Internet DRAFT - draft-ietf-scitt-architecture
draft-ietf-scitt-architecture
SCITT H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Standards Track A. Delignat-Lavaud
Expires: 5 September 2024 C. Fournet
Microsoft Research
Y. Deshpande
ARM
S. Lasker
DataTrails
4 March 2024
An Architecture for Trustworthy and Transparent Digital Supply Chains
draft-ietf-scitt-architecture-06
Abstract
Traceability of physical and digital Artifacts in supply chains is a
long-standing, but increasingly serious security concern. The rise
in popularity of verifiable data structures as a mechanism to make
actors more accountable for breaching their compliance promises has
found some successful applications to specific use cases (such as the
supply chain for digital certificates), but lacks a generic and
scalable architecture that can address a wider range of use cases.
This document defines a generic, interoperable and scalable
architecture to enable transparency across any supply chain with
minimum adoption barriers. It provides flexibility, enabling
interoperability across different implementations of Transparency
Services with various auditing and compliance requirements. Issuers
can register their Signed Statements on any Transparency Service,
with the guarantee that all Auditors and Verifiers will be able to
verify them.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-scitt-architecture/.
Discussion of this document takes place on the SCITT Working Group
mailing list (mailto:scitt@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/scitt/. Subscribe at
https://www.ietf.org/mailman/listinfo/scitt/.
Source for this draft and an issue tracker can be found at
https://github.com/ietf-wg-scitt/draft-ietf-scitt-architecture.
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Status of This Memo
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This Internet-Draft will expire on 5 September 2024.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definition of Transparency . . . . . . . . . . . . . . . . . 7
4. Architecture Overview . . . . . . . . . . . . . . . . . . . . 9
4.1. Transparency Service . . . . . . . . . . . . . . . . . . 11
4.1.1. Registration Policies . . . . . . . . . . . . . . . . 11
4.1.2. Initialization and bootstrapping . . . . . . . . . . 12
4.1.3. Append-only Log . . . . . . . . . . . . . . . . . . . 13
4.1.4. Adjacent Services . . . . . . . . . . . . . . . . . . 13
4.2. Signed Statements . . . . . . . . . . . . . . . . . . . . 13
4.2.1. Registration . . . . . . . . . . . . . . . . . . . . 17
4.3. Transparent Statements . . . . . . . . . . . . . . . . . 18
4.3.1. Validation . . . . . . . . . . . . . . . . . . . . . 20
5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22
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6. Security Considerations . . . . . . . . . . . . . . . . . . . 22
6.1. Security Guarantees . . . . . . . . . . . . . . . . . . . 24
6.2. Threat Model . . . . . . . . . . . . . . . . . . . . . . 24
6.2.1. Append-only Log . . . . . . . . . . . . . . . . . . . 25
6.2.2. Availability of Receipts . . . . . . . . . . . . . . 26
6.2.3. Confidentiality and Privacy . . . . . . . . . . . . . 26
6.2.4. Cryptographic Agility . . . . . . . . . . . . . . . . 27
6.2.5. Transparency Service Client Applications . . . . . . 27
6.2.6. Impersonation . . . . . . . . . . . . . . . . . . . . 27
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
7.1. Media Type Registration . . . . . . . . . . . . . . . . . 28
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
8.1. Normative References . . . . . . . . . . . . . . . . . . 29
8.2. Informative References . . . . . . . . . . . . . . . . . 30
Appendix A. Identifiers . . . . . . . . . . . . . . . . . . . . 32
A.1. For Binary Content . . . . . . . . . . . . . . . . . . . 33
A.2. For SCITT Messages . . . . . . . . . . . . . . . . . . . 34
A.3. For Transparent Statements . . . . . . . . . . . . . . . 34
A.4. Statements . . . . . . . . . . . . . . . . . . . . . . . 35
A.4.1. Statement URN . . . . . . . . . . . . . . . . . . . . 35
A.4.2. Statement URL . . . . . . . . . . . . . . . . . . . . 35
A.4.3. Statement Data URL . . . . . . . . . . . . . . . . . 35
A.5. Signed Statements . . . . . . . . . . . . . . . . . . . . 35
A.5.1. Signed Statement URN . . . . . . . . . . . . . . . . 35
A.5.2. Signed Statement URL . . . . . . . . . . . . . . . . 35
A.5.3. Signed Statement Data URL . . . . . . . . . . . . . . 36
A.6. Receipts . . . . . . . . . . . . . . . . . . . . . . . . 36
A.6.1. Receipt URN . . . . . . . . . . . . . . . . . . . . . 36
A.6.2. Receipt URL . . . . . . . . . . . . . . . . . . . . . 36
A.6.3. Receipt Data URL . . . . . . . . . . . . . . . . . . 36
A.7. Transparent Statements . . . . . . . . . . . . . . . . . 36
A.7.1. Transparent Statement URN . . . . . . . . . . . . . . 36
A.7.2. Transparent Statement URL . . . . . . . . . . . . . . 36
A.7.3. Transparent Statement Data URL . . . . . . . . . . . 36
Appendix B. Signing Statements Remotely . . . . . . . . . . . . 37
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
1. Introduction
This document describes the scalable, flexible, and decentralized
SCITT architecture. Its goal is to enhance auditability and
accountability across supply chains.
In supply chains, downstream artifacts are built upon upstream
artifacts. The complexity of traceability and quality control for
these supply chains increases with the number of artifacts and
parties contributing to them. There are many parties who publish
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information about artifacts: For example, the original manufacturer
may provide information about the state of the artifact when it left
the factory. The shipping company may add information about the
transport environment of the artifact. Compliance auditors may
provide information about their compliance assessment of the
artifact. Security companies may publish vulnerability information
about an artifact. Some of these parties may publish information
about their analysis or use of an artifact.
SCITT provides a way for Relying Parties to obtain this information
in a way that is "transparent", that is, parties cannot lie about the
information that they publish without it being detected. SCITT
achieves this by having producers publish information in a
Transparency Service, where Relying Parties can check the
information.
1.1. Requirements Notation
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. Terminology
The terms defined in this section have special meaning in the context
of Supply Chain Integrity, Transparency, and Trust, which are used
throughout this document. When used in text, the corresponding terms
are capitalized. To ensure readability, only a core set of terms is
included in this section.
*Editor's Note:*: _The label "394" is expected to be reserved by this
document, in the COSE Header Parameters Registry._
The terms "header", "payload", and "to-be-signed bytes" are defined
in [RFC9052].
Append-only Log (Ledger): the verifiable append-only data structure
that stores Signed Statements in a Transparency Service, often
referred to by the synonym Ledger. SCITT supports multiple Ledger
and Receipt formats to accommodate different Transparency Service
implementations, and the proof types associated with different
types of Append-only Logs.
Artifact: a physical or non-physical item that is moving along a
supply chain.
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Auditor: an entity that checks the correctness and consistency of
all Transparent Statements issued by a Transparency Service.
Client: an application making protected Transparency Service
resource requests on behalf of the resource owner and with its
authorization.
