TBD H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Standards Track A. Delignat-Lavaud
Expires: 11 June 2023 C. Fournet
Microsoft Research
Y. Deshpande
ARM
8 December 2022
An Architecture for Trustworthy and Transparent Digital Supply Chains
draft-ietf-scitt-architecture-00
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 claims on any Transparency
Service (TS), 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.
Birkholz, et al. Expires 11 June 2023 [Page 1]
Internet-Draft SCITT Architecture December 2022
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 11 June 2023.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 5
2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Software Bill of Materials (SBOM) . . . . . . . . . . . . 6
2.2. Confidential Computing . . . . . . . . . . . . . . . . . 6
2.3. Cold Chains for Seafood . . . . . . . . . . . . . . . . . 7
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Definition of Transparency . . . . . . . . . . . . . . . . . 9
5. Architecture Overview . . . . . . . . . . . . . . . . . . . . 10
5.1. Claim Issuance and Registration . . . . . . . . . . . . . 12
5.1.1. Issuer Identity . . . . . . . . . . . . . . . . . . . 12
5.1.2. Naming Artifacts . . . . . . . . . . . . . . . . . . 13
5.1.3. Claim Metadata . . . . . . . . . . . . . . . . . . . 13
5.2. Transparency Service (TS) . . . . . . . . . . . . . . . . 13
5.2.1. Service Identity, Remote Attestation, and Keying . . 14
Birkholz, et al. Expires 11 June 2023 [Page 2]
Internet-Draft SCITT Architecture December 2022
5.2.2. Registration Policies . . . . . . . . . . . . . . . . 15
5.2.3. Registry Security Requirements . . . . . . . . . . . 16
5.3. Verifying Transparent Claims . . . . . . . . . . . . . . 17
6. Claim Issuance, Registration, and Verification . . . . . . . 18
6.1. Envelope and Claim Format . . . . . . . . . . . . . . . . 18
6.2. Claim Issuance . . . . . . . . . . . . . . . . . . . . . 20
6.3. Standard registration policies . . . . . . . . . . . . . 21
6.4. Registering Signed Claims . . . . . . . . . . . . . . . . 22
6.5. Validation of Transparent Claims . . . . . . . . . . . . 23
7. Federation . . . . . . . . . . . . . . . . . . . . . . . . . 24
8. Transparency Service API . . . . . . . . . . . . . . . . . . 24
8.1. Messages . . . . . . . . . . . . . . . . . . . . . . . . 25
8.1.1. Register Signed Claims . . . . . . . . . . . . . . . 25
8.1.2. Retrieve Registration Receipt . . . . . . . . . . . . 26
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 26
10. Security Considerations . . . . . . . . . . . . . . . . . . . 27
10.1. Threat Model . . . . . . . . . . . . . . . . . . . . . . 27
10.1.1. Claim authentication and transparency. . . . . . . . 27
10.1.2. Confidentiality and privacy. . . . . . . . . . . . . 29
10.1.3. Cryptographic Assumptions . . . . . . . . . . . . . 30
10.1.4. TS Clients . . . . . . . . . . . . . . . . . . . . . 30
10.1.5. Identity . . . . . . . . . . . . . . . . . . . . . . 30
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
12.1. Normative References . . . . . . . . . . . . . . . . . . 31
12.2. Informative References . . . . . . . . . . . . . . . . . 31
Appendix A. Attic . . . . . . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
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;
3. issuers can efficiently prove to any other party the registration
of their claims; verifying this proof ensures that the issuer is
consistent and non-equivocal when making claims.
Birkholz, et al. Expires 11 June 2023 [Page 3]
Internet-Draft SCITT Architecture December 2022
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), and by requiring a
TS 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 TS implements and operates its Registry. Each service
may enforce its own policy for authorizing entities to register their
claims on the TS. Some TS 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 TS instances as the composition and
configuration of involved supply chain entities and their system
components is ever changing and always in flux.
