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An Architecture for Trustworthy and Transparent Digital Supply Chains
draft-ietf-scitt-architecture-01

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Document	Type	This is an older version of an Internet-Draft whose latest revision state is "Active".
	Authors	Henk Birkholz , Antoine Delignat-Lavaud , Cedric Fournet , Yogesh Deshpande
	Last updated	2023-03-13
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	Reviews	HTTPDIR Early review by Darrel Miller On the Right Track
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	Associated WG milestone	Dec 2023 Submit an Architecture and Terminology document to the IESG for publication
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draft-ietf-scitt-architecture-01

SCITT H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Standards Track A. Delignat-Lavaud
Expires: 15 September 2023 C. Fournet
Microsoft Research
Y. Deshpande
ARM
14 March 2023

An Architecture for Trustworthy and Transparent Digital Supply Chains
draft-ietf-scitt-architecture-01

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 memo defines a generic and scalable architecture to enable
transparency across any supply chain with minimum adoption barriers
for producers (who can register their Signed Statements on any
Transparency Service, with the guarantee that all consumers will be
able to verify them) and enough flexibility to allow different
implementations of Transparency Services with various auditing and
compliance requirements.

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

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
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This Internet-Draft will expire on 15 September 2023.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 6
2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Software Bill of Materials (SBOM) . . . . . . . . . . . . 6
2.2. Confidential Computing . . . . . . . . . . . . . . . . . 7
2.3. Cold Chains for Seafood . . . . . . . . . . . . . . . . . 8
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Definition of Transparency . . . . . . . . . . . . . . . . . 10
5. Architecture Overview . . . . . . . . . . . . . . . . . . . . 11
5.1. Signed Statement Issuance and Registration . . . . . . . 13
5.1.1. Issuer Identity . . . . . . . . . . . . . . . . . . . 13
5.1.2. Naming Artifacts . . . . . . . . . . . . . . . . . . 13
5.1.3. Signed Statement Metadata . . . . . . . . . . . . . . 14
5.2. Transparency Service . . . . . . . . . . . . . . . . . . 14
5.2.1. Service Identity, Remote Attestation, and Keying . . 15

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5.2.2. Registration Policies . . . . . . . . . . . . . . . . 16
5.2.3. Registry Security Requirements . . . . . . . . . . . 17
5.3. Verifying Transparent Statements . . . . . . . . . . . . 19
6. Signed Statement Issuance, Registration, and Verification . . 19
6.1. Envelope and Signed Statement Format . . . . . . . . . . 20
6.2. Signed Statement Issuance . . . . . . . . . . . . . . . . 21
6.3. Standard Registration Policies . . . . . . . . . . . . . 22
6.4. Registering Signed Statements . . . . . . . . . . . . . . 24
6.5. Validation of Transparent Statements . . . . . . . . . . 25
7. FederationThis section needs work. . . . . . . . . . . . . . 26
8. Transparency Service APIThis may be moved to appendix. . . . 26
8.1. Messages . . . . . . . . . . . . . . . . . . . . . . . . 26
8.1.1. Register Signed Statement . . . . . . . . . . . . . . 27
8.1.2. Retrieve Operation Status . . . . . . . . . . . . . . 28
8.1.3. Retrieve Signed Statement . . . . . . . . . . . . . . 29
8.1.4. Retrieve Registration Receipt . . . . . . . . . . . . 30
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 30
10. Security Considerations . . . . . . . . . . . . . . . . . . . 31
10.1. Threat Model . . . . . . . . . . . . . . . . . . . . . . 31
10.1.1. Signed Statement Authentication and Transparency. . 32
10.1.2. Confidentiality and privacy. . . . . . . . . . . . . 34
10.1.3. Cryptographic Assumptions . . . . . . . . . . . . . 34
10.1.4. Transparency Service Clients . . . . . . . . . . . . 35
10.1.5. Identity . . . . . . . . . . . . . . . . . . . . . . 35
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
11.1. URN Sub-namespace for SCITT (urn:ietf:params:scitt) . . 35
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
12.1. Normative References . . . . . . . . . . . . . . . . . . 36
12.2. Informative References . . . . . . . . . . . . . . . . . 37
Appendix A. Attic . . . . . . . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38

1. Introduction

This document describes a scalable and flexible decentralized
architecture to enhance auditability and accountability in various
existing and emerging supply chains. It achieves this goal by
enforcing the following complementary security guarantees:

1. statements made by issuers about supply chain artifacts must be
identifiable, authentic, and non-repudiable;

2. such statements must be registered on a secure append-only
Registry so that their provenance and history can be
independently and consistently audited;

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3. Issuers can efficiently prove to any other party the registration
of their Signed Statements; verifying this proof ensures that the
issuer is consistent and non-equivocal when producing Signed
Statements.

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 in an immutable, append-only, transparent Registry.
The last guarantee is achieved by implementing the Registry using a
verifiable data structure (such as a Merkle Tree [MERKLE]), and by
requiring a Transparency Service that operates the Registry to
endorse its state at the time of registration.

The guarantees and techniques used in this document generalize those
of Certificate Transparency [RFC9162], which can be re-interpreted as
an instance of this architecture for the supply chain of X.509
certificates. However, the range of use cases and applications in
this document is much broader, which requires much more flexibility
in how each Transparency Service implements and operates its
Registry. Each service may enforce its own policy for authorizing
entities to register their Signed Statements on the Transparency
Service. Some Transparency Services may also enforce access control
policies to limit who can audit the full Registry, or keep some
information on the Registry encrypted. Nevertheless, it is critical
to provide global interoperability for all Transparency Services
instances as the composition and configuration of involved supply
chain entities and their system components is ever-changing and
always in flux.

A Transparency Services provides visibility into Signed Statements
originally created as Statements and issued as Signed Statements by
supply chain entities and their sub-systems. These Signed Statements
(and corresponding Statement payload) are about the objects produced
by supply chain objects: Artifacts. A Transparency Service vouches
for specific and well-defined metadata about these Artifacts that is
captured in Statements. Some metadata is selected (and signed) by
the Issuer, indicating, e.g., "who issued the Statement" or "what
type of Artifact is described" or "what is the Artifact's version";
whereas additional metadata is selected (and countersigned) by the
Transparency Services, indicating, e.g., "when was the Signed
Statement about the Artifact registered in the Registry". A
Statements payload content typically is opaque to the Transparency
Services, if so desired: it is the metadata that must always be
transparent in order to warrant trust for later processing.

