| Internet-Draft | RATS CMW | December 2023 |
| Birkholz, et al. | Expires 3 June 2024 | [Page] |
This document defines two encapsulation formats for RATS conceptual messages (i.e., evidence, attestation results, endorsements and reference values.)¶
The first format uses a CBOR or JSON array with two mandatory members, one for the type, another for the value, and a third optional member complementing the type field that says which kind of conceptual message(s) are carried in the value. The other format wraps the value in a CBOR byte string and prepends a CBOR tag to convey the type information.¶
This document also defines a corresponding CBOR tag, as well as JSON Web Tokens (JWT) and CBOR Web Tokens (CWT) claims. These allow embedding the wrapped conceptual messages into CBOR-based protocols and web APIs, respectively.¶
This note is to be removed before publishing as an RFC.¶
Discussion of this document takes place on the Remote ATtestation ProcedureS Working Group mailing list (rats@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/rats/.¶
Source for this draft and an issue tracker can be found at https://github.com/thomas-fossati/draft-ftbs-rats-msg-wrap.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
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This Internet-Draft will expire on 3 June 2024.¶
Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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.¶
The RATS architecture defines a handful of conceptual messages (see Section 8 of [RFC9334]), such as evidence and attestation results. Each conceptual message can have multiple claims encoding and serialization formats (Section 9 of [RFC9334]). Throughout their lifetime, RATS conceptual messages are typically transported over different protocols. For example, EAT [I-D.ietf-rats-eat] evidence in a "background check" topological arrangement first flows from Attester to Relying Party, and then from Relying Party to Verifier, over separate protocol legs. Attestation Results for Secure Interactions (AR4SI) [I-D.ietf-rats-ar4si] payloads in "passport" mode would go first from Verifier to Attester and then, at a later point in time and over a different channel, from Attester to Relying Party.¶
It is desirable to reuse any typing information associated with the messages across such protocol boundaries in order to minimize the cost associated with type registrations and maximize interoperability.¶
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 [RFC6838] - 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.¶
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 [RFC9334].¶
This document reuses the terms defined in Section 2 of [RFC9193] (e.g., "Content-Type").¶
Two types of RATS Conceptual Message Wrapper (CMW) are specified in this document:¶
A CMW using a CBOR or JSON array (Section 3.1);¶
A CMW based on CBOR tags (Section 3.2).¶
The CMW array format is defined in Figure 1. (To improve clarity,
the Content-Type ABNF is defined separately in Appendix A.)¶
The CDDL generic JC<> is used where there is a variance between CBOR
and JSON. The first argument is the CDDL for JSON and the second is the
CDDL for CBOR.¶
cmw-array = [
type: coap-content-format / media-type
value: JC<base64-string, bytes>
? ind: uint .bits cm-type
]
coap-content-format = uint .size 2
media-type = text .abnf ("Content-Type" .cat Content-Type-ABNF)
base64-string = text .regexp "[A-Za-z0-9_-]+"
cm-type = &(
reference-values: 0
endorsements: 1
evidence: 2
attestation-results: 3
)
It is composed of three members:¶
type:Either a text string representing a Content-Type (e.g., an EAT media type [I-D.ietf-rats-eat-media-type]) or an unsigned integer corresponding to a CoAP Content-Format number (Section 12.3 of [RFC7252]).¶
value:The RATS conceptual message serialized according to the value defined in the type member.¶
ind:An optional bitmap that indicates which conceptual message types are
carried in the value field. Any combination (i.e., any value between
1 and 15, included) is allowed. This is useful only if the type is
potentially ambiguous and there is no further context available to the
CMW consumer to decide. For example, this might be the case if the base
media type is not profiled (e.g., application/eat+cwt), if the value
field contains multiple conceptual messages with different types (e.g.,
both reference values and endorsements within the same application/signed-corim+cbor), or if the same profile identifier is
shared by different conceptual messages.
Future specifications may add new values to the ind field; see Section 7.4.¶
A CMW array can be encoded as CBOR [STD94] or JSON [RFC8259].¶
When using JSON, the value field is encoded as Base64 using the URL and filename safe alphabet (Section 5 of [RFC4648]) without padding.¶
When using CBOR, the value field is encoded as a CBOR byte string.¶
CBOR Tags used as CMW may be derived from CoAP Content-Format numbers.