Envelope: metadata, created by the Issuer to produce a Signed
Statement. The Envelope contains the identity of the Issuer and
information about the Artifact, enabling Transparency Service
Registration Policies to validate the Signed Statement. A Signed
Statement is a COSE Envelope wrapped around a Statement, binding
the metadata in the Envelope to the Statement. In COSE, an
Envelope consists of a protected header (included in the Issuer's
signature) and an unprotected header (not included in the Issuer's
signature).
Equivocation: a state where it is possible for a Transparency
Service to provide different views of its Append-only log to
Relying Parties about the same Artifact [EQUIVOCATION].
Feed: A collection of Receipts, as recorded by the Transparency
Service, based on filtering of properties from the envelope
including, but not limited to the sub field of the CWT_Claims.
Relying Parties may use the Feed to ensure completeness and Non-
equivocation in supply chain evidence by identifying all
Transparent Statements linked to the Artifact they are evaluating.
Issuer: organizations, stakeholders, and users involved in creating
or attesting to supply chain artifacts, releasing authentic
Statements to a definable set of peers. An Issuer may be the
owner or author of Artifacts, or an independent third party such
as an auditor, reviewer or an endorser.
Non-equivocation: a state where it is impossible for a Transparency
Service to provide different views of its append-only log to
Relying Parties about the same Artifact. Over time, an Issuer may
register new Signed Statements about an Artifact in a Transparency
Service with new information. However, the consistency of a
collection of Signed Statements about the Artifact can be checked
by all Relying Parties.
Receipt: a cryptographic proof that a Signed Statement is included
in the Append-only Log. Receipts are based on Signed Inclusion
Proofs, such as those as described in COSE Signed Merkle Tree
Proofs [I-D.draft-ietf-cose-merkle-tree-proofs]; they can be built
on different verifiable data structures, not just binary merkle
trees. A Receipt consists of a Transparency Service-specific
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inclusion proof for the Signed Statement, a signature by the
Transparency Service of the state of the Append-only Log after the
inclusion, and additional metadata (contained in the signature's
protected headers) to assist in auditing.
Registration: the process of submitting a Signed Statement to a
Transparency Service, applying the Transparency Service's
Registration Policy, adding to the Append-only Log, and producing
a Receipt.
Registration Policy: the pre-condition enforced by the Transparency
Service before registering a Signed Statement, based on
information in the non-opaque header and metadata contained in its
COSE Envelope.
Relying Party: a relying party depends on Signed or Transparent
Statements to verify an Artifact.
Signed Statement: an identifiable and non-repudiable Statement about
an Artifact signed by an Issuer. In SCITT, Signed Statements are
encoded as COSE signed objects; the payload of the COSE structure
contains the issued Statement.
Statement: any serializable information about an Artifact. To help
interpretation of Statements, they must be tagged with a media
type (as specified in [RFC6838]). A Statement may represent a
Software Bill Of Materials (SBOM) that lists the ingredients of a
software Artifact, an endorsement or attestation about an
Artifact, indicate the End of Life (EOL), redirection to a newer
version, or any content an Issuer wishes to publish about an
Artifact. The additional Statements about an Artifact are
correlated by the Subject defined in the [CWT_CLAIMS] protected
header. The Statement is considered opaque to Transparency
Service, and MAY be encrypted.
Subject: This term has the same definition as in RFC8392, which
relies on the definition in RFC7519. The sub (subject) claim
identifies the principal that is the subject of the CWT. The
claims in a CWT are normally statements about the subject. In
SCITT, sub identifies the entity about which statements, and
receipts are made. The subject value MUST either be scoped to be
locally unique in the context of the Issuer or be globally unique.
The processing of this claim is generally application specific.
The sub value is a case-sensitive string containing a StringOrURI
value. Issuers use sub to identify the entity about which they
are making Signed Statements. Transparency Services use sub to
identify the entity about which they are issuing a Receipt.
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Transparency Service: an entity that maintains and extends the
Append-only Log, and endorses its state. A Transparency Service
can be a complex distributed system, and SCITT requires the
Transparency Service to provide many security guarantees about its
Append-only Log. The identity of a Transparency Service is
captured by a public key that must be known by Relying Parties in
order to validate Receipts.
Transparent Statement: a Signed Statement that is augmented with a
Receipt created via Registration in a Transparency Service. The
receipt is stored in the unprotected header of COSE Envelope of
the Signed Statement. A Transparent Statement remains a valid
Signed Statement, and may be registered again in a different
Transparency Service.
Relying Party: organizations, stakeholders, and users involved in
validating supply chain Artifacts. Relying Parties consume
Transparent Statements, verifying their proofs and inspecting the
Statement payload, either before using corresponding Artifacts, or
later to audit an Artifact's provenance on the supply chain.
3. Definition of Transparency
In this document, the definition of transparency is intended to build
over abstract notions of Append-only Logs and Receipts. Existing
transparency systems such as Certificate Transparency are instances
of this definition.
A Signed Statement is an identifiable and non-repudiable Statement
made by an Issuer. The Issuer selects additional metadata and
attaches a proof of endorsement (in most cases, a signature) using
the identity key of the Issuer that binds the Statement and its
metadata. Signed Statements can be made transparent by attaching a
proof of Registration by a Transparency Service, in the form of a
Receipt that countersigns the Signed Statement and witnesses its
inclusion in the Append-only Log of a Transparency Service. By
extension, the document may say an Artifact (a firmware binary) is
transparent if it comes with one or more Transparent Statements from
its author or owner, though the context should make it clear what
type of Signed Statements is expected for a given Artifact.
Transparency does not prevent dishonest or compromised Issuers, but
it holds them accountable. Any Artifact that may be verified, is
subject to scrutiny and auditing by other parties. The Transparency
Service provides a history of Statements, which may be made by
multiple Issuers, enabling Relying Parties to make informed
decisions.
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Transparency is implemented by providing a consistent, append-only,
cryptographically verifiable, publicly available record of entries.
A SCITT instance is referred to as a Transparency Service.
Implementations of Transparency Services may protect their Append-
only Log using a combination of trusted hardware, replication and
consensus protocols, and cryptographic evidence. A Receipt is an
offline, universally-verifiable proof that an entry is recorded in
the Append-only Log. Receipts do not expire, but it is possible to
append new entries (more recent Signed Statements) that subsume older
entries (less recent Signed Statements).
Anyone with access to the Transparency Service can independently
verify its consistency and review the complete list of Transparent
Statements registered by each Issuer. However, the Registrations on
a separate Transparency Service is generally disjoint, though it is
possible to take a Transparent Statement (i.e. a Signed Statement
with a Receipt in its unprotected header, from a from the first
Transparency Service ) and register it on another Transparency
Service, where the second receipt will be over the first Receipt in
the unprotected header.
Reputable Issuers are thus incentivized to carefully review their
Statements before signing them to produce Signed Statements.
Similarly, reputable Transparency Services are incentivized to secure
their Append-only Log, as any inconsistency can easily be pinpointed
by any Auditor with read access to the Transparency Service.
The building blocks defined in SCITT are intended to support
applications in any supply chain that produces or relies upon digital
artifacts, from the build and supply of software and IoT devices to
advanced manufacturing and food supply.