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Transparent Statements provide a common basis for holding Issuers
accountable for the Statement payload about Artifacts they release
and (more generally) principals accountable for auxiliary Signed
Statements from other Issuers about the original Signed Statement
about an Artifact. Hence, Issuers may register new Signed Statements
about their Artifacts, but they cannot delete or alter earlier Signed
Statements about certain Artifacts, or hide their Signed Statements
from third parties such as auditors.

Trust in the Transparency Service itself is supported both by
protecting their implementation (using, for instance, replication,
trusted hardware, and remote attestation of systems) and by enabling
independent audits of the correctness and consistency of its
Registry, thereby holding the organization accountable that operates
it. Unlike CT, where independent auditors are responsible for
enforcing the consistency of multiple independent instances of the
same global Registry, each Transparency Service is required to
guarantee the consistency of its own Registry (for instance, through
the use of a consensus algorithm between replicas of the Registry),
but assume no consistency between different Transparency Services.

The Transparency Services specified in this architecture caters to
two types of audiences:

1. Signed Statement Issuers: entities, stakeholders, and users
involved in supply chain interactions that need to release
authentic Statements to a definable set of peers; and

2. Transparent Statement Consumers: entities, stakeholders, and
users involved in supply chain interactions that need to access,
validate, and trust authentic Statements.

Signed Statement Issuers rely on being discoverable and represented
as the responsible parties for their registered Signed Statements via
Transparency Services in a believable manner. Analogously,
Transparent Statement Consumers rely on verifiable trustworthiness
assertions associated with Transparent Statements and their
processing provenance in a believable manner. If trust can be put
into the operations that record Signed Statements (i.e., a believable
notarization function) in a secure, append-only Registry via online
operations, the same trust can be put into a corresponding Receipt
that is the resulting documentation of these online operations issued
by the Transparency Services and that can be validated in offline
operations.

The Transparency Services specified in this architecture can be
implemented by various different types of services in various types
of languages provided via various variants of API layouts.

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The global interoperability enabled and guaranteed by the
Transparency Services is enabled via core components (architectural
constituents) that come with prescriptive requirements (that are
typically hidden away from the user audience via APIs). The core
components are based on the Concise Signing and Encryption standard
specified in [RFC9052], which is used to sign released Statements
about Artifacts and to build and maintain a Merkle tree that
functions as an append-only Registry for corresponding Signed
Statements. The format and verification process for Registry-based
transparency receipts are described in [I-D.birkholz-scitt-receipts].

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. Use Cases

This section presents representative and solution-agnostic use cases
to illustrate the scope of SCITT and the processing of Digital Supply
Chain Artifacts.

2.1. Software Bill of Materials (SBOM)

As the ever-increasing complexity of large software projects requires
more modularity and abstractions to manage them, keeping track of
their full Trusted Computing Base (TCB) is becoming increasingly
difficult. Each component may have its own set of dependencies and
libraries. Some of these dependencies are binaries, which means
their TCB depends not only on their source, but also on their build
environment (compilers and tool-chains). Besides, many source and
binary packages are distributed through various channels and
repositories that may not be trustworthy.

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Software Bills of Materials (SBOM) help the authors, packagers,
distributors, auditors and users of software understand its
provenance and who may have the ability to introduce a vulnerability
that can affect the supply chain downstream. However, the usefulness
of SBOM in protecting end users is limited if supply chain actors
cannot be held accountable for their contents. For instance,
consider a package repository for an open source operating system
distribution. The operator of this repository may decide to provide
a malicious version of a package only to users who live in a specific
country. They can write two equivocal SBOMs for the honest and
backdoored versions of the package, so that nobody outside the
affected country can discover the malicious version, but victims are
not aware they are being targeted.

2.2. Confidential Computing

Confidential Computing can leverage hardware-protected trusted
execution environments (TEEs) to operate cloud services that protect
the confidentiality of data that they process. It relies on remote
attestation, which allows the service to prove to remote users what
is the hash of its software, as measured and signed by the hardware.

For instance, consider a speech recognition service that implements
machine learning inference using a deep neural network model. The
operator of the service wants to prove to its users that the service
preserves the user's privacy, that is, the submitted recordings can
only be used to detect voice commands but no other purpose (such as
storing the recordings or detecting mentions of brand names for
advertisement purposes). When the user connects to the TEE
implementing the service, the TEE presents attestation evidence that
includes a hardware certificate and a software measurement for their
task; the user verifies this evidence before sending its recording.

But how can users verify the software measurement for their task?
And how can operators update their service, e.g., to mitigate
security vulnerabilities or improve accuracy, without first
convincing all users to update the measurements they trust?

A supply chain that maintains a transparent record of the successive
software releases for machine-learning models and runtimes, recording
both their software measurements and their provenance (source code,
build reports, audit reports, ...) can provide users with the
information they need to authorize these tasks, while holding the
service operator accountable for the software they release for them.

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2.3. Cold Chains for Seafood

Once seafood is caught, its quality is determined -- amongst other
criteria -- via the integrity of a cold chain that ensures a
regulatory perspective freshness mandating a continuous storing
temperature between 1 °C and 0 °C (or -18 °C and lower for frozen
seafood). The temperature is recorded by cooling units adhering to
certain compliance standards automatically. Batches of seafood can
be split or aggregated before arriving in a shelf so that each unit
can potentially have a potentially unique cold chain record whose
transparency impacts the accuracy of the shelf-life associated with
it. Especially in early links of the supply chain, Internet
connection or sophisticated IT equipment are typically not available;
sometimes temperature measurements are recorded manually; and digital
records are created in hindsight.

3. Terminology

The terms defined in this section have special meaning in the context
of Supply Chain Integrity, Transparency, and Trust 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.

Artifact: a physical or non-physical item that is moving along the
supply chain.

Auditor: an entity that checks the correctness and consistency of
all Transparent Statements registered by a Transparency Service (a
subset of potential Transparent Statement Consumers).

Consumer of Signed Statements:
// Define here.

