | +-----------------------+------------------------+----------------+ | -4294967296 | 3affffffff | Largest | | | | negative four- | | | | byte int | +-----------------------+------------------------+----------------+ | 4294967296 | 1b0000000100000000 | Smallest | | | | positive | | | | eight-byte int | +-----------------------+------------------------+----------------+ | -4294967297 | 3b0000000100000000 | Smallest | | | | negative | | | | eight-byte int | +-----------------------+------------------------+----------------+ | 18446744073709551615 | 1bffffffffffffffff | Largest | | | | positive | | | | eight-byte int | +-----------------------+------------------------+----------------+ | -18446744073709551616 | 3bffffffffffffffff | Largest | | | | negative | | | | eight-byte int | +-----------------------+------------------------+----------------+ | 18446744073709551616 | c249010000000000000000 | Smallest | | | | positive | | | | bigint | +-----------------------+------------------------+----------------+ | -18446744073709551617 | c349010000000000000000 | Smallest | | | | negative | | | | bigint | +-----------------------+------------------------+----------------+ Table 2: Integers A.2. Floating Point Numbers This _normative_ section holds a selection of [IEEE754] 16, 32, and 64-bit values, with an emphasize on edge cases. Rundgren Expires 4 September 2025 [Page 10] Internet-Draft U-CBOR March 2025 The textual representation of the values is based on the serialization method for the Number data type, defined by [ECMASCRIPT] with one change: to comply with diagnostic notation (section 8 of [RFC8949]), all values are expressed as floating point numbers. The rationale for using [ECMASCRIPT] serialization is because it supposed to generate the shortest and most correct representation of [IEEE754] numbers. +===========================+====================+==============+ | Value | CBOR Encoding | Note | +===========================+====================+==============+ | 0.0 | f90000 | Zero | +---------------------------+--------------------+--------------+ | -0.0 | f98000 | Negative | | | | zero | +---------------------------+--------------------+--------------+ | Infinity | f97c00 | Infinity | +---------------------------+--------------------+--------------+ | -Infinity | f9fc00 | -Infinity | +---------------------------+--------------------+--------------+ | NaN | f97e00 | NaN | +---------------------------+--------------------+--------------+ | 5.960464477539063e-8 | f90001 | Smallest | | | | positive | | | | _subnormal_ | | | | 16-bit float | +---------------------------+--------------------+--------------+ | 0.00006097555160522461 | f903ff | Largest | | | | positive | | | | _subnormal_ | | | | 16-bit float | +---------------------------+--------------------+--------------+ | 0.00006103515625 | f90400 | Smallest | | | | positive | | | | 16-bit float | +---------------------------+--------------------+--------------+ | 65504.0 | f97bff | Largest | | | | positive | | | | 16-bit float | +---------------------------+--------------------+--------------+ | 1.401298464324817e-45 | fa00000001 | Smallest | | | | positive | | | | _subnormal_ | | | | 32-bit float | +---------------------------+--------------------+--------------+ | 1.1754942106924411e-38 | fa007fffff | Largest | | | | positive | | | | _subnormal_ | Rundgren Expires 4 September 2025 [Page 11] Internet-Draft U-CBOR March 2025 | | | 32-bit float | +---------------------------+--------------------+--------------+ | 1.1754943508222875e-38 | fa00800000 | Smallest | | | | positive | | | | 32-bit float | +---------------------------+--------------------+--------------+ | 3.4028234663852886e+38 | fa7f7fffff | Largest | | | | positive | | | | 32-bit float | +---------------------------+--------------------+--------------+ | 5.0e-324 | fb0000000000000001 | Smallest | | | | positive | | | | _subnormal_ | | | | 64-bit float | +---------------------------+--------------------+--------------+ | 2.225073858507201e-308 | fb000fffffffffffff | Largest | | | | positive | | | | _subnormal_ | | | | 64-bit float | +---------------------------+--------------------+--------------+ | 2.2250738585072014e-308 | fb0010000000000000 | Smallest | | | | positive | | | | 64-bit float | +---------------------------+--------------------+--------------+ | 1.7976931348623157e+308 | fb7fefffffffffffff | Largest | | | | positive | | | | 64-bit float | +---------------------------+--------------------+--------------+ | -0.0000033333333333333333 | fbbecbf647612f3696 | Randomly | | | | selected | | | | number | +---------------------------+--------------------+--------------+ | 295147905179352830000.0 | fa61800000 | ~2^68 | +---------------------------+--------------------+--------------+ | 2.0 | f94000 | Number | | | | without a | | | | fractional | | | | part | +---------------------------+--------------------+--------------+ | -5.960464477539063e-8 | f98001 | Smallest | | | | negative | | | | _subnormal_ | | | | 16-bit float | +---------------------------+--------------------+--------------+ | -5.960464477539062e-8 | fbbe6fffffffffffff | Close to | | | | smallest | | | | negative | | | | _subnormal_ | Rundgren Expires 4 September 2025 [Page 12] Internet-Draft U-CBOR March 2025 | | | 16-bit float | +---------------------------+--------------------+--------------+ | -5.960464477539064e-8 | fbbe70000000000001 | "" | +---------------------------+--------------------+--------------+ | -5.960465188081798e-8 | fab3800001 | "" | +---------------------------+--------------------+--------------+ | 0.0000609755516052246 | fb3f0ff7ffffffffff | Close to | | | | largest | | | | _subnormal_ | | | | 16-bit float | +---------------------------+--------------------+--------------+ | 0.000060975551605224616 | fb3f0ff80000000001 | "" | +---------------------------+--------------------+--------------+ | 0.000060975555243203416 | fa387fc001 | "" | +---------------------------+--------------------+--------------+ | 0.00006103515624999999 | fb3f0fffffffffffff | Close to | | | | smallest | | | | 16-bit float | +---------------------------+--------------------+--------------+ | 0.00006103515625000001 | fb3f10000000000001 | "" | +---------------------------+--------------------+--------------+ | 0.00006103516352595761 | fa38800001 | "" | +---------------------------+--------------------+--------------+ | 65503.99999999999 | fb40effbffffffffff | Close to | | | | largest | | | | 16-bit float | +---------------------------+--------------------+--------------+ | 65504.00000000001 | fb40effc0000000001 | "" | +---------------------------+--------------------+--------------+ | 65504.00390625 | fa477fe001 | "" | +---------------------------+--------------------+--------------+ | 1.4012984643248169e-45 | fb369fffffffffffff | Close to | | | | smallest | | | | _subnormal_ | | | | 32-bit float | +---------------------------+--------------------+--------------+ | 1.4012984643248174e-45 | fb36a0000000000001 | "" | +---------------------------+--------------------+--------------+ | 1.175494210692441e-38 | fb380fffffbfffffff | Close to | | | | largest | | | | _subnormal_ | | | | 32-bit float | +---------------------------+--------------------+--------------+ | 1.1754942106924412e-38 | fb380fffffc0000001 | "" | +---------------------------+--------------------+--------------+ | 1.1754943508222874e-38 | fb380fffffffffffff | Close to | | | | smallest | | | | 32-bit float | Rundgren Expires 4 September 2025 [Page 13] Internet-Draft U-CBOR March 2025 +---------------------------+--------------------+--------------+ | 1.1754943508222878e-38 | fb3810000000000001 | "" | +---------------------------+--------------------+--------------+ | 3.4028234663852882e+38 | fb47efffffdfffffff | Close to | | | | largest | | | | 32-bit float | +---------------------------+--------------------+--------------+ | 3.402823466385289e+38 | fb47efffffe0000001 | "" | +---------------------------+--------------------+--------------+ Table 3: Floating Point Numbers A.3. Invalid Encodings The following table holds a selection of valid CBOR objects, not permitted by U-CBOR. Rundgren Expires 4 September 2025 [Page 14] Internet-Draft U-CBOR March 2025 +==========================+=======================+===============+ | CBOR Encoding | Diagnostic Notation | Note | +==========================+=======================+===============+ | a2616200616101 | {"b":0,"a":1} | Improper map | | | | key ordering | | | | [1] | +--------------------------+-----------------------+---------------+ | 1900ff | 255 | Number with | | | | leading zero | | | | bytes [1] | +--------------------------+-----------------------+---------------+ | c34a00010000000000000000 | -18446744073709551617 | Number with | | | | leading zero | | | | bytes [1] | +--------------------------+-----------------------+---------------+ | Fa41280000 | 10.5 | Not in | | | | shortest | | | | encoding [1] | +--------------------------+-----------------------+---------------+ | fa7fc00000 | NaN | Not in | | | | shortest | | | | encoding [1] | +--------------------------+-----------------------+---------------+ | c243010000 | 65536 | Incorrect | | | | value for | | | | bigint [1] | +--------------------------+-----------------------+---------------+ | f97e01 | NaN | NaN with | | | | payload [1] | +--------------------------+-----------------------+---------------+ | f7 | undefined | Unsupported | | | | simple type | +--------------------------+-----------------------+---------------+ | f0 | simple(16) | Unsupported | | | | simple type | +--------------------------+-----------------------+---------------+ | 5f4101420203ff | (_ h'01', h'0203') | Unsupported | | | | indefinite | | | | length object | +--------------------------+-----------------------+---------------+ Table 4: Invalid Encodings 1. See also Appendix C. Rundgren Expires 4 September 2025 [Page 15] Internet-Draft U-CBOR March 2025 Appendix B. Enveloped Signatures This is a _non-normative_ appendix showing how U-CBOR can be used for supporting enveloped signatures. The primary advantages with enveloped signatures compared to the approach used by COSE [RFC9052] include: * Keeping the _structure_ of the original (unsigned) data intact, by simply making signatures an additional attribute. * Permitting signing CBOR data and associated security attributes (aka "headers"), _in one go_, without having to wrap data in CBOR "bstr" objects. Enveloped signatures are for example featured in Verified Credentials [CREDENTIALS]. A drawback with designs based on JSON [RFC8259] is that they rely on _canonicalization schemes_ like JCS [RFC8785], that require specialized encoders and decoders, whereas U-CBOR works "straight out of the box". Although this specification is not "married" to any particular signature schema, the example uses the CBOR Signature Format [CSF]. For the sake of simplicity, the example uses an HMAC (see Appendix B.4) as signature algorithm. B.1. Unsigned Data Imagine you have a CBOR map object like the following that you want to sign: { 1: "data", 2: "more data" } Then continue to the next section (Appendix B.2)... B.2. Signature Process This section describes the steps required for adding an enveloped signature to the CBOR map object in Appendix B.1. 1. Add an empty CSF container (a CBOR map) to the unsigned CBOR map using an _application-defined_ label (-1). 2. Add the designated signature algorithm to the CSF container using the CSF algorithm label (1). Rundgren Expires 4 September 2025 [Page 16] Internet-Draft U-CBOR March 2025 3. _Optional_. Add other signature meta data to the CSF container. Not used in the example. 4. Generate a signature by invoking a (hypothetical) signature method with the following arguments: * the designated signature key. * the designated signature algorithm. * the _deterministic encoding_ of the current CBOR object in its _entirety_. In the example that would be a301646461746102696d6f7265206461746120a10105, if expressed in hex code. 5. Add the returned signature value to the CSF container using the CSF signature label (6). The result after the final step (using the parameters from Appendix B.4), should match the following CBOR object: { 1: "data", 2: "more data", -1: { 1: 5, 6: h'4853d7730cc1340682b1748dc346cf627a5e91ce62c67fff15c40257ed2a37a1' } } Note that the signature covers the _entire_ CBOR object except for the CSF signature value and label (6). B.3. Validation Process In order to validate the enveloped signature created in the Appendix B.2, the following steps are performed: 1. Fetch a _reference_ to the CSF container using the _application- defined_ label (-1). Next perform the following operations using the reference: 1. Retrieve the signature algorithm using the CSF algorithm label (1). 