RATS Conceptual Messages Wrapper

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Table of Contents

1. Introduction

The RATS architecture defines a handful of conceptual messages (see Section 8 of [I-D.ietf-rats-architecture]), such as evidence and attestation results. Each conceptual message can have multiple claims encoding and serialization formats (Section 9 of [I-D.ietf-rats-architecture]). Such serialized message may have to be transported via different protocols - for example, evidence using an EAT [I-D.ietf-rats-eat] encoding serialized as a CBOR payload in a "background check" topological arrangement, or attestation results as Attestation Results for Secure Interactions (AR4SI) [I-D.ietf-rats-ar4si] payloads in "passport" mode.¶

In order to minimize the cost associated with registration and maximize interoperability, it is desirable to reuse their typing information across such boundaries.¶

This document defines two encapsulation formats for RATS conceptual messages that aim to achieve the goals stated above.¶

These encapsulation formats are designed to be:¶

• Self-describing - which removes the dependency on the framing provided by the embedding protocol (or the storage system) to convey exact typing information.¶
• Based on media types - which allows amortising their registration cost across many different usage scenarios.¶

A protocol designer could use these formats, for example, to convey evidence, endorsements or reference values in certificates and CRLs extensions ([DICE-arch]), to embed attestation results or evidence as first class authentication credentials in TLS handshake messages [I-D.fossati-tls-attestation], to transport attestation-related payloads in RESTful APIs, or for stable storage of attestation results in form of file system objects.¶

2. Conventions and Definitions

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.¶

In this document, CDDL [RFC8610] [RFC9165] is used to describe the data formats.¶

The reader is assumed to be familiar with the vocabulary and concepts defined in [I-D.ietf-rats-architecture].¶

3. Conceptual Message Wrapper Encodings

Two types of RATS Conceptual Message Wrapper (CMW) are specified in this document:¶

1. a CMW using a CBOR or a JSON array (Section 3.1)¶
2. a CMW based on CBOR tags (Section 3.2).¶

cmw = [ type, value ]

type = coap-content-format / media-type

coap-content-format = uint .size 2
media-type = text .abnf ("media-type" .cat RFC6838)

value = cbor-bytes / base64-string

cbor-bytes = bytes
base64-string = text .regexp "[A-Za-z0-9_-]+"

RFC6838 = '
media-type = type-name "/" subtype-name *1("+" suffix) parameters

type-name = restricted-name
subtype-name = restricted-name

; see https://www.iana.org/assignments/media-type-structured-suffix/
suffix = "xml" / "json" / "ber" / "cbor" / "der" / "fastinfoset" /
         "wbxml" / "zip" / "tlv" / "json-seq" / "sqlite3" / "jwt" /
         "gzip" / "cbor-seq" / "zstd"

parameters = *(";" parameter-name "=" parameter-value)

parameter-name = restricted-name
parameter-value = *VCHAR

restricted-name = restricted-name-first *126restricted-name-chars
restricted-name-first  = ALPHA / DIGIT
restricted-name-chars  = ALPHA / DIGIT / "!" / "#" / "$" / "&" / "-" /
                         "^" / "_"
restricted-name-chars =/ "." ; Characters before first dot always
                             ; specify a facet name
restricted-name-chars =/ "+" ; Characters after last plus always
                             ; specify a structured syntax suffix

VCHAR = %x21-7E            ; Visible (printing) characters
ALPHA = %x41-5A / %x61-7A  ; A-Z / a-z
DIGIT = %x30-39            ; 0-9
'

4. Examples

The (equivalent) examples below assume the media-type application/vnd.example.rats-conceptual-msg has been registered alongside a corresponding CoAP content format 30001. The CBOR tag 1668576818 is derived applying the TN transform as described in Section 3.2.¶

[
  30001,
  h'abcdabcd'
]

[
  "application/vnd.example.rats-conceptual-msg",
  "q82rzQ"
]

1668576818(h'abcdabcd')

5. Security Considerations

This document defines two encapsulation formats for RATS conceptual messages. The messages themselves and their encoding ensure security protection. For this reason there are no further security requirements raised by the introduction of this encapsulation.¶

Changing the encapsulation of a payload by an adversary will result in incorrect processing of the encapsulated messages and this will subsequently lead to a processing error.¶

6. IANA Considerations

When registering a new media type for evidence, in addition to its syntactical description, the author SHOULD provide a public and stable description of the signing and appraisal procedures associated with the data format.¶

Acknowledgments

TODO acknowledge.¶

Authors' Addresses