Envelope: metadata and an Issuer's signature is added to a Statement
via a COSE envelope by the Issuer to produce a Signed Statement.
An Envelope contains the identity of the Issuer and other
information to help components responsible for validation that are
part of a Transparency Services to identify the software Artifact
referred to in a Signed Statement. In essence, a Signed Statement
is a COSE Envelope wrapped around a Statement binding the metadata
included in the Envelope to a 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).

Feed: an identifier chosen by the Issuer for the Artifact. For

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every Issuer and Feed, the Registry on a Transparency Service
contains a sequence of Signed Statements about the same Artifact.
In COSE, Feed is a dedicated header attribute in the protected
header of the Envelope.

Issuer: an entity that creates Signed Statements about software
Artifacts in the supply chain. An Issuer may be the owner or
author of software Artifacts, or an independent third party such
as a reviewer or an endorser.

Receipt: a Receipt is a special form of COSE countersignature for
Signed Statements that embeds cryptographic evidence that the
Signed Statement is recorded in the Registry. A Receipt consists
of a Registry-specific inclusion proof, a signature by the
Transparency Service of the state of the Registry, and additional
metadata (contained in the countersignature'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, storing it in the Registry, producing a
Receipt, and returning it to the submitting Issuer.

Registration Policy: the pre-condition enforced by the Transparency
Service before registering a Signed Statement, rendering it a
Signed Statement, based on metadata contained in its COSE Envelope
(notably the identity of its Issuer) and on prior Signed
Statements already added to a Registry.

Registry: the verifiable append-only data structure that stores
Signed Statements in a Transparency Service often referred to by
the synonym log or ledger. SCITT supports multiple Registry and
Receipt formats to accommodate different Transparency Service
implementations, such as historical Merkle Trees and sparse Merkle
Trees.

Signed Statement: an identifiable and non-repudiable Statement about
an Artifact made 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]). For example, a statement may
represent a Software Bill Of Materials (SBOM) that lists the
ingredients of a software Artifact, or some endorsement or
attestation about an Artifact.

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Transparency Service: an entity that maintains and extends the
Registry, and endorses its state. A Transparency Service is often
referred to by its synonym Notary. A Transparency Service can be
a complex distributed system, and SCITT requires the Transparency
Service to provide many security guarantees about its Registry.
The identity of a Transparency Service is captured by a public key
that must be known by Verifiers in order to validate Receipts.

Transparent Statement: a Signed Statement that is augmented with a
Receipt created via registration at a Transparency Services
(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.

Verifier: an entity that consumes Transparent Statements (a
specialization of Signed Statement Consumer), verifying their
proofs and inspecting their Statement payload, either before using
corresponding Artifacts, or later to audit an Artifact's
provenance on the supply chain.

4. Definition of Transparency

In this document, the definition of transparency is indented to build
over abstract notions of Registry 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 Registry of a Transparency Service. By extension,
the document may say an Artifact (e.g., a firmware binary) is
transparent if it comes with one or more Transparent Signed
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 used to target a
particular user that checks for Receipts must have been recorded in
the tamper-proof Registry, and will be subject to scrutiny and
auditing by other parties.

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Transparency is implemented by a Registry that provides a consistent,
append-only, cryptographically verifiable, publicly available record
of entries. Implementations of Transparency Service may protect
their Registry 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 Registry. 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 Registry can independently verify its
consistency and review the complete list of Transparent Statements
registered by each Issuer. However, the Registries of separate
Transparency Services are generally disjoint, though it is possible
to take a Transparent Statement from one Registry and register it
again on another (if its policy allows it), so the authorization of
the Issuer and of the Registry by the Verifier of the Receipt are
generally independent.

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 Registry, as any inconsistency can easily be pinpointed by any
auditor with read access to the Registry. Some Registry formats may
also support consistency auditing (Section 5.2.3.2) through Receipts,
that is, given two valid Receipts the Transparency Service may be
asked to produce a cryptographic proof that they are consistent.
Failure to produce this proof can indicate that the Transparency
Services operator misbehaved.

5. Architecture Overview

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.----------.
| Artifact |
'----+-----'
v
.----+----. .----------. Decentralized Identifier
Issuer --> | Statement || Envelope +<------------------.
'----+----' '-----+----' |
| | +--------------+---+
'----. .----' | DID Key Manifest |
| | |
v +-------+------+---+
.----+----. | |
| Signed | COSE Signing | |
| Statement +<-------------------' |
'----+----' |
| +--------------+ |
.-' '------------->+ Transparency | |
| .-------. | | |
Transparency --> | | Receipt +<-----+ Service | |
Service | '---+---' +------------+-+ |
'-. .-' | |
| | |
v | |
.-----+-----. | |
| Transparent | | |
| Statement | | |
'-----+-----' | |
| | |
|'-------. .-------------)---'
| | | |
| v v |
| .----+---+-----------. |
Verifier --> | / Verify Transparent / |
| / Statement / |
| '--------------------' |
v v
.--------+---------. .-----------+-----.
Auditor --> / Collect Receipts / / Replay Log /
'------------------' '-----------------'

The SCITT architecture consists of a very loose federation of
Transparency Services, and a set of common formats and protocols for
issuing, registering 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 Registry algorithm. Most of the details of the

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Receipt's contents are specific to the Registry algorithm. The
[I-D.birkholz-scitt-receipts] document defines two initial Registry
algorithms (for historical and sparse Merkle Trees), but other
Registry formats (such as blockchains, or hybrid historical and
indexed Merkle Trees) may be proposed later.

In this section, a high level the three main roles and associated
processes in SCITT: Issuers and the Signed Statement issuance
process, transparency Registry and the Transparent Statement
Registration process, as well as Verifiers and the Receipt validation
process.

5.1. Signed Statement Issuance and Registration

5.1.1. Issuer Identity

Before an Issuer is able to produce Signed Statements, it must first
create its decentralized identifier [DID-CORE] (also known as a DID).
A DID can be _resolved_ into a _key manifest_ (a list of public keys
indexed by a _key identifier_) using many different DID methods.

Issuers MAY choose the DID method they prefer, but with no guarantee
that all Transparency Services will be able to register their Signed
Statements. To facilitate interoperability, all Transparency Service
implementations SHOULD support the did:web method [DID-WEB]. For
instance, if the Issuer publishes its manifest at
https://sample.issuer/user/alice/did.json, the DID of the Issuer is
did:web:sample.issuer:user:alice.