A TS provides visibility into claims issued by supply chain entities
and their sub-systems. These claims are called Digital Supply Chain
Artifacts (DSCA). A TS vouches for specific and well-defined
metadata about these DSCAs. Some metadata is selected (and signed)
by the issuer, indicating, e.g., "who issued the DSCA" or "what type
of DSCA is described" or "what is the DSCA version"; whereas
additional metadata is selected (and countersigned) by the TS,
indicating, e.g., "when was the DSCA registered in the Registry".
The DSCA contents can be opaque to the TS, if so desired: it is the
metadata that must always be transparent in order to warrant trust.
Transparent claims provide a common basis for holding issuers
accountable for the DSCA they release and (more generally) principals
accountable for auxiliary claims they make about DSCAs. Hence,
issuers may register new claims about their artifacts, but they
cannot delete or alter earlier claims, or hide their claims from
third parties such as auditors.
Trust in the TS 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, we
Birkholz, et al. Expires 11 June 2023 [Page 4]
Internet-Draft SCITT Architecture December 2022
require each TS 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 TS specified in this architecture caters to two types of
audiences:
1. DSCA Issuers: entities, stakeholders, and users involved in
supply chain interactions that need to release DSCAs to a
definable set of peers; and
2. DSCA Consumers: entities, stakeholders, and users involved in
supply chain interactions that need to access, validate, and
trust DSCAs.
DSCA Issuers rely on being discoverable and represented as the
responsible parties for released DSCAs by the TS in a believable
manner. Analogously, DSCA Consumers rely on verifiable
trustworthiness assertions associated with DSCAs and their processing
in a believable manner. If trust can be put into the operations that
record DSCAs in a secure, append-only Registry via an online
operation, the same trust can be put into a corresponding receipt
that is the result of these online operations issued by the TS and
that can be validated in offline operations.
The TS 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 global interoperability enabled and guaranteed by the TS 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 [RFC8152], which
is used to sign released DSCAs and to build and maintain a Merkle
tree that functions as the append-only Registry for DSCAs. 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.
Birkholz, et al. Expires 11 June 2023 [Page 5]
Internet-Draft SCITT Architecture December 2022
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.
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.
Birkholz, et al. Expires 11 June 2023 [Page 6]
Internet-Draft SCITT Architecture December 2022
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.
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
and 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
Birkholz, et al. Expires 11 June 2023 [Page 7]
Internet-Draft SCITT Architecture December 2022
supply chain.
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.
Claim: an identifiable and non-repudiable Statement about an
Artifact made by an Issuer. In SCITT, Claims are encoded as COSE
signed objects; the payload of the COSE structure contains the
Statement.
Issuer: an entity that makes Claims about Artifacts in the supply
chain. The Issuer may be the owner or author of the Artifact, or
an independent third party such as a reviewer or an endorser.
Envelope: the metadata added to the Statement by the Issuer to make
it a Claim. It contains the identity of the Issuer and other
information to help Verifiers identify the Artifact referred in
the Statement. A Claim binds the Envelope to the Statement. In
COSE, the Envelope consists of protected headers.
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 Claims about the same Artifact. In COSE,
Feed is a dedicated header attribute in the protected header of
the Envelope.
Registry: the verifiable append-only data structure that stores
Claims 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.
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 TS to provide
many security guarantees about its Registry . The identity of a TS
is captured by a public key that must be known by Verifiers in
order to validate Receipts.
Receipt: a Receipt is a special form of COSE countersignature for
Birkholz, et al. Expires 11 June 2023 [Page 8]
Internet-Draft SCITT Architecture December 2022
Claims that embeds cryptographic evidence that the Claim is
recorded in the Registry . It 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 protected headers) to assist in auditing.
Registration: the process of submitting a Claim to a Transparency
Service, applying its registration policy, storing it in the
Registry, producing a Receipt, and returning it to the submitter.