SCITT is a generalization of Certificate Transparency [RFC9162],
which can be interpreted as a transparency architecture for the
supply chain of X.509 certificates. Considering CT in terms of
SCITT:
* CAs (Issuers) sign X.509 TBSCertificates (Artifacts) to produce
X.509 certificates (Signed Statements)
* CAs submit the certificates to one or more CT logs (Transparency
Services)
* CT logs produce Signed Certificate Timestamps (Transparent
Statements)
* Signed Certificate Timestamps are checked by Relying Parties
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* The Append-only Log can be checked by Auditors
4. Architecture Overview
The SCITT architecture consists of a very loose federation of
Transparency Services, and a set of common formats and protocols for
issuing and registering Signed Statements, and auditing Transparent
Statements.
In order to accommodate as many Transparency Service implementations
as possible, this document only specifies the format of Signed
Statements (which must be used by all Issuers) and a very thin
wrapper format for Receipts, which specifies the Transparency Service
identity and the agility parameters for the Signed Inclusion Proofs.
Most of the details of the Receipt's contents are specified in the
COSE Signed Merkle Tree Proof document
[I-D.draft-ietf-cose-merkle-tree-proofs].
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.----------.
| Artifact |
'----+-----'
v
.----+----. .----------. Identifiers
| Statement || Envelope +<-------------.
'----+----' '-----+----' |
| | .--------+--.
'----. .----' | Identity |
| | Documents +---.
v '------+----' |
.----+----. | |
| Signed | COSE Signing | |
| Statement +<-------------------+ |
'----+----' | |
| +--------+------+ |
.-' '--------------->+ Transparency | |
| .--------. | | |
| | Receipt +<------+ Service +-+ |
| | +. +--+------------+ | |
| '-+------' | | Transparency | |
| | Receipt +<-------+ | |
| '------+' | Service | |
'-------. .-' +------------+-+ |
| | |
v | |
.-----+-----. | |
| Transparent | | |
| Statement | | |
'-----+-----' | |
| | |
|'-------. .------------)--'
| | | |
| v v |
| .----+---+-----------. |
| / Verify Transparent / |
| / Statement / |
| '--------------------' |
v v
.--------+---------. .----------+-----.
/ Collect Receipts / / Replay Log /
'------------------' '----------------'
This section describes at a high level, the three main roles and
associated processes in SCITT: Issuers and Signed Statements,
Transparency Service and the Signed Statement Registration process,
as well as Relying Parties of the Transparent Statements and the
Receipt validation process.
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4.1. Transparency Service
Transparency Services MUST feature an Append-only Log. The Append-
only Log is the verifiable data structure that records Signed
Statements and supports the production of Receipts.
All Transparency Services MUST expose APIs for the registration of
Signed Statements and issuance of Receipts.
Transparency Services MAY support additional APIs for auditing, for
instance, to query the history of Signed Statements.
Typically a Transparency Service has a single Issuer identity which
is present in the iss claim of Receipts for that service.
Multi-tenant support can be enabled through the use of identifiers in
the iss claim, for example, ts.example may have a distinct Issuer
identity for each sub domain, such as customer1.ts.example and
customer2.ts.example.
4.1.1. Registration Policies
Registration Policies refer to additional checks over and above the
Mandatory Registration Checks that are performed before a Signed
Statement is accepted to be registered to the Append-only Log.
Transparency Services MUST maintain Registration Policies.
Transparency Services MUST also maintain a list of trust anchors used
to authenticate Issuers, which MAY be included in a registration
policy statement. For instance, a trust anchor could be an X.509
root certificate, the discovery URL of an OpenID Connect identity
provider, or any other COSE compatible PKI trust anchor.
Registration Policies and trust anchors MUST be made transparent and
available to all Relying Parties of the Transparency Service by
registering them as Signed Statements on the Append-only Log, and
distributing the associated Receipts.
This specification leaves implementation, encoding and documentation
of Registration Policies and trust anchors to the operator of the
Transparency Service.
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4.1.1.1. Mandatory Registration Checks
During registration, a Transparency Service MUST, at a minimum,
authenticate the Issuer of the Signed Statement by validating the
COSE signature and checking the identity of the issuer against one of
its currently configured trust anchors, using the x5t (34),
x5chain(33) or kid(4) protected headers of the Signed Statement as
hints. For instance, in order to authenticate X.509 Signed
Statements, the Transparency Service MUST build and validate a
complete certificate chain from the Issuer's certificate identified
by x5t, to one of the root certificates most recently registered as a
trust anchor of the Transparency Service.
The Transparency Service MUST apply the Registration Policy that was
most recently added to the Append-only Log at the time of
registration.
4.1.1.2. Auditability of Registration
The operator of a Transparency Service MAY update the Registration
Policy or the trust anchors of a Transparency Service at any time.
Transparency Services MUST ensure that for any Signed Statement they
register, enough information is made available to Auditors (either in
the Append-only Log and retrievable through audit APIs, or included
in the Receipt) to authenticate and retrieve the Transparent
Statements describing the registration policy and trust anchors that
apply to this registration.
4.1.2. Initialization and bootstrapping
Since the mandatory registration checks rely on having registered
Signed Statements for the registration policy and trust anchors,
Transparency Services MUST support at least one of the three
following bootstrapping mechanisms:
* A built-in default Registration Policy and default trust anchors;
* Acceptance of a first Signed Statement whose payload is a valid
Registration Policy, without performing registration checks
* An out-of-band authenticated management interface
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4.1.3. Append-only Log
The security properties of the Append-only Log are determined by the
choice of the verifiable data structure used by the Transparency
Service to implement the Log. This verifiable data structure MUST
support the following security requirements:
Append-Only: once included in the verifiable data structure, a
Signed Statement cannot be modified, deleted, or reordered; hence
its Receipt provides an offline verifiable proof of registration.
Non-equivocation: there is no fork in the Append-only Log. Everyone
with access to its content sees the same collection of Signed
Statements and can check that it is consistent with any Receipts
they have verified.
Replayability: the Append-only Log includes sufficient information
to enable authorized actors with access to its content to check
that each included Signed Statement has been correctly registered.
In addition to Receipts, some verifiable data structures might
support additional proof types, such as proofs of consistency, or
proofs of non inclusion.
Specific verifiable data structures, such those describes in
[RFC9162] and [I-D.draft-ietf-cose-merkle-tree-proofs], and the
review of their security requirements for SCITT are out of scope for
this document.
4.1.4. Adjacent Services
Transparency Services can be deployed along side other database or
object storage technologies. For example, a Transparency Service
that is supporting a software package management system, might be
referenced from the APIs exposed for package management. Providing
an ability to request a fresh receipt for a given software package,
or to request a list of Signed Statements and Artifacts associated
with a software package.
4.2. Signed Statements
This specification prioritizes conformance to [RFC9052] and its
required and optional properties. Profiles and implementation
specific choices should be used to determine admissability of
conforming messages. This specification is left intentionally open
to allow implementations to make the restrictions that make the most
sense for their operational use cases.
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At least one identifier for an identity document MUST be included in
the protected header of the COSE envelope, as one of x5t, x5chain or
kid.
* Support for x5t is mandatory to implement.
* Support for kid and x5chain is optional.
When x5t or x5chain is present, iss MUST be a string with a value
between 1 and 8192 characters in length that fits the regular
expression of a distinguished name.
The mechanisms for how Transparency Services obtain identity
documents is out-of-scope of this document.