Issuers SHOULD use consistent decentralized identifiers for all their
Statements about Artifacts, to simplify authorization by Verifiers
and auditing. They MAY update their DID manifest, for instance to
refresh their signing keys or algorithms, but they SHOULD NOT remove
or change any prior keys unless they intend to revoke all Signed
Statements that are registered as Transparent Statements issued with
those keys. This DID appears in the Issuer protected header of
Signed Statements' Envelopes, while the version of the key from the
manifest used to sign the Signed Statement is written in the kid
header.

5.1.2. Naming Artifacts

Many Issuers issue Signed Statements about different Artifacts under
the same DID, so it is important for everyone to be able to
immediately recognize by looking at the Envelope of a Signed
Statements what Artifact it is referring to. This information is
stored in the Feed header of the Envelope. Issuers MAY use different
signing keys (identified by kid in the resolved key manifest) for

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different Artifacts, or sign all Signed Statements under the same
key.

5.1.3. Signed Statement Metadata

Besides Issuer, Feed and kid, the only other mandatory metadata in a
Signed Statement is the type of the Payload, indicated in the cty
(content type) Envelope header. However, this set of mandatory
metadata is not sufficient to express many important Registration
policies. For example, a Registry may only allow a Signed Statement
to be registered, if it was signed recently. While the Issuer is
free to add any information in the payload of the Signed Statements,
the Transparency Services (and most of its auditors) can only be
expected to interpret information in the Envelope.

Such metadata, meant to be interpreted by the Transparency Services
during Registration policy evaluation, should be added to the
reg_info header. While the header MUST be present in all Signed
Statements, its contents consist of a map of named attributes. Some
attributes (such as the Issuer's timestamp) are standardized with a
defined type, to help uniformize their semantics across Transparency
Services. Others are completely customizable and may have arbitrary
types. In any case, all attributes are optional; so the map MAY be
empty.

5.2. Transparency Service

The role of Transparency Service can be decomposed into several major
functions. The most important is maintaining a Registry, the
verifiable data structure that records Signed Statements, and
enforcing a Registration policy. It also maintains a service key,
which is used to endorse the state of the Registry in Receipts. All
Transparency Services MUST expose standard endpoints for Registration
of Signed Statements and Receipt issuance, which is described in
Section 8.1. Each Transparency Services also defines its
Registration policy, which MUST apply to all entries in the Registry.

The combination of Registry, identity, Registration policy
evaluation, and Registration endpoint constitute the trusted part of
the Transparency Service. Each of these components SHOULD 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 policy evaluation, Registry
extension and endorsement may be protected by running in a TEE; the
Registry may be replicated and a consensus algorithm such as
Practical Byzantine Fault Tolerance (pBFT [PBFT]) may be used to
protect against malicious or vulnerable replicas; threshold
signatures may be use to protect the service key, etc.

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Beyond the trusted components, Transparency Services may operate
additional endpoints for auditing, for instance to query for the
history of Transparent Statements registered by a given Issuer via a
certain Feed. Implementations of Transparency Services SHOULD avoid
using the service identity and extending the Registry in auditing
endpoints; as much as practical, the Registry SHOULD contain enough
evidence to re-construct verifiable proofs that the results returned
by the auditing endpoint are consistent with a given state of the
Registry.

5.2.1. Service Identity, Remote Attestation, and Keying

Every Transparency Services MUST have a public service identity,
associated with public/private key pairs for signing on behalf of the
service. In particular, this identity must be known by Verifiers
when validating a Receipt.

This identity should be stable for the lifetime of the service, so
that all Receipts remain valid and consistent. The Transparency
Service operator MAY use a distributed identifier as their public
service identity if they wish to rotate their keys, if the Registry
algorithm they use for their Receipt supports it. Other types of
cryptographic identities, such as parameters for non-interactive
zero-knowledge proof systems, may also be used in the future.

A Transparency Services SHOULD provide evidence that it is securely
implemented and operated, enabling remote authentication of the
hardware platforms and/or software TCB that run the Transparency
Service. This additional evidence SHOULD be recorded in the Registry
and presented on demand to Verifiers and auditors. Examples for
Statements that can improve trustworthy assessments of Transparency
Services are RATS Conceptual Messages, such as Evidence,
Endorsements, or corresponding Attestation Results (see [RFC9334].

For example, consider a Transparency Services implemented using a set
of replicas, each running within its own hardware-protected trusted
execution environments (TEEs). Each replica SHOULD provide a recent
attestation report for its TEE, binding their hardware platform to
the software that runs the Transparency Service, the long-term public
key of the service, and the key used by the replica for signing
Receipts. This attestation evidence SHOULD be supplemented with
transparency Receipts for the software and configuration of the
service, as measured in its attestation report.

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5.2.2. Registration Policies

A Transparency Services that accepts to register any valid Signed
Statement offered by an Issuer would end up providing only limited
value to verifiers. In consequence, a baseline transparency
guarantee policing the registration of Signed Statements is required
to ensure completeness of audit, which can help detect equivocation.
Most advanced SCITT scenarios rely on the Transparency Service
performing additional domain-specific checks before a Signed
Statement is accepted: Transparency Services may only allow trusted
authenticated users to register Signed Statements, Transparency
Services may try to check that a new Signed Statement is consistent
with previous Signed Statements from the same Issuers or that Signed
Statements are registered in the correct order and cannot be re-
played; some Transparency Services may even interpret and validate
the payload of Signed Statements.

In general, registration policies are applied at the discretion of
the Transparency Services, and verifiers use Receipts as witnesses
that confirm that the registration policy of the Transparency
Services was satisfied at the time of creating a Transparent
Statement via Signed Statement registration. Transparency Service
implementations SHOULD make their full registration policy public and
auditable, e.g. by recording stateful policy inputs at evaluation
time in the registry to ensure that policy can be independently
validated later. From an interoperability point of view, the policy
that was applied by the Transparency Services is opaque to the
verifier, which is forced to trust the associated registration
policy. If the policy of the Transparency Services evolves over
time, or is different across Issuers, the assurances derived from
Receipt validation may not be uniform across all Signed Statements
over time.