Registration Policy: the pre-condition enforced by the TS before
registering a Claim, based on its Envelope (notably the identity
of its Issuer) and on prior claims already in the Registry.
Transparent Claim: a Claim that is augmented with a Receipt of its
registration. A Transparent Claim remains a valid Claim (as the
Receipt is carried in the countersignature), and may be registered
again in a different TS.
Verifier: an entity that consumes Transparent Claims (a
specialization of Claim Consumer), verifying their proofs and
inspecting their Statements, either before using their Artifacts,
or later to audit their provenance on the supply chain.
Auditor: an entity that checks the correctness and consistency of
all Claim registered by a TS (a specialization of Claim Consumer).
4. Definition of Transparency
In this document, we use a definition of transparency built over
abstract notions of Registry and Receipts. Existing transparency
systems such as Certificate Transparency are instances of this
definition.
A Claim 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. Claims can be
made transparent by attaching a proof of Registration by a TS, in the
form of a Receipt that countersigns the Claim and witnesses its
inclusion in the Registry of a TS. By extension, we may say an
Artifact (e.g. a firmware binary) is transparent if it comes with one
or more Transparent Claims from its author or owner, though the
context should make it clear what type of Claim is expected for a
given Artifact.
Birkholz, et al. Expires 11 June 2023 [Page 9]
Internet-Draft SCITT Architecture December 2022
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 TS 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 that subsume
older entries.
Anyone with access to the Registry can independently verify its
consistency and review the complete list of Claims registered by each
Issuer. However, the Registry of separate Transparency Services are
generally disjoint, though it is possible to take a Claim 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 into Claims. Similarly, reputable TS
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 through
Receipts, that is, given two valid Receipts the TS may be asked to
produce a cryptographic proof that they are consistent. Failure to
produce this proof can indicate that the TS operator misbehaved.
5. Architecture Overview
Birkholz, et al. Expires 11 June 2023 [Page 10]
Internet-Draft SCITT Architecture December 2022
.----------.
| Artifact |
'----+-----'
v
.----+----. .----------.
Issuer --> | Statement || Envelope |
'----+----' '-----+----'
| | +------------------+
'----. .----' | DID Key Manifest |
| | (decentralized) |
v +---+----------+---+
.--+--. Sign Claim | |
| Claim +<------------' |
'--+--' |
| +--------------+ |
.-' '---------->+ Transparency | |
| .-------. | | |
Transparency --> | | Receipt |<--+ Registry | |
Service | '---+---' +-------+------+ |
'-. .-' | |
| | |
v | |
.-+-------+-. | |
| Transparent | | |
| Claim | | |
'-----+-----' | |
| | |
|'------. .------)-------'
| | | |
| v v |
| .----+---+-----. |
Verifier --> | / Verify Claim / |
| '--------------' |
v v
.--------+---------. .-------+---------.
Auditor --> / Collect Receipts / / Replay Registry /
'------------------' '-----------------'
Birkholz, et al. Expires 11 June 2023 [Page 11]
Internet-Draft SCITT Architecture December 2022
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 Claims. In order to accommodate as
many TS implementations as possible, this document only specifies the
format of Claims (which must be used by all Issuers) and a very thin
wrapper format for Receipts, which specifies the TS identity and the
Registry algorithm. Most of the details of the 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, we describe at a high level the three main roles and
associated processes in SCITT: Issuers and the Claim issuance
process, transparency Registry and the Claim Registration process,
and Verifiers and the Receipt validation process.
5.1. Claim Issuance and Registration
5.1.1. Issuer Identity
Before an Issuer is able to produce Claims, it must first create its
decentralized identifier (https://www.w3.org/TR/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 TS will be able to register their Claim. To facilitate
interoperability, all Transparency Service implementations SHOULD
support the did:web method from [https://w3c-ccg.github.io/did-
method-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
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 Claims issued with those keys.