The kid header parameter MUST be present when neither x5t nor x5chain
are present. Key discovery protocols are out-of-scope of this
document.
The protected header of a Signed Statement and a Receipt MUST include
the CWT Claims header parameter as specified in Section 2 of
[CWT_CLAIMS_COSE]. The CWT Claims value MUST include the Issuer
Claim (Claim label 1) and the Subject Claim (Claim label 2)
[IANA.cwt].
A Receipt is a Signed Statement, (cose-sign1), with addition claims
in its protected header related to verifying the inclusion proof in
its unprotected header. See
[I-D.draft-ietf-cose-merkle-tree-proofs].
Figure 1 illustrates a normative CDDL definition for of the protected
header for Signed Statements and Receipts.
Everything that is optional in the following CDDL can potentially be
discovered out of band and Registration Policies are not assured on
the presence of these optional fields. A Registration Policy that
requires an optional field to be present MUST reject any Signed
Statements or Receipts that are invalid according to the policy.
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Signed_Statement = #6.18(COSE_Sign1)
Receipt = #6.18(COSE_Sign1)
COSE_Sign1 = [
protected : bstr .cbor Protected_Header,
unprotected : Unprotected_Header,
payload : bstr / nil,
signature : bstr
]
Protected_Header = {
&(CWT_Claims: 15) => CWT_Claims
? &(alg: 1) => int
? &(content_type: 3) => tstr / uint
? &(kid: 4) => bstr
? &(x5chain: 33) => COSE_X509
? &(x5t: 34) => COSE_CertHash
* int => any
}
CWT_Claims = {
&(iss: 1) => tstr
&(sub: 2) => tstr
* int => any
}
Unprotected_Header = {
? &(receipts: 394) => [+ Receipt]
}
Figure 1: CDDL definition for Signed Statements and Receipts
{ / Protected /
1: -7, / Algorithm /
3: application/example+json, / Content type /
4: h'50685f55...50523255', / Key identifier /
15: { / CWT Claims /
1: software.vendor.example, / Issuer /
2: vendor.product.example, / Subject /
}
}
Figure 2: CBOR Extended Diagnostic Notation example of a Signed
Statement's Protected Header
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18( / COSE Sign 1 /
[
h'a4012603...6d706c65', / Protected /
{}, / Unprotected /
nil, / Detached payload /
h'79ada558...3a28bae4' / Signature /
]
)
Figure 3: CBOR Extended Diagnostic Notation example of a Signed
Statement
Figure 3 illustrates a payload that is detached. This is to support
very large supply chain artifacts, and to ensure that Transparent
Statements can integrate with existing file systems.
There are many types of Statements (such as SBOMs, malware scans,
audit reports, policy definitions) that Issuers may want to turn into
Signed Statements. An Issuer must first decide on a suitable format
(3: payload type) to serialize the Statement payload. For a software
supply chain, payloads describing the software artifacts may include:
* [COSWID]
* [CycloneDX]
* [in-toto]
* [SPDX-CBOR]
* [SPDX-JSON]
* [SLSA]
* [SWID]
Once all the Envelope headers are set, an Issuer MUST use a standard
COSE implementation to produce an appropriately serialized Signed
Statement (the SCITT tag of COSE_Sign1_Tagged is outside the scope of
COSE, and used to indicate that a signed object is a Signed
Statement).
Issuers may produce Signed Statements about different Artifacts under
the same Identity. Issuers and Relying Parties must be able to
recognize the Artifact to which the statements pertain by looking at
the Signed Statement. The iss and sub claims, within the CWT_Claims
protected header, are used to identify the Artifact the statement
pertains to.
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See Subject under Section 2 Terminology.
Issuers MAY use different signing keys (identified by kid in the
resolved key manifest) for different Artifacts, or sign all Signed
Statements under the same key.
An Issuer can make multiple Statements about the same Artifact. For
example, an Issuer can make amended Statements about the same
Artifact as their view changes over time.
Multiple Issuers can make different, even conflicting Statements,
about the same Artifact. Relying Parties can choose which Issuers
they trust.
Multiple Issuers can make the same Statement about a single Artifact,
affirming multiple Issuers agree.
4.2.1. Registration
To register a Signed Statement, the Transparency Service performs the
following steps:
1. *Client authentication:* A Client authenticates with the
Transparency Service, to Register Signed Statements on behalf of
one or more issuers. Authentication and authorization is
implementation-specific, and out of scope of the SCITT
Architecture.
2. *Issuer Verification:* The Transparency Service MUST perform
resolution of the Issuer's identity, which may be different than
the Client identity. This step may require that the service
retrieves the Issuer ID in real-time, or rely on a cache of
recent resolutions. For auditing, during Registration, the
Transparency Service MUST store evidence of the lookup, including
if it was resolved from a cache.
3. *Signature verification:* The Transparency Service MUST verify
the signature of the Signed Statement, as described in [RFC9360],
using the signature algorithm and verification key of the Issuer.
4. *Signed Statement validation:* The Transparency Service MUST
check that the Signed Statement includes the required protected
headers listed above. The Transparency Service MAY verify the
Statement payload format, content and other optional properties.
5. *Apply Registration Policy:* The Transparency Service MUST check
the attributes required by a policy are present in the protected
headers. Custom Signed Statements are evaluated given the
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current Transparency Service state and the entire Envelope, and
may use information contained in the attributes of named
policies.
6. *Register the Signed Statement* to the append-only log.
7. *Return the Receipt*, which MAY be asynchronous from
registration. The Transparency Service MUST be able to provide a
receipt for all registered Statements. A receipt for a Signed
Statement MAY be provided asynchronously. Details about
generating Receipts are described in Section 4.3.
The last two steps may be shared between a batch of Signed Statements
recorded in the Append-only Log.
A Transparency Service MUST ensure that a Signed Statement is
registered before releasing its Receipt.
The same Signed Statement may be independently registered in multiple
Transparency Services, producing multiple, independent Receipts. The
multiple receipts may be attached to the unprotected header of the
Signed Statement, creating a Transparent Statement.
4.3. Transparent Statements
The Client (which is not necessarily the Issuer) that registers a
Signed Statement and receives a Receipt can produce a Transparent
Statement by adding the Receipt to the Unprotected Header of the
Signed Statement. Client applications MAY register Signed Statements
on behalf of one or more Issuers. Client applications MAY request
Receipts regardless of the identity of the Issuer of the associated
Signed Statement.
When a Signed Statement is registered by a Transparency Service a
Receipt becomes available. When a Receipt is included in a Signed
Statement a Transparent Statement is produced.
Receipts are based on Signed Inclusion Proofs as described in COSE
Signed Merkle Tree Proofs ([I-D.draft-ietf-cose-merkle-tree-proofs]).
The registration time is defined as the timestamp at which the
Transparency Service has added this Signed Statement to its Append-
only Log.
*Editor's Note:* The WG is discussing if existing CWT claims might
better support these design principles.
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Figure 4 illustrates a normative CDDL definition of Transparent
Statements.