To help verifiers interpret the semantics of Signed Statement
registration, the SCITT Architecture defines a standard mechanism to
include signals the Signed Statement itself which policies have been
applied by the Transparency Service from a defined set of
registration policies with standardized semantics. Each policy that
is expected to be enforced by the Transparency Service is represented
by an entry in the registration policy info map (reg_info) in the
COSE Envelope of the Signed Statement. The key of the map entry
corresponds to the name of the policy, while its value (including its
type) is policy-specific. For instance, the register_by policy
defines the maximum timestamp by which a Signed Statement can be
registered, hence the associated value contains an unsigned integer.

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While this design ensures that all verifiers get the same guarantee
regardless of where a Transparent Statement is registered, its main
downside is that it requires the Issuer to include the necessary
policies in the Envelope when the Signed Statement is produced.
Furthermore, it makes it impossible to register the same Signed
Statement on two different Transparency Services, if their required
registration policies are incompatible.

| *Editor's note*
|
| The technical design for signalling and verifying registration
| policies is a work in progress. An alternative design would be
| to include the registration policies in the receipt/
| countersignature rather than in the envelope. This improves
| the portability of Signed Statements but requires the verifier
| to be more aware of the particular policies at the Transparency
| Service where the Signed Statement is registered.

5.2.3. Registry Security Requirements

There are many different candidate verifiable data structures that
may be used to implement the Registry, such as chronological Merkle
Trees, sparse/indexed Merkle Trees, full blockchains, and many other
variants. The Registry is only required to support concise Receipts
(i.e., whose size grows at most logarithmically in the number of
entries in the Registry). This does not necessarily rule out
blockchains as a Registry, but may necessitate advanced Receipt
schemes that use arguments of knowledge and other verifiable
computing techniques.

Since the details of how to verify a Receipt are specific to the data
structure, no particular Registry format is specified in this
document. Instead, two initial formats for Registry in
[I-D.birkholz-scitt-receipts] using historical and sparse Merkle
Trees are proposed. Beyond the format of Receipts, generic
properties that should be satisfied by the components in the
Transparency Services that have the ability to write to the Registry
are required.

5.2.3.1. Finality

A Registry is append-only: once a Signed Statement is registered and
becomes a Transparent Statement, it cannot be modified, deleted, or
moved. In particular, once a Receipt is returned for a given Signed
Statement, the registered Signed Statement and any preceding entry in
the Registry become immutable, and the Receipt provides universally-
verifiable evidence of this property.

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5.2.3.2. Consistency

There is no fork in the Registry: everyone with access to its
contents sees the same sequence of entries, and can check its
consistency with any Receipts they have collected. Transparency
Service implementations SHOULD provide a mechanism to verify that the
state of the Registry encoded in an old Receipt is consistent with
the current Registry state.

5.2.3.3. Replayability and Auditing

Everyone with access to the Registry can check the correctness of its
contents. In particular,

* the Transparency Service defines and enforces deterministic
Registration policies that can be re-evaluated based solely on the
contents of the Registry at the time of registration, and must
then yield the same result.

* the ordering of entries, their cryptographic contents, and the
Registry governance may be non-deterministic, but they must be
verifiable.

* a Transparency Services SHOULD store evidence about the resolution
of distributed identifiers into manifests.

* a Transparency Service MAY additionally support verifiability of
client authentication and access control.

5.2.3.4. Governance and Bootstrapping

The Transparency Service needs to support governance, with well-
defined procedures for allocating resources to operate the Registry
(e.g., for provisioning trusted hardware and registering their
attestation materials in the Registry) and for updating its code
(e.g., relying on Transparent Statement about code updates, secured
on the Registry itself, or on some auxiliary Transparency Service).

Governance procedures, their auditing, and their transparency are
implementation specific. A Transparency Service SHOULD document
them.

* Governance may be based on a consortium of members that are
jointly responsible for the Transparency Services, or automated
based on the contents of an auxiliary governance Transparency
Service.

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* Governance typically involves additional records in the Registry
to enable its auditing. Hence, the Registry may contain both
Transparent Statements and governance entries.

* Issuers, Verifiers, and third-party auditors may review the
Transparency Service governance before trusting the service, or on
a regular basis.

5.3. Verifying Transparent Statements

For a given Artifact, Verifiers take as trusted inputs:

1. the distributed identifier of the Issuer (or its resolved key
manifest),

2. the expected name of the Artifact (i.e., the Feed),

3. the list of service identities of trusted Transparency Services.

When presented with a Transparent Statement for an Artifact,
consumers verify its Issuer identity, signature, and Receipt. They
may additionally apply a validation policy based on the protected
headers present both in the Envelope, the Receipt, or the Statement
itself, which may include security-critical or Artifact-specific
details.

Some Verifiers may systematically resolve Issuer DIDs to fetch the
latest corresponding DID documents. This behavior strictly enforces
the revocation of compromised keys: once the Issuer has updated its
Statement to remove a key identifier, all Signed Statements include
the corresponding kid will be rejected. However, others may delegate
DID resolution to a trusted third party and/or cache its results.

Some Verifiers may decide to skip the DID-based signature
verification, relying on the Transparency Service's Registration
policy and the scrutiny of other Verifiers. Although this weakens
their guarantees against key revocation, or against a corrupt
Transparency Services, they can still keep the Receipt and blame the
Issuer or the Transparency Services at a later point.

6. Signed Statement Issuance, Registration, and Verification

This section details the interoperability requirements for
implementers of Signed Statements issuance and validation libraries,
and of Transparency Services.

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6.1. Envelope and Signed Statement Format

The formats of Signed Statements and Receipts are based on CBOR
Object Signing and Encryption (COSE [RFC9052]). The choice of CBOR
[RFC8949] is a trade-off between safety (in particular, non-
malleability: each Signed Statement has a unique serialization), ease
of processing and availability of implementations.

At a high-level that is the context of this architecture, a Signed
Statement is a COSE single-signed object (i.e., a COSE_Sign1) that
contains the correct set of protected headers. Although Issuers and
relaying parties may attach unprotected headers to Signed Statements,
Transparency Services and Verifiers MUST NOT rely on the presence or
value of additional unprotected headers in Signed Statements during
Registration and validation.