This DID appears in the Issuer header of the Claim's Envelope, while
the version of the key from the manifest used to sign the Claim is
written in the kid header.
Birkholz, et al. Expires 11 June 2023 [Page 12]
Internet-Draft SCITT Architecture December 2022
5.1.2. Naming Artifacts
Many Issuers issue Claims 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 Claim 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
Claims under the same key.
5.1.3. Claim Metadata
Besides Issuer, Feed and kid, the only other mandatory metadata in
the Claim is the type of the Payload, indicated in the cty 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 Claim to be registered if it was signed
recently. While the Issuer is free to add any information in the
payload of the Claim, the TS (and most of its auditor) can only be
expected to interpret information in the Envelope.
Such metadata, meant to be interpreted by the TS during Registration
policy evaluation, should be added to the reg_info header. While the
header MUST be present in all Claims, 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 TS. 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 (TS)
The role of TS can be decomposed into several major functions. The
most important is maintaining a Registry, the verifiable data
structure that records Claims, and enforcing a Registration policy.
It also maintains a service key, which is used to endorse the state
of the Registry in Receipts. All TS MUST expose standard endpoints
for Registration of Claims and Receipt issuance, which is described
in Section 8.1. Each TS 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 TS. Each of these components SHOULD be carefully protected
against both external attacks and internal misbehavior by some or all
of the operators of the TS. 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
Birkholz, et al. Expires 11 June 2023 [Page 13]
Internet-Draft SCITT Architecture December 2022
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 Claims made by a given Issuer and Feed. Implementations
of TS 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 TS 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 TS 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.
The TS SHOULD provide evidence that it is securely implemented and
operated, enabling remote authentication of the hardware platforms
and/or software TCB that run the TS. This additional evidence SHOULD
be recorded in the Registry and presented on demand to Verifiers and
auditors.
For example, consider a TS 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.
Birkholz, et al. Expires 11 June 2023 [Page 14]
Internet-Draft SCITT Architecture December 2022
5.2.2. Registration Policies
A TS that accepts to register any valid claim offered by an issuer
would end up providing only limited value to verifiers. In
consequence, a baseline transparency guarantee policing the
registration of claims is required to ensure completeness of audit,
which can help detect equivocation. Most advanced SCITT scenarios
rely on the TS performing additional domain-specific checks before a
claim is accepted: TS may only allow trusted authenticated users to
register claims, TS may try to check that a new claim is consistent
with previous claims from the same issuers or that claims are
registered in the correct order and cannot be re-played; some TS may
even interpret and validate the payload of claims.
In general, registration policies are applied at the discretion of
the TS, and verifiers use receipts as witnesses that confirm that the
registration policy of the TS was satisfied at the time claim
registration. TS 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 TS is opaque to the
verifier, who is forced to trust the associated registration policy.
If the policy of the TS evolves over time, or is different across
issuers, the guarantee derived from receipt validation may not be
uniform across all claims over time.
To help verifiers interpret the semantics of claim registration,
SCITT defines a standard mechanism for signalling in the claim itself
which policies have been applied by the TS from a defined set of
registration policies with standardized semantics. Each policy that
is expected to be enforced by the TS is represented by an entry in
the registration policy info map (reg_info) in the envelope. The key
of the map 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 claim can
be registered, hence the associated value contains an unsigned
integer.
While this design ensures that all verifiers get the same guarantee
regardless of where a claim is registered, its main downside is that
it requires the issuer to include the necessary policies in the
envelope when the claim is signed. Furthermore, it makes it
impossible to register the same claim on two different TS if their
required registration policies are incompatible.
*Editor's note*
Birkholz, et al. Expires 11 June 2023 [Page 15]
Internet-Draft SCITT Architecture December 2022
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
claims but requires the verifier to be more aware of the
particular policies at the TS where the claim 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. We only require the Registry 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, we do not specify any particular Registry format in this
document. Instead, we propose two initial formats for Registry in
[I-D.birkholz-scitt-receipts] using historical and sparse Merkle
Trees. Beyond the format of Receipts, we require generic properties
that should be satisfied by the components in the TS that have the
ability to write to the Registry.