Transparent_Statement = #6.18(COSE_Sign1)
Unprotected_Header = {
&(receipts: 394) => [+ Receipt]
}
Figure 4: CDDL definition for a Transparent Statement
18( / COSE Sign 1 /
[
h'a4012603...6d706c65', / Protected /
{ / Unprotected /
394: [ / Receipts (1) /
h'd284586c...4191f9d2' / Receipt 1 /
]
},
nil, / Detached payload /
h'79ada558...3a28bae4' / Signature /
]
)
Figure 5: CBOR Extended Diagnostic Notation example of a
Transparent Statement
Figure 5 illustrates a payload that is detached.
The unprotected header can contain multiple receipts.
{ / Protected /
1: -7, / Algorithm /
4: h'50685f55...50523255', / Key identifier /
-111: 1, / Verifiable Data Structure /
15: { / CWT Claims /
1: transparency.vendor.example, / Issuer /
2: vendor.product.example, / Subject /
}
}
Figure 6: CBOR Extended Diagnostic Notation example of a
Receipt's Protected Header
Notice the verifiable data structure used is RFC9162_SHA256 in this
case. We know from the COSE Verifiable Data Structure Registry that
RFC9162_SHA256 is value 1, and that it supports -1 (inclusion proofs)
and -2 (consistency proofs).
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18( / COSE Sign 1 /
[
h'a4012604...6d706c65', / Protected /
{ / Unprotected /
-222: { / Proofs /
-1: [ / Inclusion proofs (1) /
h'83080783...32568964', / Inclusion proof 1 /
]
},
},
nil, / Detached payload /
h'10f6b12a...4191f9d2' / Signature /
]
)
Figure 7: CBOR Extended Diagnostic Notation example of a Receipt
Notice the unprotected header contains verifiable data structure
proofs, see the protected header for details regarding the specific
verifiable data structure used.
[ / Inclusion proof 1 /
8, / Tree size /
7, / Leaf index /
[ / Inclusion hashes (3) /
h'c561d333...f9850597' / Intermediate hash 1 /
h'75f177fd...2e73a8ab' / Intermediate hash 2 /
h'0bdaaed3...32568964' / Intermediate hash 3 /
]
]
Figure 8: CBOR Extended Diagnostic Notation example of a
Receipt's Inclusion Proof
This is a decoded inclusion proof for RFC9162_SHA256, other
verifiable data structures might encode inclusion proofs differently.
4.3.1. Validation
Relying Parties MUST apply the verification process as described in
Section 4.4 of RFC9052.
APIs exposing verification logic for Transparent Statements may
provide more details than a single boolean result. For example, an
API may indicate if the signature on the Receipt or Signed Statement
is valid, if claims related to the validity period are valid, or if
the inclusion proof in the Receipt is valid.
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The algorithm-specific details of checking inclusion proofs are
covered in [I-D.draft-ietf-cose-merkle-tree-proofs]. The pseudo-code
for validation of a transparent statement is as follows:
let verify_transparent_statement(t) =
let receipt = t.unprotected.scitt-receipt
let version = receipt.protected.scitt-version or fail "Missing SCITT Receipt version"
assert(version == 1)
let leaf = COSE.serialize(t with .unprotected = {
334 => receipt.unprotected.statement-registration-info
})
let vds = receipt.protected.verifiable-data-structure of fail "Missing verifiable data structure"
let root = verify_inclusion_proof(vds, receipt.unprotected.scitt-inclusion-proof, leaf)
or fail "Failed to verify inclusion proof"
// Statement registration info has been authenticated by the inclusion proof
receipt.protected.statement-registration-info = receipt.unprotected.statement-registration-info
return COSE.verify(receipt, detached_payload=root)
Before checking a Transparent Statement, the Verifier must be
configured with one or more identities of trusted Transparency
Services.
Relying Parties MAY be configured to re-verify the Issuer's Signed
Statement locally, but this requires a fresh resolution of the
Issuer's verification keys, which MAY fail if the key has been
revoked.
Some Relying Parties MAY decide to locally re-apply some or all of
the Registration Policies, if they have limited trust in the
Transparency Services. In addition, Relying Parties MAY apply
arbitrary validation policies after the Transparent Statement has
been verified and validated. Such policies may use as input all
information in the Envelope, the Receipt, and the Statement payload,
as well as any local state.
Relying Parties MAY offer options to store or share the Receipt of
the Transparent Statement for auditing the Transparency Services in
case a dispute arises.
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5. Privacy Considerations
Transparency Services are often publicly accessible. Issuers should
treat Signed Statements (rendering them as Transparent Statements) as
publicly accessible. In particular, a Signed Statement Envelope and
Statement payload should not carry any private information in
plaintext.
Transparency Services can have an authorization policy controlling
who can access the Append-only Log. While this can be used to limit
who can read the Log, it may also limit the usefulness of the system.
Some jurisdictions have a Right to be Forgotten. However, once a
Signed Statement is inserted into the Append-only Log maintained by a
Transparency Service, it cannot be removed from the Log.
6. Security Considerations
On its own, verifying a Transparent Statement does not guarantee that
its Envelope or contents are trustworthy. Just that they have been
signed by the apparent Issuer and counter-signed by the Transparency
Service. If the Verifier trusts the Issuer, it can infer that an
Issuer's Signed Statement was issued with this Envelope and contents,
which may be interpreted as the Issuer saying the Artifact is fit for
its intended purpose. If the Verifier trusts the Transparency
Service, it can independently infer that the Signed Statement passed
the Transparency Service Registration Policy and that has been
persisted in the Append-only Log. Unless advertised in the
Transparency Service Registration Policy, the Verifier cannot assume
that the ordering of Signed Statements in the Append-only Log matches
the ordering of their issuance.
Similarly, the fact that an Issuer can be held accountable for its
Transparent Statements does not on its own provide any mitigation or
remediation mechanism in case one of these Transparent Statements
turned out to be misleading or malicious. Just that signed evidence
will be available to support them.
An Issuer that knows of a changed state of quality for an Artifact,
SHOULD Register a new Signed Statement, using the same 15 CWT iss and
sub claims.
Issuers MUST ensure that the Statement payloads in their Signed
Statements are correct and unambiguous, for example by avoiding ill-
defined or ambiguous formats that may cause Relying Parties to
interpret the Signed Statement as valid for some other purpose.
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Issuers and Transparency Services MUST carefully protect their
private signing keys and avoid these keys being used for any purpose
not described in this architecture document. In cases where key re-
use is unavoidable, keys MUST NOT sign any other message that may be
verified as an Envelope as part of a Signed Statement.
Each of these functions MUST be carefully protected against both
external attacks and internal misbehavior by some or all of the
operators of the Transparency Service.
For instance, the code for the Registration Policy evaluation and
endorsement may be protected by running in a Trusted Execution
Environment (TEE).
The Transparency Service may be replicated with a consensus
algorithm, such as Practical Byzantine Fault Tolerance [PBFT] and may
be used to protect against malicious or vulnerable replicas.
Threshold signatures may be use to protect the service key, etc.
Issuers and Transparency Services MUST rotate verification keys for
signature checking in well-defined cryptoperiods (see
[KEY-MANAGEMENT]).