All Signed Statements MUST include the following protected headers:

* algorithm (label: 1): Asymmetric signature algorithm used by the
Issuer of a Signed Statement, as an integer, for example -35 for
ECDSA with SHA-384, see COSE Algorithms registry [IANA.cose];

* Issuer (label: TBD, temporary: 391): DID (Decentralized Identifier
[DID-CORE]) of the signer, as a string, for example
did:web:example.com;

* Feed (label: TBD, temporary: 392): the Issuer's name for the
Artifact, as a string;

* payload type (label: 3): media-type of Statement payload as a
string, for example application/spdx+json

* Registration policy info (label: TBD, temporary: 393): a map of
additional attributes to help enforce Registration policies;

* Key ID (label: 4): Key ID, as a bytestring.

Additionally, Signed Statements MAY carry the following unprotected
headers:

* Receipts (label: TBD, temporary: 394): Array of Receipts, defined
in [I-D.birkholz-scitt-receipts]

In CDDL [RFC8610] notation, the Envelope is defined as follows:

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SCITT_Envelope = COSE_Sign1_Tagged

COSE_Sign1_Tagged = #6.18(COSE_Sign1)

COSE_Sign1 = [
protected : bstr .cbor Protected_Header,
unprotected : Unprotected_Header,
payload : bstr,
signature : bstr
]

Reg_Info = {
? "register_by": uint .within (~time),
? "sequence_no": uint,
? "issuance_ts": uint .within (~time),
? "no_replay": null,
* tstr => any
}

; All protected headers are mandatory, to protect against faulty implementations of COSE
; that may accidentally read a missing protected header from the unprotected headers.
Protected_Header = {
1 => int ; algorithm identifier
3 => tstr ; payload type
4 => bstr ; Key ID
; TBD, Labels are temporary
391 => tstr ; DID of Issuer
392 => tstr ; Feed
393 => Reg_Info ; Registration policy info
}

Unprotected_Header = {
; TBD, Labels are temporary
? 394 => [+ SCITT_Receipt]
}

6.2. Signed Statement Issuance

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
to serialize the Statement payload, such as:

* JSON-SPDX

* CBOR-SPDX

* SWID

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* CoSWID

* CycloneDX

* in-toto

* SLSA

Once the Statement is serialized with the correct media-type/content-
format, an Issuer should fill in the attributes for the Registration
policy information header. From the Issuer's perspective, using
attributes from named policies ensures that the Signed Statement may
only be registered on Transparency Services that implement the
associated policy. For instance, if a Signed Statement is frequently
updated, and it is important for Verifiers to always consider the
latest version, Issuers SHOULD use the sequence_no or issuer_ts
attributes.

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).

6.3. Standard Registration Policies

| *Editor's note*
|
| The technical design for signaling and verifying registration
| policies is a work in progress. We expect that once the
| formats and semantics of the registration policy headers are
| finalized, standardized policies may be moved to a separate
| draft. For now, we inline some significant policies to
| illustrate the most common use cases.

Transparency Service implementations MUST indicate their support for
registration policies and MUST check that all the policies indicated
as defined in the reg_info map are supported and are satisfied before
a Signed Statement can be registered. Any unsupported types of
Signed Statements MUST be indicated separately and corresponding
unknown policy entries in the map of a Signed Statement MUST be
rejected. This is to ensure that all verifiers get the same
guarantee out of the registration policies regardless of where it is
registered.

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Table 1: An Initial Set of Named Policies

6.4. Registering Signed Statements

The same Signed Statement may be independently registered in multiple
Transparency Services. To register a Signed Statement, the service
performs the following steps:

1. Client authentication. This is implementation-specific and MAY
be unrelated to the Issuer identity. Signed Statements may be
registered by a different party than their Issuer.

2. Issuer identification. The Transparency Service MUST store
evidence of the DID resolution for the Issuer protected header of
the Envelope and the resolved key manifest at the time of
Registration for auditing. This MAY require that the service
resolves the Issuer DID and record the resulting document, or
rely on a cache of recent resolutions.

3. Envelope signature verification, as described in COSE signature,
using the signature algorithm and verification key of the Issuer
DID document.

4. Envelope validation. The service MUST check that the Envelope
includes a Statement payload and the protected headers listed
above. The service MAY additionally verify the Statement payload
format and content.

5. Apply Registration policy: for named policies, the Transparency
Service should check that the required Registration info
attributes are present in the Envelope and apply the check
described in Table 1. A Transparency Service MUST reject Signed
Statements that contain an attribute used for a named policy that
is not enforced by the service. Custom Signed Statements are
evaluated given the current Registry state and the entire
Envelope, and MAY use information contained in the attributes of
named policies.

6. Commit (register) the new Signed Statement to the Registry

7. Sign and return the Receipt.

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The last two steps MAY be shared between a batch of Signed Statements
recorded in the Registry.

A Transparency Service MUST ensure that a Signed Statement is
registered before releasing its Receipt, so that it can always back
up the Receipt by releasing the corresponding entry (the now
Transparent Statement) in the Registry. Conversely, the service MAY
re-issue Receipts for the Registry content, for instance after a
transient fault during Signed Statement Registration.

6.5. Validation of Transparent Statements

This section provides additional implementation considerations. The
high-level validation algorithm is described in Section 5.3; the
Registry-specific details of checking Receipts are covered in
[I-D.birkholz-scitt-receipts].

Before checking a Transparent Statement, the Verifier must be
configured with one or more identities of trusted Transparency
Services. If more than one service is configured, the Verifier MUST
return which service the Transparent Statement is registered on.

In some scenarios, the Verifier already expects a specific Issuer and
Feed for the Transparent Statement, while in other cases they are not
known in advance and can be an output of validation. Verifiers
SHOULD offer a configuration to decide if the Issuer's signature
should be locally verified (which may require a DID resolution, and
may fail if the manifest is not available or if the key is revoked),
or if it should trust the validation done by the Transparency Service
during Registration.

Some Verifiers MAY decide to locally re-apply some or all of the
Registration policies, if they have limited trust in the Transparency
Services. In addition, Verifiers MAY apply arbitrary validation
policies after the signature and Receipt have been checked. Such
policies may use as input all information in the Envelope, the
Receipt, and the Statement payload, as well as any local state.

Verifiers SHOULD offer options to store or share Receipts in case
they are needed to audit the Transparency Services in case of a
dispute.

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7. Federation
// This section needs work.
//
// -- Henk

Editor's note: This section needs work.

Multiple, independently-operated Transparency Services can help
secure distributed supply chains, without the need for a single,
centralized service trusted by all parties. For example, multiple
Transparency Service instances may be governed and operated by
different organizations that do not trust one another.