If a CoAP content format exists for a RATS conceptual message, the
TN() transform defined in Appendix B of [RFC9277] can be used to
derive a corresponding CBOR tag in range [1668546817, 1668612095].¶
The RATS conceptual message is first serialized according to the Content-Format number associated with the CBOR tag and then encoded as a CBOR byte string, to which the tag is prepended.¶
The CMW CBOR Tag is defined in Figure 2.¶
cmw-cbor-tag<bytes> = #6.<cbor-tag-numbers>(bytes) cbor-tag-numbers = 0..18446744073709551615
After removing any external framing (for example, the ASN.1 OCTET STRING if the CMW is carried in a certificate extension [DICE-arch]), the CMW decoder does a 1-byte lookahead, as illustrated in the following pseudo code, to decide how to decode the remainder of the byte buffer:¶
func CMWTypeSniff(b []byte) (CMW, error) {
if len(b) == 0 {
return Unknown
}
if b[0] == 0x82 || b[0] == 0x83 {
return CBORArray
} else if b[0] >= 0xc0 && b[0] <= 0xdb {
return CBORTag
} else if b[0] == 0x5b {
return JSONArray
}
return Unknown
}
¶
The (equivalent) examples in Section 4.1, Section 4.2, and Section 4.3 assume that
the Media-Type-Name application/vnd.example.rats-conceptual-msg has been
registered alongside a corresponding CoAP Content-Format number 30001. The
CBOR tag 1668576818 is derived applying the TN() transform as described in
Section 3.2.¶
The example in Section 4.4 is a signed CoRIM payload with an explicit CM
indicator 0b0000_0011 (3), meaning that the wrapped message contains both
Reference Values and Endorsements.¶
[ "application/vnd.example.rats-conceptual-msg", "q82rzQ" ]¶
Note that a CoAP Content-Format number can also be used with the JSON array form. That may be the case when it is known that the receiver can handle CoAP Content-Formats and it is crucial to save bytes.¶
[ 30001, h'abcdabcd' ]¶
with the following wire representation:¶
82 # array(2)
19 7531 # unsigned(30001)
44 # bytes(4)
abcdabcd # "\xABͫ\xCD"
¶
Note that a Media-Type-Name can also be used with the CBOR array form, for example if it is known that the receiver cannot handle CoAP Content-Formats, or (unlike the case in point) if a CoAP Content-Format number has not been registrered.¶
[ "application/vnd.example.rats-conceptual-msg", h'abcdabcd' ]¶
1668576818(h'abcdabcd')¶
with the following wire representation:¶
da 63747632 # tag(1668576818)
44 # bytes(4)
abcdabcd # "\xABͫ\xCD"
¶
[ "application/signed-corim+cbor", h'd28443a10126a1', 3 ]¶
with the following wire representation:¶
83 # array(3)
78 1d # text(29)
6170706c69636174696f6e2f7369676e65642d636f72696d2b63626f72
# "application/signed-corim+cbor"
47 # bytes(7)
d28443a10126a1 # "҄C\xA1\u0001&\xA1"
03 # unsigned(3)
¶
This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.¶
According to [RFC7942], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".¶
The organization responsible for this implementation is Project Veraison, a Linux Foundation project hosted at the Confidential Computing Consortium.¶
The software, hosted at https://github.com/veraison/cmw, provides a Golang package that allows encoding and decoding of CMW payloads. The implementation covers all the features presented in this draft. The maturity level is alpha. The license is Apache 2.0. The developers can be contacted on the Zulip channel: https://veraison.zulipchat.com/#narrow/stream/383526-CMW/.¶
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.¶
RFC Editor: replace "RFCthis" with the RFC number assigned to this document.¶
cmw Claim Registration
IANA is requested to add a new cmw claim to the "CBOR Web Token (CWT) Claims" registry [IANA.cwt] as follows:¶
Claim Name: cmw¶
Claim Description: A RATS Conceptual Message Wrapper¶
Claim Key: TBD¶
Claim Value Type(s): CBOR Array, or CBOR Tag¶
Change Controller: IETF¶
Specification Document(s): Section 3.1 and Section 3.2 of RFCthis¶
The suggested value for the Claim Key is 299.¶
cmw Claim Registration
IANA is requested to add a new cmw claim to the "JSON Web Token Claims" sub-registry of the "JSON Web Token (JWT)" registry [IANA.jwt] as follows:¶
IANA is requested to add the following tag to the "CBOR Tags" [IANA.cbor-tags] registry.¶
| CBOR Tag | Data Item | Semantics | Reference |
|---|---|---|---|
| TBD | CBOR array, CBOR tag | RATS Conceptual Message Wrapper | Section 3.1 and Section 3.2 of RFCthis |
This specification defines a new "RATS Conceptual Message Wrapper (CMW) Indicators" registry, with the policy "Expert Review" (Section 4.5 of [BCP26]).¶
The objective is to have Indicators values registered for all RATS Conceptual Messages (Section 8 of [RFC9334]).¶
The expert is instructed to add the values incrementally.¶
Acceptable values are those corresponding to RATS Conceptual Messages defined by the RATS architecture [RFC9334] and any of its updates.¶
Each entry in the registry must include:¶
A number corresponding to the bit position in the cm-ind bitmap.¶
A text string describing the RATS conceptual message this indicator corresponds to.¶
A reference to a document, if available, or the registrant.¶
The initial registrations for the registry are detailed in Table 2.¶
| Indicator value | Conceptual Message name | Reference |
|---|---|---|
| 0 | Reference Values | RFCthis |
| 1 | Endorsements | RFCthis |
| 2 | Evidence | RFCthis |
| 3 | Attestation Results | RFCthis |
; from RFC9193
Content-Type-ABNF = '
Content-Type = Media-Type-Name *( *SP ";" *SP parameter )
parameter = token "=" ( token / quoted-string )
token = 1*tchar
tchar = "!" / "#" / "$" / "%" / "&" / "\'" / "*"
/ "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
/ DIGIT / ALPHA
quoted-string = %x22 *( qdtext / quoted-pair ) %x22
qdtext = SP / %x21 / %x23-5B / %x5D-7E
quoted-pair = "\" ( SP / VCHAR )
Media-Type-Name = type-name "/" subtype-name
type-name = restricted-name
subtype-name = restricted-name
restricted-name = restricted-name-first *126restricted-name-chars
restricted-name-first = ALPHA / DIGIT
restricted-name-chars = ALPHA / DIGIT / "!" / "#" /
"$" / "&" / "-" / "^" / "_"
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
DIGIT = %x30-39 ; 0 - 9
POS-DIGIT = %x31-39 ; 1 - 9
ALPHA = %x41-5A / %x61-7A ; A - Z / a - z
SP = %x20
VCHAR = %x21-7E ; printable ASCII (no SP)
'
¶
Figure 3 describes the registration preconditions for using CMWs in either array or CBOR tag forms.¶
The list of currently open issues for this documents can be found at https://github.com/thomas-fossati/draft-ftbs-rats-msg-wrap/issues.¶
Note to RFC Editor: please remove before publication.¶
The authors would like to thank Carl Wallace and Carsten Bormann for their reviews and suggestions.¶