An Architecture for Trustworthy and Transparent Digital Supply Chains
draft-ietf-scitt-architecture-02
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| Authors | Henk Birkholz , Antoine Delignat-Lavaud , Cedric Fournet , Yogesh Deshpande , Steve Lasker | ||
| Last updated | 2023-07-10 (Latest revision 2023-03-13) | ||
| Replaces | draft-birkholz-scitt-architecture | ||
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draft-ietf-scitt-architecture-02
SCITT H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Standards Track A. Delignat-Lavaud
Expires: 11 January 2024 C. Fournet
Microsoft Research
Y. Deshpande
ARM
S. Lasker
Lasker Consulting
10 July 2023
An Architecture for Trustworthy and Transparent Digital Supply Chains
draft-ietf-scitt-architecture-02
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.
Producers can register their Signed Statements on any Transparency
Service, with the guarantee that all Consumers 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
This Internet-Draft is submitted in full conformance with the
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Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 11 January 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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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
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Definition of Transparency . . . . . . . . . . . . . . . . . 8
5. Architecture Overview . . . . . . . . . . . . . . . . . . . . 10
5.1. Signed Statement Issuance and Registration . . . . . . . 12
5.1.1. Issuer Identity . . . . . . . . . . . . . . . . . . . 12
5.1.2. Naming Artifacts . . . . . . . . . . . . . . . . . . 16
5.1.3. Signed Statement Metadata . . . . . . . . . . . . . . 16
5.2. Transparency Service . . . . . . . . . . . . . . . . . . 16
5.2.1. Service Identity, Remote Attestation, and Keying . . 17
5.2.2. Registration Policies . . . . . . . . . . . . . . . . 18
5.2.3. Registry Security Requirements . . . . . . . . . . . 19
5.3. Verifying Transparent Statements . . . . . . . . . . . . 20
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6. Signed Statement Issuance, Registration, and Verification . . 21
6.1. Envelope and Signed Statement Format . . . . . . . . . . 21
6.2. Receipts . . . . . . . . . . . . . . . . . . . . . . . . 23
6.3. Signed Statement Issuance . . . . . . . . . . . . . . . . 25
6.4. Registering Signed Statements . . . . . . . . . . . . . . 26
6.5. Validation of Transparent Statements . . . . . . . . . . 27
7. Federation . . . . . . . . . . . . . . . . . . . . . . . . . 28
8. Transparency Service API . . . . . . . . . . . . . . . . . . 28
8.1. Messages . . . . . . . . . . . . . . . . . . . . . . . . 28
8.1.1. Register Signed Statement . . . . . . . . . . . . . . 29
8.1.2. Retrieve Operation Status . . . . . . . . . . . . . . 30
8.1.3. Retrieve Signed Statement . . . . . . . . . . . . . . 31
8.1.4. Retrieve Registration Receipt . . . . . . . . . . . . 32
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 32
10. Security Considerations . . . . . . . . . . . . . . . . . . . 33
10.1. Threat Model . . . . . . . . . . . . . . . . . . . . . . 33
10.1.1. Signed Statement Authentication and Transparency. . 34
10.1.2. Confidentiality and privacy. . . . . . . . . . . . . 36
10.1.3. Cryptographic Assumptions . . . . . . . . . . . . . 36
10.1.4. Transparency Service Clients . . . . . . . . . . . . 37
10.1.5. Identity . . . . . . . . . . . . . . . . . . . . . . 37
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
11.1. URN Sub-namespace for SCITT (urn:ietf:params:scitt) . . 37
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
12.1. Normative References . . . . . . . . . . . . . . . . . . 38
12.2. Informative References . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
This document describes a scalable and flexible, decentralized
architecture to enhance auditability and accountability across
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 Log,
so that their provenance and history can be independently and
consistently audited;
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.
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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 immutable, append-only Log. The next guarantee
is achieved by implementing the append-only Log using a verifiable
data structure (such as a Merkle Tree [MERKLE]). Lastly, the
Transparency Service verifies the identity of the Issuer, and
conformance to a Registration Policy associated with the instance of
the Transparency Service. As the Issuer of the Signed Statement and
conformance to the Registration Policy are confirmed, an endorsement
is made as the Signed Statement is added to the append-only Log.
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 is implemented and operates. Each
service MAY enforce its own Registration Policies for authorizing
entities to register their Signed Statements to the append-only Log.
Some Transparency Services may also enforce authorization policies
limiting who can write, read and audit specific Feeds or the full
registry. It is critical to provide 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, so it is implausible to expect all
participants to choose a single vendor or registry.