A Transparency Service MAY provide additional authenticity assurances
about its secure implementation and operation, enabling remote
attestation of the hardware platforms and/or software Trusted
Computing Bases (TCB) that run the Transparency Service. If present,
these additional authenticity assurances MUST be registered in the
Append-only Log and MUST always be exposed by the Transparency
Services' APIs. An example of Signed Statement's payloads that can
improve authenticity assurances are trustworthiness assessments that
are RATS Conceptual Messages, such as Evidence, Endorsements, or
corresponding Attestation Results (see [RFC9334]).
For example, if a Transparency Service is implemented using a set of
redundant replicas, each running within its own hardware-protected
trusted execution environments (TEEs), then each replica can provide
fresh Evidence or fresh Attestation Results about its TEEs. The
respective Evidence can show, for example, the binding of the
hardware platform to the software that runs the Transparency Service,
the long-term public key of the service, or the key used by the
replica for signing Receipts. The respective Attestation Result, for
example, can show that the remote attestation Evidence was appraised
by a trusted Verifier and complies with well-known Reference Values
and Endorsements.
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6.1. Security Guarantees
SCITT provides the following security guarantees:
1. Statements made by Issuers about supply chain Artifacts are
identifiable, authentic, and non-repudiable
2. Statement provenance and history can be independently and
consistently audited
3. Issuers can efficiently prove that their Statement is logged by a
Transparency Service
The first guarantee is achieved by requiring Issuers to sign their
Statements and associated metadata using a distributed public key
infrastructure. The second guarantee is achieved by storing the
Signed Statement on an Append-only Log. The third guarantee is
achieved by implementing the Append-only Log using a verifiable data
structure (such as a Merkle Tree [MERKLE]).
6.2. Threat Model
The document provides a generic threat model for SCITT, describing
its residual security properties when some of its actors (identity
providers, Issuers, Transparency Services, and Auditors) are corrupt
or compromised.
This model may need to be refined to account for specific supply
chains and use cases.
SCITT primarily supports checking of Signed Statement authenticity,
both from the Issuer (authentication) and from the Transparency
Service (transparency). These guarantees are meant to hold for
extensive periods of time, possibly decades.
It can never be assumed that some Issuers and some Transparency
Services will not be corrupt.
SCITT entities explicitly trust one another on the basis of their
long-term identity, which maps to shorter-lived cryptographic
credentials. A Verifier SHOULD validate a Transparent Statement
originating from a given Issuer, registered at a given Transparency
Service (both identified in the Verifier's local authorization
policy) and would not depend on any other Issuer or Transparency
Services.
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Authorized supply chain actors (Issuers) cannot be stopped from
producing Signed Statements including false assertions in their
Statement payload (either by mistake or by corruption), but these
Issuers can made accountable by ensuring their Signed Statements are
systematically registered at a trustworthy Transparency Service.
Similarly, providing strong residual guarantees against faulty/
corrupt Transparency Services is a SCITT design goal. Preventing a
Transparency Service from registering Signed Statements that do not
meet its stated Registration Policy, or to issue Receipts that are
not consistent with their Append-only Log is not possible. In
contrast Transparency Services can be held accountable and they can
be called out by any Auditor that replays their Append-only Log
against any contested Receipt. Note that the SCITT Architecture does
not require trust in a single centralized Transparency Service.
Different actors may rely on different Transparency Services, each
registering a subset of Signed Statements subject to their own
policy.
In both cases, the SCITT Architecture provides generic, universally-
verifiable cryptographic proof to individually blame Issuers or the
Transparency Service. On one hand, this enables valid actors to
detect and disambiguate malicious actors who employ Equivocation with
Signed Statements to different entities. On the other hand, their
liability and the resulting damage to their reputation are
application specific, and out of scope of the SCITT Architecture.
Relying Parties and Auditors need not be trusted by other actors. In
particular, so long as actors maintain proper control of their
signing keys and identity infrastructure they cannot "frame" an
Issuer or a Transparency Service for Signed Statements they did not
issue or register.
6.2.1. Append-only Log
If a Transparency Service is honest, then a Transparent Statement
including a correct Receipt ensures that the associated Signed
Statement passed its Registration Policy and was recorded
appropriately.
Conversely, a corrupt Transparency Service may:
1. refuse or delay the Registration of Signed Statements
2. register Signed Statements that do not pass its Registration
Policy (e.g., Signed Statement with Issuer identities and
signatures that do not verify)
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3. issue verifiable Receipts for Signed Statements that do not match
its Append-only Log
4. refuse access to its Transparency Service (e.g., to Auditors,
possibly after storage loss)
An Auditor granted (partial) access to a Transparency Service and to
a collection of disputed Receipts will be able to replay it, detect
any invalid Registration (2) or incorrect Receipt in this collection
(3), and blame the Transparency Service for them. This ensures any
Verifier that trusts at least one such Auditor that (2, 3) will be
blamed to the Transparency Service.
Due to the operational challenge of maintaining a globally consistent
Append-only Log, some Transparency Services may provide limited
support for historical queries on the Signed Statements they have
registered, and accept the risk of being blamed for inconsistent
Registration or Issuer Equivocation.
Relying Parties and Auditors may also witness (1, 4) but may not be
able to collect verifiable evidence for it.
6.2.2. Availability of Receipts
Networking and Storage are trusted only for availability.
Auditing may involve access to data beyond what is persisted in the
Transparency Services. For example, the registered Transparency
Service may include only the hash of a detailed SBOM, which may limit
the scope of auditing.
Resistance to denial-of-service is implementation specific.
Actors may want to independently keep their own record of the Signed
Statements they issue, endorse, verify, or audit.
6.2.3. Confidentiality and Privacy
According to Zero Trust Principles any location in a network is never
trusted. All contents exchanged between actors is protected using
secure authenticated channels (e.g., TLS) but may not exclude network
traffic analysis.
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The Transparency Service is trusted with the confidentiality of the
Signed Statements presented for Registration. Some Transparency
Services may publish every Signed Statement in their logs, to
facilitate their dissemination and auditing. Transparency Services
MAY return Receipts to client applications synchronously or
asynchronously.
A collection of Signed Statements must not leak information about the
contents of other Signed Statements registered on the Transparency
Service.
Issuers must carefully review the inclusion of private/confidential
materials in their Statements. For example, Issuers must remove
Personally Identifiable Information (PII) as clear text in the
statement. Alternatively, Issuers may include opaque cryptographic
statements, such as hashes.
The confidentiality of queries is implementation-specific, and
generally not guaranteed. For example, while offline Envelope
validation of Signed Statements is private, a Transparency Service
may monitor which of its Transparent Statements are being verified
from lookups to ensure their freshness.
6.2.4. Cryptographic Agility
The SCITT Architecture supports cryptographic agility. The actors
depend only on the subset of signing and Receipt schemes they trust.
This enables the gradual transition to stronger algorithms, including
e.g. post-quantum signature algorithms.
6.2.5. Transparency Service Client Applications
Authentication of Client applications is out of scope for this
document. Transparency Services MUST authenticate both client
applications and the Issuer of signed statements in order to ensure
that implementation specific authentication and authorization
policies are enforced. The specification of authentication and
authorization policies is out of scope for this document.
6.2.6. Impersonation
The identity resolution mechanism is trusted to associate long-term
identifiers with their public signature-verification keys.
Transparency Services and other parties may record identity-
resolution evidence to facilitate its auditing.