This may involve registering the same Signed Statements at different
Transparency Services, each with their own purpose and registration
policy. This may also involve attaching multiple Receipts to the
same Signed Statements, each Receipt endorsing the Issuer signature
and a subset of prior Receipts, and each Transparency Service
verifying prior Receipts as part of their registration policy.

For example, a supplier's Transparency Service may provide a
complete, authoritative Registry for some kind of Signed Statements,
whereas a consumer's Transparency Service may collect different kinds
of Signed Statements to ensure complete auditing for a specific use
case, and possibly require additional reviews before registering some
of these Signed Statements.

8. Transparency Service API
// This may be moved to appendix.
//
// -- Henk

Editor's Note: This may be moved to appendix.

8.1. Messages

All messages are sent as HTTP GET or POST requests.

If the transparency service cannot process a client's request, it
MUST return an HTTP 4xx or 5xx status code, and the body SHOULD be a
JSON problem details object ([RFC7807]) containing:

* type: A URI reference identifying the problem. To facilitate
automated response to errors, this document defines a set of
standard tokens for use in the type field within the URN namespace
of: "urn:ietf:params:scitt:error:".

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* detail: A human-readable string describing the error that
prevented the transparency service from processing the request,
ideally with sufficient detail to enable the error to be
rectified.

Error responses SHOULD be sent with the Content-Type: application/
problem+json HTTP header.

As an example, submitting a signed statement with an unsupported
signature algorithm would return a 400 Bad Request status code and
the following body:

{
"type": "urn:ietf:params:scitt:error:badSignatureAlgorithm",
"detail": "The statement was signed with an algorithm the server does not support"
}

Most error types are specific to the type of request and are defined
in the respective subsections below. The one exception is the
"malformed" error type, which indicates that the transparency service
could not parse the client's request because it did not comply with
this document:

* Error code: malformed (The request could not be parsed).

Clients SHOULD treat 500 and 503 HTTP status code responses as
transient failures and MAY retry the same request without
modification at a later date. Note that in the case of a 503
response, the transparency service MAY include a Retry-After header
field per [RFC7231] in order to request a minimum time for the client
to wait before retrying the request. In the absence of this header
field, this document does not specify a minimum.

8.1.1. Register Signed Statement

8.1.1.1. Request

POST <Base URL>/entries

Headers:

* Content-Type: application/cose

Body: SCITT COSE_Sign1 message

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8.1.1.2. Response

One of the following:

* Status 201 - Registration is successful.

- Header Location: <Base URL>/entries/<Entry ID>

- Header Content-Type: application/json

- Body { "entryId": "<Entry ID"> }

* Status 202 - Registration is running.

- Header Location: <Base URL>/operations/<Operation ID>

- Header Content-Type: application/json

- (Optional) Header: Retry-After: <seconds>

- Body { "operationId": "<Operation ID>", "status": "running" }

* Status 400 - Registration was unsuccessful due to invalid input.

- Error code badSignatureAlgorithm

If 202 is returned, then clients should wait until registration
succeeded or failed by polling the registration status using the
Operation ID returned in the response. Clients should always obtain
a receipt as a proof that registration has succeeded.

8.1.2. Retrieve Operation Status

8.1.2.1. Request

GET <Base URL>/operations/<Operation ID>

8.1.2.2. Response

One of the following:

* Status 200 - Registration is running

- Header: Content-Type: application/json

- (Optional) Header: Retry-After: <seconds>

- Body: { "operationId": "<Operation ID>", "status": "running" }

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* Status 200 - Registration was successful

- Header: Location: <Base URL>/entries/<Entry ID>

- Header: Content-Type: application/json

- Body: { "operationId": "<Operation ID>", "status": "succeeded",
"entryId": "<Entry ID>" }

* Status 200 - Registration failed

- Header Content-Type: application/json

- Body: { "operationId": "<Operation ID>", "status": "failed",
"error": { "type": "<type>", "detail": "<detail>" } }

- Error code: badSignatureAlgorithm

* Status 404 - Unknown Operation ID

- Error code: operationNotFound

- This can happen if the operation ID has expired and been
deleted.

If an operation failed, then error details SHOULD be embedded as a
JSON problem details object in the "error" field.

If an operation ID is invalid (i.e., it does not correspond to any
submit operation), a service may return either a 404 or a running
status. This is because differentiating between the two may not be
possible in an eventually consistent system.

8.1.3. Retrieve Signed Statement

8.1.3.1. Request

GET <Base URL>/entries/<Entry ID>

Query parameters:

* (Optional) embedReceipt=true

If the query parameter embedReceipt=true is provided, then the signed
statement is returned with the corresponding registration receipt
embedded in the COSE unprotected header.

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8.1.3.2. Response

One of the following:

* Status 200.

- Header: Content-Type: application/cose

- Body: COSE_Sign1

* Status 404 - Entry not found.

- Error code: entryNotFound

8.1.4. Retrieve Registration Receipt

8.1.4.1. Request

GET <Base URL>/entries/<Entry ID>/receipt

8.1.4.2. Response

One of the following:

* Status 200.

- Header: Content-Type: application/cbor

- Body: SCITT_Receipt

* Status 404 - Entry not found.

- Error code: entryNotFound

The retrieved Receipt may be embedded in the corresponding COSE_Sign1
document in the unprotected header, see draft-birkholz-scitt-receipts
(TODO (more error codes to be defined, see [#17](https://github.com/
ietf-wg-scitt/draft-ietf-scitt-architecture/issues/17)): replace with
final reference).

9. Privacy Considerations

Unless advertised by a Transparency Service, every Issuer should
treat Signed Statements it registered (rendering them Transparent
Statements) as public. In particular, Signed Statement's Envelopes
and Statement payload should not carry any private information in
plaintext.

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10. 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 Registry. Unless advertised in the Transparency
Service Registration policy, the Verifier should not assume that the
ordering of Transparent Statements in the Registry 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.

Issuers SHOULD 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 Verifiers to interpret
the Signed Statement as valid for some other purpose.

Issuers and Transparency Services SHOULD carefully protect their
private signing keys and avoid these keys 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.

10.1. 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.

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10.1.1. Signed Statement Authentication and Transparency.

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 the
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. Hence, a Verifier would usually 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.