A Transparency Service provides visibility into Signed Statements
associated with various supply chains and their sub-systems. These
Signed Statements (and corresponding Statement payload) make claims
about the Artifacts produced by a supply chain. A Transparency
Service endorses 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". Producing a Transparent Statement may be considered a
form of notarization. A Statements payload content MAY be encrypted
and opaque to the Transparency Services, if so desired: however the
metadata MUST be transparent in order to warrant trust for later
processing. 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. Issuers may Register new Signed
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Statements about Artifacts, but they cannot delete or alter Signed
Statements previously added to the append-only Log. A Transparency
Service may restrict access to Signed Statements through access
control policies. However, third parties (such as Auditors) would be
granted access as needed to attest to the validity of the Artifact,
Feed or the entirety of the Transparency Service.
Trust in the Transparency Service itself is supported both by
protecting their implementation (using, for instance, replication,
trusted hardware, and remote attestation of a system's operational
state) and by enabling independent audits of the correctness and
consistency of its Registry, thereby holding the organization that
operates it accountable. 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.
Breadth of access is critical so the Transparency Service specified
in this architecture cater to two types of audiences:
1. Producers: organizations, stakeholders, and users involved in
creating or attesting to supply chain artifacts, releasing
authentic Statements to a definable set of peers; and
2. Consumers: organizations, stakeholders, and users involved in
validating supply chain artifacts, but can only do so if the
Statements are known to be authentic. Consumers MAY be
producers, providing additional Signed Statements, attesting to
conformance of various compliance requirements.
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. The issuer of a Signed
Statement should be authenticated and authorized according to the
registration policy of the Transparency Service. 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 in a secure,
append-only log via online operations, the same trust can be put into
the resulting transparent statement, issued by the Transparency
Services and that can be validated in offline operations.
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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.
The interoperability 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 but can be relied upon as non functional
requirements). Many of the data elements processed by the core
components are based on the Concise Signing and Encryption standard
specified in [RFC9052], which is used to produce Signed Statements
about Artifacts and to build and maintain a Merkle tree that
functions as an append-only Log for corresponding Signed Statements.
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
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.
Detailed use cases are maintained in a separate document
[I-D.ietf-scitt-software-use-cases].
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 issued by a Transparency Service.
Consumer of Signed Statements: Define here.
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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
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 Artifacts, or an independent third party such as a
reviewer or an endorser.
Append-only Log (converges Ledger and Registry): the verifiable
append-only data structure that stores Signed Statements in a
Transparency Service. SCITT supports multiple Log and Receipt
formats to accommodate different Transparency Service
implementations, such as historical Merkle Trees and sparse Merkle
Trees.
Receipt: a Receipt is a cryptographic proof that a Signed Statement
is recorded in the Registry. Receipts are based on COSE Signed
Merkle Tree Proofs [I-D.draft-steele-cose-merkle-tree-proofs];
they consist of a Registry-specific inclusion proof, a signature
by the Transparency Service of the state of the Registry, 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, 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.
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Registry: the verifiable append-only data structure that stores
Signed Statements in a Transparency Service often referred to by
the synonym log or ledger. Since COSE Signed Merkle Tree Proofs
([I-D.draft-steele-cose-merkle-tree-proofs]) support multiple
Merkle Tree algorithms, SCITT supports different Transparency
Service implementations of the Registry, such as historical Merkle
Trees or 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.
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 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.
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 intended to build
over abstract notions of Registry and Receipts. Existing
transparency systems such as Certificate Transparency are instances
of this definition.
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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.
Transparency is implemented by a Registry that provides a consistent,
append-only, cryptographically verifiable, publicly available record
of entries. Implementations of Transparency Services 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.
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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 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
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wrapper format for Receipts, which specifies the Transparency Service
identity and the agility parameters for the Merkle Tree Proof. Most
of the details of the Receipt's contents are specified in the COSE
Signed Merkle Tree Proof document
[I-D.draft-steele-cose-merkle-tree-proofs].
This section describes at a high level, the three main roles and
associated processes in SCITT: Issuers and the Signed Statement
issuance process, Transparency Service and the Signed Statement
Registration process, as well as Verifiers of the Transparent
Statements 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.
kid MUST either be an absolute URL, or a relative URL. Relative URL
MUST be relative to an iss value. When relative URL is used, iss
MUST also be present in the protected header.
Resolving kid MUST return an identity document of a registered
content type (a set of public keys). In the case of kid being an
absolute DID URL, the identity document is called a DID Document, and
is expected ot have content type application/did+json.
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To dereference a DID URL, it first MUST be resolved. After that the
fragment is processed according to the media type.