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If one of the credentials of an Issuer gets compromised, the SCITT
Architecture still guarantees the authenticity of all Signed
Statements signed with this credential that have been registered on a
Transparency Service before the compromise. It is up to the Issuer
to notify Transparency Services of credential revocation to stop
Relying Parties from accepting Signed Statements signed with
compromised credentials.
7. IANA Considerations
TBD; Section 3.
7.1. Media Type Registration
This section requests registration of the following media types
[RFC2046] in the "Media Types" registry [IANA.media-types] in the
manner described in [RFC6838].
To indicate that the content is an scitt configuration represented as
JSON:
* Type name: application
* Subtype name: scitt-configuration+json
* Required parameters: n/a
* Optional parameters: n/a
* Encoding considerations: binary; application/scitt-
configuration+json values are represented as a JSON Object; UTF-8
encoding SHOULD be employed for the JSON object.
* Security considerations: See the Security Considerations section
of TBD.
* Interoperability considerations: n/a
* Published specification: TBD
* Applications that use this media type: TBD
* Fragment identifier considerations: n/a
* Additional information:
- Magic number(s): n/a
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- File extension(s): n/a
- Macintosh file type code(s): n/a
* Person & email address to contact for further information: TBD
* Intended usage: COMMON
* Restrictions on usage: none
* Author: TBD
* Change Controller: IETF
* Provisional registration? No
8. References
8.1. Normative References
[COSWID] Birkholz, H., Fitzgerald-McKay, J., Schmidt, C., and D.
Waltermire, "Concise Software Identification Tags",
RFC 9393, DOI 10.17487/RFC9393, June 2023,
<https://www.rfc-editor.org/rfc/rfc9393>.
[CWT_CLAIMS_COSE]
Looker, T. and M. B. Jones, "CBOR Web Token (CWT) Claims
in COSE Headers", Work in Progress, Internet-Draft, draft-
ietf-cose-cwt-claims-in-headers-10, 29 November 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-cose-
cwt-claims-in-headers-10>.
[IANA.cwt] IANA, "CBOR Web Token (CWT) Claims",
<http://www.iana.org/assignments/cwt>.
[IANA.media-types]
IANA, "Media Types",
<http://www.iana.org/assignments/media-types>.
[IANA.named-information]
IANA, "Named Information",
<http://www.iana.org/assignments/named-information>.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/rfc/rfc2046>.
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[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/rfc/rfc2119>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
<https://www.rfc-editor.org/rfc/rfc6570>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/rfc/rfc6838>.
[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/rfc/rfc8174>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/rfc/rfc9052>.
[RFC9360] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Header Parameters for Carrying and Referencing X.509
Certificates", RFC 9360, DOI 10.17487/RFC9360, February
2023, <https://www.rfc-editor.org/rfc/rfc9360>.
8.2. Informative References
[CWT_CLAIMS]
"CBOR Web Token (CWT) Claims", n.d.,
<https://www.iana.org/assignments/cwt/cwt.xhtml>.
[CycloneDX]
"CycloneDX", n.d.,
<https://cyclonedx.org/specification/overview/>.
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[EQUIVOCATION]
Chun, B., Maniatis, P., Shenker, S., and J. Kubiatowicz,
"Attested append-only memory: making adversaries stick to
their word", Association for Computing Machinery (ACM),
ACM SIGOPS Operating Systems Review vol. 41, no. 6, pp.
189-204, DOI 10.1145/1323293.1294280, October 2007,
<https://doi.org/10.1145/1323293.1294280>.
[I-D.draft-ietf-core-href]
Bormann, C. and H. Birkholz, "Constrained Resource
Identifiers", Work in Progress, Internet-Draft, draft-
ietf-core-href-14, 9 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
href-14>.
[I-D.draft-ietf-cose-merkle-tree-proofs]
Steele, O., Birkholz, H., Delignat-Lavaud, A., and C.
Fournet, "COSE Receipts", Work in Progress, Internet-
Draft, draft-ietf-cose-merkle-tree-proofs-04, 2 March
2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
cose-merkle-tree-proofs-04>.
[in-toto] "in-toto", n.d., <https://in-toto.io/>.
[KEY-MANAGEMENT]
Barker, E. and W. Barker, "Recommendation for key
management:: part 2 -- best practices for key management
organizations", National Institute of Standards and
Technology, DOI 10.6028/nist.sp.800-57pt2r1, May 2019,
<https://doi.org/10.6028/nist.sp.800-57pt2r1>.
[MERKLE] Merkle, R., "A Digital Signature Based on a Conventional
Encryption Function", Springer Berlin Heidelberg, Advances
in Cryptology — CRYPTO ’87 pp. 369-378,
DOI 10.1007/3-540-48184-2_32, ISBN ["9783540187967",
"9783540481843"], 1988,
<https://doi.org/10.1007/3-540-48184-2_32>.
[PBFT] Castro, M. and B. Liskov, "Practical byzantine fault
tolerance and proactive recovery", Association for
Computing Machinery (ACM), ACM Transactions on Computer
Systems vol. 20, no. 4, pp. 398-461,
DOI 10.1145/571637.571640, November 2002,
<https://doi.org/10.1145/571637.571640>.
[RFC2397] Masinter, L., "The "data" URL scheme", RFC 2397,
DOI 10.17487/RFC2397, August 1998,
<https://www.rfc-editor.org/rfc/rfc2397>.
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[RFC8141] Saint-Andre, P. and J. Klensin, "Uniform Resource Names
(URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,
<https://www.rfc-editor.org/rfc/rfc8141>.
[RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate
Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
December 2021, <https://www.rfc-editor.org/rfc/rfc9162>.
[RFC9334] Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote ATtestation procedureS (RATS)
Architecture", RFC 9334, DOI 10.17487/RFC9334, January
2023, <https://www.rfc-editor.org/rfc/rfc9334>.
[SLSA] "SLSA", n.d., <https://slsa.dev/>.
[SPDX-CBOR]
"SPDX Specification", n.d.,
<https://spdx.dev/use/specifications/>.
[SPDX-JSON]
"SPDX Specification", n.d.,
<https://spdx.dev/use/specifications/>.
[SWID] "SWID Specification", n.d.,
<https://csrc.nist.gov/Projects/Software-Identification-
SWID/guidelines>.
[URLs] "URL Living Standard", n.d.,
<https://url.spec.whatwg.org/>.
Appendix A. Identifiers
This section provides informative examples of identifiers for
statements, signed statements, and receipts.
SCITT Identifiers are primarily meant to be understood by humans and
secondarily meant to be understood by machines, as such we define
text encodings for message identifiers first, and then provide binary
translations according to standard transformations for URLs and URNs
to binary formats.
SCITT Identifiers for URLs and URNs that are not Data URLs MUST be
represented in binary using [I-D.draft-ietf-core-href].
For each SCITT conceptual message, we define a Data URL format
according to [RFC2397], a URN format according to [RFC8141] and a URL
format according to [URLs].
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Note that Data URLs require base64 encoding, but the URN definitions
require base64url encoding.
Resolution and dereferencing of these identifiers is out of scope for
this document, and can be implemented by any concrete api
implementing the abstract interface defined as follows:
resource: content-type = dereference(identifier: identifier-type)
These identifiers MAY be present in a tstr field that does not
otherwise restrict the string in ways that prevent a URN or URL from
being present.