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 Registry is not possible. In
contrast Transparency Services can be hold accountable and they can
be called out by any Auditor that replays their Registry 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 the one hand, this enables valid actors to
detect and disambiguate malicious actors who issue contradictory
Signed Statements to different entities (Verifiers, Auditors,
Issuers). On the other hand, their liability and the resulting
damage to their reputation are application specific, and out of scope
of the SCITT Architecture.

Verifiers and Auditors need not be trusted by other actors. In
particular, they cannot "frame" an Issuer or a Transparency Service
for Signed Statements they did not issue or register.

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10.1.1.1. Append-only Log

If a Transparency Service is honest, then a Transparent Statement
including a correct Receipt ensures that the Transparent 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), 3. issue
verifiable Receipts for Signed Statements that do not match its
Registry, or 4. refuse access to its Registry (e.g., to Auditors,
possibly after storage loss).

An Auditor granted (partial) access to a Registry 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 Registry, some Transparency Services may provide limited
support for historical queries on the Transparent Statements they
have registered, and accept the risk of being blamed for inconsistent
Registration or Issuer equivocation.

Verifier and Auditors may also witness (1,4) but may not be able to
collect verifiable evidence for it.

10.1.1.2. Availability of Transparent Signed Statement

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 should independently keep their own record of the Signed
Statements they issue, endorse, verify, or audit.

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10.1.2. 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, as usual, this may not
exclude network traffic analysis.

10.1.2.1. Signed Statements and Their Registration

The Transparency Service is trusted with the confidentiality of the
Signed Statements presented for registration. Some Transparency
Services may publish every Transparent Statement in their logs, to
facilitate their dissemination and auditing. Others may just return
Receipts to clients that present Singed Statements for registration,
and disclose the ledger only to auditors trusted with the
confidentiality of its contents.

A collection of Transparent Statements leaks no information about the
contents of other Transparent Statements registered at the
Transparency Service.

Nonetheless, Issuers should carefully review the inclusion of
private/confidential materials in their issued Signed Statements;
they may for instance remove any PII, or include instead opaque
cryptographic commitments, such as hashes.

10.1.2.2. Queries to the Registry

The confidentiality of queries is implementation-specific, and
generally not guaranteed. For example, while offline Envelope
validation of Signed Statements is private, a Transparency Services
may monitor which of its Transparent Statements are being verified
from lookups to ensure their freshness.

10.1.3. Cryptographic Assumptions

SCITT relies on standard cryptographic security for signing schemes
(EUF-CMA: for a given key, given the public key and any number of
signed messages, an attacker cannot forge a valid signature for any
other message) and for Receipts schemes (log collision-resistance:
for a given commitment such as a Merkle-tree root, there is a unique
log such that any valid path authenticates a Signed Statement in this
log.)

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.

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10.1.4. Transparency Service Clients

Trust in clients that submit Signed Statements for registration is
implementation-specific. Hence, an attacker may attempt to register
any Signed Statement it has obtained, at any Transparency Service
that accepts them, possibly multiple times and out of order. This
may be mitigated by a Transparency Services that enforces restrictive
access control and Registration policies.

10.1.5. Identity

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.)

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
Verifiers from accepting Signed Statements signed with compromised
credentials.

The confidentiality of any identity lookup during Signed Statement
Registration or Transparent Statement Verification is out of scope.

11. IANA Considerations

TBD; Section 4.

11.1. URN Sub-namespace for SCITT (urn:ietf:params:scitt)

IANA is requested to register the URN sub-namespace
urn:ietf:params:scitt in the "IETF URN Sub-namespace for Registered
Protocol Parameter Identifiers" registry [IANA.params], following the
template in [RFC3553]:

Registry name: scitt

Specification: [RFCthis]

Repository: http://www.iana.org/assignments/scitt

Index value: No transformation needed.

12. References

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12.1. Normative References

[DID-CORE] W3C, "Decentralized Identifiers (DIDs) v1.0", 22 July
2022, <https://www.w3.org/TR/did-core/>.

[DID-WEB] "did:web Decentralized Identifiers Method Spec", n.d.,
<https://w3c-ccg.github.io/did-method-web/>.

[IANA.cose]
IANA, "CBOR Object Signing and Encryption (COSE)",
<https://www.iana.org/assignments/cose>.

[IANA.params]
IANA, "Uniform Resource Name (URN) Namespace for IETF
Use", <https://www.iana.org/assignments/params>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://doi.org/10.17487/RFC2119>.

[RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
IETF URN Sub-namespace for Registered Protocol
Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
2003, <https://doi.org/10.17487/RFC3553>.

[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://doi.org/10.17487/RFC6838>.

[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://doi.org/10.17487/RFC7231>.

[RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP
APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
<https://doi.org/10.17487/RFC7807>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://doi.org/10.17487/RFC8174>.

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[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://doi.org/10.17487/RFC8610>.

[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://doi.org/10.17487/RFC8949>.

[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://doi.org/10.17487/RFC9052>.

[RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate
Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
December 2021, <https://doi.org/10.17487/RFC9162>.

12.2. Informative References

[I-D.birkholz-scitt-receipts]
Birkholz, H., Riechert, M., Delignat-Lavaud, A., and C.
Fournet, "Countersigning COSE Envelopes in Transparency
Services", Work in Progress, Internet-Draft, draft-
birkholz-scitt-receipts-02, 24 October 2022,
<https://datatracker.ietf.org/doc/html/draft-birkholz-
scitt-receipts-02>.

[MERKLE] Merkle, R., "A Digital Signature Based on a Conventional
Encryption Function", DOI 10.1007/3-540-48184-2_32,
Advances in Cryptology - CRYPTO '87 pp. 369-378, 1988,
<https://doi.org/10.1007/3-540-48184-2_32>.

[PBFT] Castro, M. and B. Liskov, "Practical byzantine fault
tolerance and proactive recovery",
DOI 10.1145/571637.571640, ACM Transactions on Computer
Systems vol. 20, no. 4, pp. 398-461, November 2002,
<https://doi.org/10.1145/571637.571640>.

[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://doi.org/10.17487/RFC9334>.

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Appendix A. Attic

Not ready to throw these texts into the trash bin yet.

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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An Architecture for Trustworthy and Transparent Digital Supply Chains draft-ietf-scitt-architecture-01

An Architecture for Trustworthy and Transparent Digital Supply Chains
draft-ietf-scitt-architecture-01