For example, when resolving did:example:123#key-42, first, the
identity document for did:example:123 is resolved as content type
application/did+json, next, the fragment #key-2 is dereferenced to a
verification method that contains a publicKeyJwk property.
The content type of publicKeyJwk is expected to be application/
jwk+json.
The details of both DID resolution and DID dereferencing are out of
scope for this document.
The iss or kid, might not be DID URLs, however the following
interfaces MUST be satisfied in order to ensure issuer identity
documents, and associated keys are discoverable in a consistent
manner.
5.1.1.1. Resolving Identity Documents
The value of id might be found the iss or sub claims if they are
present in the protected header or payload.
resolve = (id: string, accept: content_type = 'application/did+json')
=> idDocument (of content type application/did+json).
For example:
did:example:123
Might resolve to:
{ "id": "did:example:123", "verificationMethod": [{ "id": "#key-42",
"type": "JsonWebkey", "controller": "did:example:123",
"publicKeyJwk": { "kty": "EC", "crv": "P-384", "alg": "ES384", "x":
"LCeAt2sW36j94wuFP0gNEIHDzqR6Nh_Udu2ObLer3cKFBCaAHY1svmbPV69bP3RH",
"y":
"zz2SkcOGYM6PbYlw19tcbpzo6bEMYHIwGBnN5rd8QWykAprstPdxx4U0uScvDcYd" }
}] }
Editor note, we might wish to eliminate this intermediate identity
document content type, by treating it as an alterative encoding of
application/jwk-set+json or application/cose-key-set.
However, there is no media type fragment processing directive that
would enable dereferencing the known key set content types, listed
above.
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5.1.1.1.1. Comment on OIDC
For well known token types, such as id_token or access_token.
iss MUST be a URL, and it MUST have keys discoverable in the
following way:
iss can be used to build a .well-known URL to discovery the issuer's
configuration.
For example, iss contoso.example will have the following open id
connect configuration URL.
https://contoso.example/.well-known/openid-configuration.
This URL will resolve to a JSON document which contains the property:
jwks_uri, for example https://contoso.example/.well-known/jwks.json
This URL will resolve to a JSON document of content type application/
jwk-set+json, which will contain specific keys... for example:
```json { "keys": [ { "alg": "RS256", "kty": "RSA", "use": "sig",
"n": "wW9TkSbcn5FV3iUJ-812sqTvwTGCFrDm6vD2U-g23gn6rrBdFZQbf2bgEnSkolp
h6CanOYTQ1lKVhKjHLd6Q4MDVGidbVBhESxib2YIzJVUS-
0oQgizkBEJxyHI4Zl3xX_sdA_yegLUi-Ykt_gaMPSw_vpxe-pBxu-jd14i-jDfwoPJUdF
8ZJGS9orCPRiHCYLDgOscC9XibH9rUbTvG8q4bAPx9Ox6malx4OLvU3pXVjew6LG3iBi2
YhpCWe6voMvZJYXqC1n5Mk_KOdGcCFtDgu3I56SGSfsF7-tI7qG1ZO8RMuzqH0LkJViru
jYzXrnMZ7WgbMPXmHU8i4z04zw", "e": "AQAB", "kid":
"NTBGNTJEMDc3RUE3RUVEOTM4NDcyOEFDNzEyOTY5NDNGOUQ4OEU5OA", "x5t":
"NTBGNTJEMDc3RUE3RUVEOTM4NDcyOEFDNzEyOTY5NDNGOUQ4OEU5OA", "x5c": [ "M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" ] }, { "alg": "RS256", "kty": "RSA", "use": "sig",
"n": "ylgVZbNR4nlsU_AbU8Zd7ZhVfmYuwq-RB1_YQWHY362pAed-qgSXV1QmKwCukQ2
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WDsPHWgpPuEf3O_acmJcCiSxhctpBr5WKkji5o50YX2FqC3xymGkYW5NilvFznKaKU45u
lBVByrcb3Vt8BqqBAhaD4YywZZKo7mMudcq_M__f0_tB4fHsHHe7ehWobWtzAW7_NRP0_
FjB4Kw4PiqJnChPvfbuxTCEUcIYrshRwD6GF4D_oLdeR44dwx4wtEgvPOtkQ5XIGrhQC_
sgWcb2jh7YXauVUjuPezP-
VkK7Wm9mZRe758q43SWxwT3afo5BLa3_YLWazqcpWRXn9QEDWw", "e": "AQAB",
"kid": "aMIKy_brQk3nLd0PKd9ln", "x5t": "-xcTyx47q3ddycG7LtE6QCcETbs",
"x5c": [ "MIIC/TCCAeWgAwIBAgIJH62yWyX7VxxQMA0GCSqGSIb3DQEBCwUAMBwxGjA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" ] } ] }
```
If SCITT wanted to be interoperable with OIDC, we would define key
dereferencing in a way that was compatible with how OIDC handles it
today.