2. Retrieve the signature value using the CSF algorithm label (6). Rundgren Expires 4 September 2025 [Page 17] Internet-Draft U-CBOR March 2025 3. Remove the CSF algorithm label (6) and its associated value. Now we should have exactly the same CBOR object as we had _before_ step #4 in Appendix B.2. That is: { 1: "data", 2: "more data", -1: { 1: 5 } } 2. Validate the signature data by invoking a (hypothetical) signature validation method with the following arguments: * the designated signature key (in the example taken from Appendix B.4). * the signature algorithm retrieved in step #1. * the signature value retrieved in step #1. * the _deterministic encoding_ of the current CBOR object in its _entirety_. Note: this is a "bare-bones" validation process, lacking the ruggedness of a real-world implementation. B.4. Example Parameters The signature and validation processes depend on the COSE [RFC9053] algorithm "HMAC 256/256" and an associated 256-bit key, here provided in hex code: 7fdd851a3b9d2dafc5f0d00030e22b9343900cd42ede4948568a4a2ee655291a B.5. Code Example Using the JavaScript implementation mentioned in Appendix E, basic signature validation of the signed CBOR object created in Appendix B.2, could be performed by the following code: Rundgren Expires 4 September 2025 [Page 18] Internet-Draft U-CBOR March 2025 // The variable cborBinary is supposed to contain CBOR let object = CBOR.decode(cborBinary); // Decode let csf = object.get(CBOR.Int(-1)); // Get CSF container let alg = csf.get(CBOR.Int(1)).getInt(); // Read algorithm let sig = csf.remove(CBOR.Int(6)).getBytes(); // Read and remove signature value let key = CBOR.fromHex('7fdd851a3b9d2dafc5f0d00030e22b9343900cd42ede4948568a4a2ee655291a'); // Hypothetical HMAC validation method: hmacValidate(alg, sig, key, object.encode()); // Note that object.encode() // reserializes all but sig. // Validated object, access the "payload": let param = object.get(CBOR.Int(1)).getString(); // param should now contain "data" Note that this code depends heavily on the CBOR tool features outlined in Section 2.3. Appendix C. Supporting Existing Systems It is assumed that _most_ systems using CBOR are able to process an (_application specific_), selection of CBOR data items that are encoded in compliance with [RFC8949]. Since the deterministic encoding scheme mandated by U-CBOR, also is compliant with [RFC8949], there should be no major interoperability issues. That is, if the previous assumption actually is correct 😏 However, in the _other_ direction (U-CBOR tools processing data from Systems using "legacy" CBOR encoding schemes), the situation is likely to be considerably more challenging since deterministic encoding "by design" is _strict_. Due to this potential obstacle, implementers of U-CBOR tools, are RECOMMENDED to offer _decoder_ options that permit "relaxing" the rigidness of deterministic encoding with respect to: * Numbers. Numbers MUST still be compliant with [RFC8949]. * Sorted maps. Duplicate keys MUST still be rejected. Note that regardless of the format of received CBOR data, a compliant U-CBOR implementation MUST maintain deterministic encoding. See also Appendix A.3. Appendix D. Compatible Online Tools For testing and learning about U-CBOR, there are currently a number of compatible online tools (subject to availability...). Browser-based CBOR "playground": https://cyberphone.github.io/CBOR.js/doc/playground.html Rundgren Expires 4 September 2025 [Page 19] Internet-Draft U-CBOR March 2025 Server-based CBOR and [CSF] test system: https://test.webpki.org/csf-lab Appendix E. Compatible Implementations For using U-CBOR in applications, there are currently a number of compatible ibraries. JavaScript-based implementation: https://github.com/cyberphone/CBOR.js Java-based implementation that also supports [CSF] and [CEF]: https://github.com/cyberphone/openkeystore Android Java-based implementation that also supports [CSF] and [CEF]: https://github.com/cyberphone/android-cbor Document History * 00. First cut. * 01. Editorial. Changed order of columns in invalid encoding. Acknowledgements This work was inspired by a CBOR based project known as [GordianEnvelope], pioneered by Wolf McNally and Christopher Allen. This project also exploits the ability to hash "raw" (non-wrapped) CBOR data, enabled by the use of a deterministic encoding scheme. Author's Address Anders Rundgren (editor) Independent Montpellier France Email: anders.rundgren.net@gmail.com URI: https://www.linkedin.com/in/andersrundgren/ Rundgren Expires 4 September 2025 [Page 20]