This includes iss, and sub which are used to express the Issuer and
subject of a signed statement or receipt.
This also includes kid which is used to express a hint for which
public key should be used to verify a signature.
All SCITT identifiers share common parameters to promote
interoperability:
Let hash-name be an algorithm name registered in
[IANA.named-information].
To promote interoperability, the hash-name MUST be "sha-256".
Let base-encoding, be a base encoding defined in [RFC4648].
To promote interoperability, the base encoding MUST be "base64url".
In the blocks and examples that follow, note '' line wrapping per RFC
8792.
A.1. For Binary Content
Identifiers for binary content, such as Statements, or even Artifacts
themselves are computed as follows:
Let the base64url-encoded-bytes-digest for the message be the
base64url encoded digest with the chosen hash algorithm of bytes /
octets.
Let the SCITT name for the message be the URN constructed from the
following URI template, according to [RFC6570]:
Let the message-type, be "statement" for Statements and Artifacts.
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urn:ietf:params:scitt:\
{message-type}:\
{hash-name}:{base-encoding}:\
{base64url-encoded-bytes-digest}
A.2. For SCITT Messages
Identifiers for COSE Sign 1 based messages, such as identifiers for
Signed Statements and Receipts are computed as follows:
Let the base64url-encoded-to-be-signed-bytes-digest for the message
be the base64url encoded digest with the chosen hash algorithm of the
"to-be-signed bytes", according to Section 8.1 of [RFC9052].
Let the SCITT name for the message be the URN constructed from the
following URI template, according to [RFC6570]:
Let the message-type, be "signed-statement" for Signed Statements,
and "receipt" for Receipts.
urn:ietf:params:scitt:\
{message-type}:\
{hash-name}:{base-encoding}:\
{base64url-encoded-to-be-signed-bytes-digest}
Note that this means the content of the signature is not included in
the identifier, even though signature related claims, such as
activation or expiration information in protected headers are
included.
As a result, an attacker may construct a new signed statement that
has the same identifier as a previous signed statement, but has a
different signature.
A.3. For Transparent Statements
Identifiers for Transparent Statements are defined as identifiers for
binary content, but with "transparent-statement" as the message-type.
urn:ietf:params:scitt:\
{message-type}:\
{hash-name}:{base-encoding}:\
{base64url-encoded-bytes-digest}
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Note that because this identifier is computed over the unprotected
header of the Signed Statement, any changes to the unprotected
header, such as changing the order of the unprotected header map key
value pairs, adding additional receipts, or adding additional proofs
to a receipt, will change the identifier of a transparent statement.
Note that because this identifier is computed over the signatures of
the signed statement and signatures in each receipt, any
canonicalization of the signatures after the fact will produce a
distinct identifier.
A.4. Statements
A.4.1. Statement URN
urn:ietf:params:scitt:statement:sha-256:base64url:5i6UeRzg1...qnGmr1o
Figure 9: Example Statement URN
A.4.2. Statement URL
https://transparency.example/api/identifiers\
/urn:ietf:params:scitt:statement:sha-256:base64url:5i6UeRzg1...qnGmr1o
Figure 10: Example Statement URL
A.4.3. Statement Data URL
data:application/json;base64,SGVsb...xkIQ==
Figure 11: Example Statement Data URL
A.5. Signed Statements
A.5.1. Signed Statement URN
urn:ietf:params:scitt:\
signed-statement:sha-256:base64url:5i6UeRzg1...qnGmr1o
Figure 12: Example Signed Statement URN
A.5.2. Signed Statement URL
https://transparency.example/api/identifiers\
/urn:ietf:params:scitt:\
signed-statement:sha-256:base64url:5i6UeRzg1...qnGmr1o
Figure 13: Example Signed Statement URL
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A.5.3. Signed Statement Data URL
data:application/cose;base64,SGVsb...xkIQ==
Figure 14: Example Signed Statement Data URL
A.6. Receipts
A.6.1. Receipt URN
urn:ietf:params:scitt:receipt:sha-256:base64url:5i6UeRzg1...qnGmr1o
Figure 15: Example Receipt URN
A.6.2. Receipt URL
https://transparency.example/api/identifiers\
/urn:ietf:params:scitt:receipt:sha-256:base64url:5i6UeRzg1...qnGmr1o
Figure 16: Example Receipt URL
A.6.3. Receipt Data URL
data:application/cose;base64,SGVsb...xkIQ==
Figure 17: Example Receipt Data URL
A.7. Transparent Statements
A.7.1. Transparent Statement URN
urn:ietf:params:scitt:\
transparent-statement:sha-256:base64url:5i6UeRzg1...qnGmr1o
Figure 18: Example Transparent Statement URN
A.7.2. Transparent Statement URL
https://transparency.example/api/identifiers\
/urn:ietf:params:scitt:\
transparent-statement:sha-256:base64url:5i6UeRzg1...qnGmr1o
Figure 19: Example Transparent Statement URL
A.7.3. Transparent Statement Data URL
data:application/cose;base64,SGVsb...xkIQ==
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Figure 20: Example Transparent Statement Data URL
Appendix B. Signing Statements Remotely
Statements, such as digital artifacts or structured data regarding
artifacts, can be too large or too sensitive to be send to a remote
Transparency Services over the Internet. In these cases a statement
can also be hash, which becomes the payload included in COSE to-be-
signed bytes. A Signed Statement (cose-sign1) MUST be produced from
the to-be-signed bytes according to Section 4.4 of [RFC9052].
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.----+-----.
| Artifact |
'+-+-------'
| |
.-' v
| .--+-------.
| | Hash +-+
| '----------' | /\
'-. | / \ .----------.
| +-->+ OR +-->+ Payload |
v | \ / '--------+-'
.+--------. | \/ |
| Statement +--+ |
'---------' |
|
|
... Producer Network ... |
...
... Issuer Network ... |
|
|
.---------. |
| Identity | (iss, x5t) |
| Document +--------------------+ |
`----+----` | |
^ | |
.----+-------. | |
| Private Key | | |
'----+-------' v |
| .----+---. |
| | Header | |
| '----+---' |
v v v
.-+-----------. .------+------+--.
/ / / \
/ Sign +<------+ To Be Signed Bytes |
/ / \ /
'-----+-------' '----------------'
v
.----+-------.
| COSE Sign 1 |
'------------'
Contributors
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Orie Steele
Transmute
United States
Email: orie@transmute.industries
Orie contributed to improving the generalization of COSE building
blocks and document consistency.
Authors' Addresses
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Germany
Email: henk.birkholz@sit.fraunhofer.de
Antoine Delignat-Lavaud
Microsoft Research
21 Station Road
Cambridge
CB1 2FB
United Kingdom
Email: antdl@microsoft.com
Cedric Fournet
Microsoft Research
21 Station Road
Cambridge
CB1 2FB
United Kingdom
Email: fournet@microsoft.com
Yogesh Deshpande
ARM
110 Fulbourn Road
Cambridge
CB1 9NJ
United Kingdom
Email: yogesh.deshpande@arm.com
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Steve Lasker
DataTrails
Seattle, WA 98199
United States
Email: steve.lasker@datatrails.ai
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