5.1.1.2. Dereferencing Public Keys
kid is always present in the protected header.
If iss is also present, kid MUST be a relative URL to iss, otherwise
kid MUST be an absolute URL that starts with iss.
id = kid if iss is undefined, or iss + # + kid when iss is defined.
See also draft-ietf-cose-cwt-claims-in-headers
(https://datatracker.ietf.org/doc/draft-ietf-cose-cwt-claims-in-
headers/).
dereference = (id: string, accept: content_type = 'application/
jwk+json') => publicKeyJwk (of content type application/jwk+json).
For example, when DIDs are used:
did:example:123#key-42
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Might dereference to:
{ "kty": "EC", "crv": "P-384", "alg": "ES384", "x":
"LCeAt2sW36j94wuFP0gNEIHDzqR6Nh_Udu2ObLer3cKFBCaAHY1svmbPV69bP3RH",
"y":
"zz2SkcOGYM6PbYlw19tcbpzo6bEMYHIwGBnN5rd8QWykAprstPdxx4U0uScvDcYd" }
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
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
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of Signed Statements and Receipt issuance, which is described in
Section 8.1. Each Transparency Service also defines its own
Registration Policies, 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.
Beyond the trusted components, Transparency Services may operate
additional endpoints for auditing, for instance to query for the
history of Signed 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 Service 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.
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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 Service 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.
5.2.2. Registration Policies
A Transparency Service that accepts to register any valid Signed
Statement offered by anonymous Issuers would only provide limited
value, or no value, to verifiers. As a consequence, some form of
authorization is needed prior to registration of Signed Statements to
ensure completeness of audit. More advanced use case will rely on
the Transparency Service performing additional domain-specific checks
before a Signed Statement is accepted. For example, some
Transparency Services may validate the content of Signed Statements.
We use the term "registration policies" to refer to the checks that
are performed before a Signed Statement is registered given a set of
input values. This baseline specification leaves the implementation
of the registration policy to the provider of the Transparency
Services and its users.
As a minimum we expect that a deployment authenticates the Issuer of
the Signed Statement, which requires some form of trust anchor. As
defined in [RFC6024], "A trust anchor represents an authoritative
entity via a public key and associated data. The public key is used
to verify digital signatures, and the associated data is used to
constrain the types of information for which the trust anchor is
authoritative." The Trust Anchor may be a certificate, a raw public
key or other structure, as appropriate. It can be a non-root
certificate when it is a certificate.
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A provider of a Transparency Service is, however, expected to
indicate what registration policy is used in a given deployment and
inform its users about changes to the registration policy.
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) that can be encoded as a COSE Signed Merkle
Tree Proof.
It is possible to offer multiple signature algorithms for the COSE
signature of receipts' Signed Merkle Tree, or to change the signing
algorithm at later points. However, the Merkle Tree algorithm
(including its internal hash function) cannot easily be changed
without breaking the consistency of the Registry. It is possible to
maintain separate Registries for each algorithm in parallel but the
Transparency Service is then responsible for proving their mutual
consistency.
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.
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,
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* 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 Service 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.
* 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),
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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.
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
relying 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:
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* 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
below. This allows the Receipt to be attached to the Signed
Statement, thus making a Transparent Statement.
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 => [+ Receipt]
}
6.2. Receipts
Receipts are based on COSE Signed Merkle Tree Proofs
([I-D.draft-steele-cose-merkle-tree-proofs]) with an additional
wrapper structure that adds the following information:
* version: Receipt version number; this should be set to 0 for
implementation of this document. We envision that future version
of SCITT may add support for more complex receipts; for instance,
registrations on multiple TS, receipts for dependency graphs and
endorsements of Signed Claims, etc.
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* ts_identifier: The DID of the Transparency Service that issued the
claim. Verifiers MAY use this DID as a key discovery mechanism to
verify the COSE Merkle Root signature; in this case the
verification is the same as for Signed Claims and the signer
should include the Key ID header. Verifiers MUST support the
did:web method, all other methods are optional.
We also introduce the following requirements for the COSE signature
of the Merkle Root:
* The SCITT version header MUST be included and its value match the
version field of the Receipt stucture.
* The DID of issuer header (like in Signed Claims) MUST be included
and its value match the ts_identifier field of the Receipt
structure.
