Internet-Draft CDEP August 2023
Rundgren Expires 20 February 2024 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-rundgren-deterministic-cbor-20
Published:
Intended Status:
Informational
Expires:
Author:
A. Rundgren, Ed.
Independent

CBOR Deterministic Encoding Profile (CDEP)

Abstract

This document describes CDEP, a deterministic encoding profile for CBOR, intended for usage in high-end computing platforms like mobile phones, Web browsers, and Web servers. In addition to enhancing interoperability, deterministic encoding also enables performing cryptographic operations like signing "raw" CBOR data items, something which otherwise would require wrapping such data in byte strings, or introduce dependencies on non-standard canonicalization procedures.

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 20 February 2024.

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Table of Contents

1. Introduction

This specification introduces a deterministic encoding profile for data expressed in the CBOR [RFC8949] format. This profile is subsequently referred to as CDEP.

Note that this document is not on the IETF standards track. However, a conformant implementation is supposed to adhere to the specified behavior for security and interoperability reasons.

1.1. Background

[RFC8949] supports a number of deterministic encoding options. Some of these options are not necessarily interoperable, like Rule 1‑3 in Section 4.2.2. This could potentially hamper large scale rollout of applications depending on deterministically encoded CBOR.

1.2. Objectives

The main objective of CDEP is providing an interoperable CBOR encoding profile, primarily targeting high-end computing platforms like mobile phones, Web browsers, and Web servers. In addition, due to the underpinning deterministic representation of data, CDEP also enables performing cryptographic operations like signatures over "raw" (unwrapped) CBOR data items since signatures depend on a unified representation of the data to be signed. Furthermore, building on the same foundation, CDEP also permits decoded CBOR data to be subjected to simple and secure transformation and reencoding operations.

The deterministic encoding profile described in this document is characterized by being bidirectional also when CBOR is provided in diagnostic notation (Section 8 of [RFC8949]), making CDEP comparatively easy to understand, debug, and implement.

Although CDEP is a deterministic encoding profile, the intent is that the encoding scheme should be equally useful for applications that do not depend on this particular feature. See also Appendix C

In spite of the enhanced functionality, this specification retains full compatibility with [RFC8949].

See also [I-D.mcnally-deterministic-cbor] which represents an alternative approach to deterministic encoding.

1.3. 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.

2. Specification

The CDEP deterministic encoding profile builds on Section 4.2 of [RFC8949].

The following sections contain some additional clarifications and explicit choices, in order to facilitate an interoperable encoding scheme.

2.1. General Requirements

Occurrences of unknown or malformed CBOR data items MUST be rejected.

Map keys MUST only be compared and sorted based on their bytewise lexicographic order of their deterministic encoding.

For applications that depend on deterministic reencoding of CBOR data items, compliant decoder implementations MUST be able to recreate such data in its original form. This means for example that the string component of date items (tag 0) MUST be preserved "as is" in order to maintain consistency.

The optional numerical extensions described in Section 3.4.4 of [RFC8949] MUST be treated as distinct data items as well as not be subjected to any transformations at the encoding level.

2.2. CBOR Data Items

Compliant CDEP implementations SHOULD as a minimum support the following CBOR data items:

Table 1: CBOR Data Items
Data Item Encoding
integer Major type 0 and 1
bignum 0xc2 and 0xc3
floating point 0xf9, 0xfa and 0xfb
byte string Major type 2
text string Major type 3
false 0xf4
true 0xf5
null 0xf6
array Major type 4
map Major type 5
tag Major type 6

See also Appendix B.

2.3. Encoding of Numbers

The following sub sections hold examples of numeric values expressed in diagnostic notation (Section 8 of [RFC8949]) and their CDEP encoded counterpart (expressed in hexadecimal). See also Appendix A.

To achieve a fixed and bidirectional representation of numbers, Rule 2 in Section 4.2.2 of [RFC8949] MUST be adhered to. In addition, integer and floating point data items MUST use preferred serialization as described in Section 4.2.1.

Note that the values and encodings are supposed to work in both directions.

2.3.1. Integer Numbers

The following table holds a set of integers. Note that bignum data items MUST use preferred serialization as described in Section 3.4.3 of [RFC8949].

Table 2: Integer Numbers
Value Encoding
0 00
-1 20
23 17
24 1818
-24 37
-25 3818
255 18ff
256 190100
-256 38ff
-257 390100
65535 19ffff
65536 1a00010000
1099511627775 1b000000ffffffffff
18446744073709551615 1bffffffffffffffff
18446744073709551616 c249010000000000000000
-18446744073709551616 3bffffffffffffffff
-18446744073709551617 c349010000000000000000

2.3.2. Special Floating Point Numbers

The following table holds the set of special IEEE 754 [IEEE754] values. Note that "signaling" NaN values MUST NOT be present.

Table 3: Special Floating Point Numbers
Value Encoding
0.0 f90000
-0.0 f98000
Infinity f97c00
-Infinity f9fc00
NaN f97e00

2.3.3. "Ordinary" Floating Point Numbers

The following table holds a set of "ordinary" IEEE 754 [IEEE754] values including some edge cases. Note that subnormal floating point values MUST be supported.

Table 4: "Ordinary" Floating Point Numbers
Value Encoding
-5.960464477539062e-8 fbbe6fffffffffffff
-5.960464477539063e-8 f98001
-5.960464477539064e-8 fbbe70000000000001
-5.960465188081798e-8 fab3800001
0.00006097555160522461 f903ff
65504.0 f97bff
65504.00390625 fa477fe001
65536.0 fa47800000
10.559998512268066 fa4128f5c1
10.559998512268068 fb40251eb820000001
3.4028234663852886e+38 fa7f7fffff
3.402823466385289e+38 fb47efffffe0000001
1.401298464324817e-45 fa00000001
1.1754942106924411e-38 fa007fffff
5.0e-324 fb0000000000000001
-1.7976931348623157e+308 fbffefffffffffffff

3. IANA Considerations

This document has no IANA actions.

4. Security Considerations

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

Applications that exploit the uniqueness of deterministic encoding should verify that the used decoder actually rejects incorrectly formatted CBOR data items.

5. References

5.1. Normative References

[IEEE754]
IEEE, "IEEE Standard for Floating-Point Arithmetic", IEEE 754-2019, DOI 10.1109/IEEESTD.2019.8766229, <https://ieeexplore.ieee.org/document/8766229>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <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, , <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, , <https://www.rfc-editor.org/info/rfc8949>.

5.2. Informative References

[I-D.mcnally-deterministic-cbor]
McNally, W. and C. Allen, "Gordian dCBOR: Deterministic CBOR Implementation Practices", Work in Progress, Internet-Draft, draft-mcnally-deterministic-cbor-01, , <https://datatracker.ietf.org/doc/html/draft-mcnally-deterministic-cbor-01>.

Appendix A. Incorrectly Encoded Numbers

The following table holds a few examples of numeric CBOR data items that MUST be rejected because their respective encoding does not conform to CDEP. "PS" in the table is a short form for "Preferred Serialization".

Table 5: Incorrectly Encoded Numbers
Encoded Error Description
f97e01 NaN "signaling"
f97c01 Invalid NaN
fb7ff8000000000000 PS: f97e00
fb8000000000000000 PS: f98000
faff800000 PS: f9fc00
fa477fe000 PS: f97bff
fab3800000 PS: f98001
fbbe70000000000000 PS: f98001
fa00000000 PS: f90000
fb36a0000000000000 PS: fa00000001
fb380fffffc0000000 PS: fa007fffff
1800 PS: 00
1817 PS: 17
1900ff PS: 18ff
1a000000ff PS: 18ff
1a0000ffff PS: 19ffff
1b00000000ffffffff PS: 1affffffff
3b00000000ffffffff PS: 3affffffff
c2488000000000000000 PS: 1b8000000000000000
c348ffffffffffffffff PS: 3bffffffffffffffff
c24a00800000000000000000 PS: c249800000000000000000

Appendix B. Implementation Constraints

This section is non-normative.

Note that even if an application does not support (or need) bignum or floating point data items, CDEP is still applicable, since a strict subset is upwardly compatible with full-blown implementations. Low-end platforms typically also restrict CBOR map keys to integer and text string data items. Since these issues are application specific, they are out of scope for this specification.

Appendix C. Decoder Considerations

This section is non-normative.

To not unnecessarily create incompatibilities with the existing CBOR ecosystem, CDEP decoders may benefit from supporting a non-deterministic mode, where the map key ordering and preferred serialization checks are disabled.

Appendix D. Reference Implementations

This section is non-normative.

Reference implementations that conform to this specification include:

Appendix E. Online Tools

This section is non-normative.

The following online tools enable testing CDEP without installing any software:

Acknowledgements

This document incorporates much appreciated suggestions and feedback by Eliot Lear, Wolf McNally, Laurence Lundblade, Joe Hildebrand, and Carsten Bormann.

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Author's Address

Anders Rundgren (editor)
Independent
Montpellier
France
URI: https://www.linkedin.com/in/andersrundgren/