* TS MAY include the Registration policy info header to indicate to
verifiers what policies have been applied at the registration of
this claim.
* Since [I-D.draft-steele-cose-merkle-tree-proofs] uses optional
headers, the crit header (id: 2) MUST be included and all SCITT-
specific headers (version, DID of TS and Registration Policy) MUST
be marked critical.
The TS may include the registration time to help verifiers decide
about the trustworthiness of the Transparent Statement. The
registration time is defined as the timestamp at which the TS has
added this Signed Statement to its Registry.
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Receipt = [
version: int,
ts_identifier: tstr,
proof: SignedMerkleTreeProof
]
; Additional protected headers in the COSE signed_tree_root of the SignedMerkleTreeProof
Protected_Header = {
390 => int ; SCITT Receipt Version
394 => tstr ; DID of Transparency Service (required)
? 395 => RegistrationInfo ; Registration policy information (optional)
; Other COSE Signed Merkle Tree headers
; (e.g. tree algorithm, tree size)
; Additional standard COSE headers
2 => [+ label] ; Critical headers
? 4 => bstr ; Key ID (optional)
? 33 => COSE_X509 ; X.509 chain (optional)
}
; Details of the registration info, as provided by the TS
RegistrationInfo = {
? "registration_time": uint .within (~time),
* tstr => any
}
6.3. 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. For a software supply chain,
payloads describing the software artifacts may, for example, include
* JSON-SPDX
* CBOR-SPDX
* SWID
* CoSWID
* CycloneDX
* in-toto
* SLSA
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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).
6.4. 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
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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.
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.draft-steele-cose-merkle-tree-proofs].
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.
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Verifiers SHOULD offer options to store or share Receipts in case
they are needed to audit the Transparency Services in case of a
dispute.
7. Federation
This topic is still under discussion, see issue 79
(https://github.com/ietf-wg-scitt/draft-ietf-scitt-architecture/
issues/79)
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
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
- TBD: more error codes to be defined, see #17
(https://github.com/ietf-wg-scitt/draft-ietf-scitt-
architecture/issues/17)
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
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- (Optional) Header: Retry-After: <seconds>
- Body: { "operationId": "<Operation ID>", "status": "running" }
* 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
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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.
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.
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 Statements' 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 Signed 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 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.
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
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 Log 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 one hand, this enables valid actors to
detect and disambiguate malicious actors who issue contradictory
Signed Statements to different entities (Verifiers, Auditors,
Issuers), otherwise known as 'equivocation'. On the other hand,
their liability and the resulting damage to their reputation are
application specific, and out of scope of the SCITT Architecture.
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Verifiers 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.
10.1.1.1. 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), 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 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.
Verifiers 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.
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Actors should independently keep their own record of the Signed
Statements they issue, endorse, verify, or audit.
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 Signed 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 Append-only Log only to Auditors trusted with the
confidentiality of its contents.
A collection of Signed Statements must not leak information about the
contents of other Signed Statements registered on the Transparency
Service.
Nonetheless, Issuers should carefully review the inclusion of
private/confidential materials in their Statements. For example,
issuers should remove Personally Identifiable Information (PII) as
clear text in the statement. Alternatively, Issuers may include
opaque cryptographic statements, 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 Service
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.)
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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.
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 Service 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]:
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Registry name: scitt
Specification: [RFCthis]
Repository: http://www.iana.org/assignments/scitt
Index value: No transformation needed.
12. References
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>.
[RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management
Requirements", RFC 6024, DOI 10.17487/RFC6024, October
2010, <https://doi.org/10.17487/RFC6024>.
[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>.
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[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>.
[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.draft-steele-cose-merkle-tree-proofs]
Steele, O., Birkholz, H., Delignat-Lavaud, A., and C.
Fournet, "Concise Encoding of Signed Merkle Tree Proofs",
Work in Progress, Internet-Draft, draft-steele-cose-
merkle-tree-proofs-01, 10 July 2023,
<https://datatracker.ietf.org/doc/html/draft-steele-cose-
merkle-tree-proofs-01>.
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[I-D.ietf-scitt-software-use-cases]
Birkholz, H., Deshpande, Y., Brooks, D., Martin, B., and
B. Knight, "Detailed Software Supply Chain Uses Cases for
SCITT", Work in Progress, Internet-Draft, draft-ietf-
scitt-software-use-cases-00, 14 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-scitt-
software-use-cases-00>.
[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>.
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
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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
Steve Lasker
Lasker Consulting
Seattle,
United States
Email: stevenlasker@hotmail.com
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