5.2.3.1. Finality
The Registry is append-only: once a Claim is registered, it cannot be
modified, deleted, or moved. In particular, once a Receipt is
returned for a given Claim, the Claim 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. TS
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,
Birkholz, et al. Expires 11 June 2023 [Page 16]
Internet-Draft SCITT Architecture December 2022
* the TS defines and enforces deterministic Registration policies
that can be re-evaluated based solely on the contents of the
Registry at the time of registraton, 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.
* The TS SHOULD store evidence about the resolution of distributed
identifiers into manifests.
* The TS MAY additionally support verifiability of client
authentication and access control.
5.2.3.4. Governance and Bootstrapping
The TS 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
Claims about code updates, secured on the Registry itself, or on some
auxiliary TS).
Governance procedures, their auditing, and their transparency are
implementation specific. The TS SHOULD document them.
* Governance may be based on a consortium of members that are
jointly responsible for the TS, or automated based on the contents
of an auxiliary governance TS.
* Governance typically involves additional records in the Registry
to enable its auditing. Hence, the Registry may contain both
Transparent Claims and governance entries.
* Issuers, Verifiers, and third-party auditors may review the TS
governance before trusting the service, or on a regular basis.
5.3. Verifying Transparent Claims
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 TS.
Birkholz, et al. Expires 11 June 2023 [Page 17]
Internet-Draft SCITT Architecture December 2022
When presented with a Transparent Claim for the Artifact, they verify
its Issuer identity, signature, and Receipt. They may additionally
apply a validation policy based on the protected headers present both
in the Envelope or in the countersignature and the Statement itself,
which may include security-critical Artifact-specific details.
Some Verifiers may systematically resolve the Issuer DID to fetch
their latest DID document. This strictly enforces the revocation of
compromised keys: once the Issuer has updated its document to remove
a key identifier, all Claims signed with this 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 TS's Registration policy and the
scrutiny of other Verifiers. Although this weakens their guarantees
against key revocation, or against a corrupt TS, they can still keep
the Receipt and blame the Issuer or the TS at a later point.
6. Claim Issuance, Registration, and Verification
This section details the interoperability requirements for
implementers of Claim issuance and validation libraries, and of
Transparency Services.
6.1. Envelope and Claim Format
The formats of Claims and Receipts are based on CBOR Object Signing
and Encryption (COSE). The choice of CBOR is a trade-off between
safety (in particular, non-malleability: each Claim has a unique
serialization), ease of processing and availability of
implementations.
At a high-level that is the context of this architecture, a Claim is
a COSE single-signed object (i.e. COSE_Sign1) that contains the
correct set of protected headers. Although Issuers and relays may
attach unprotected headers to Claims, Transparency Services and
Verifiers MUST NOT rely on the presence or value of additional
unprotected headers in Claims during Registration and validation.
All Claims MUST include the following protected headers:
* algorithm (label: 1): Asymmetric signature algorithm used by the
Claim Issuer, as an integer, for example -35 for ECDSA with SHA-
384, see COSE Algorithms registry
(https://www.iana.org/assignments/cose/cose.xhtml);
Birkholz, et al. Expires 11 June 2023 [Page 18]
Internet-Draft SCITT Architecture December 2022
* Issuer (label: TBD, temporary: 391): DID (Decentralized
Identifier, see W3C Candidate Recommendation
(https://www.w3.org/TR/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 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, Claims 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:
Birkholz, et al. Expires 11 June 2023 [Page 19]
Internet-Draft SCITT Architecture December 2022
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,
? "sequence_no": uint,
? "issuance_ts": uint,
* 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. Claim Issuance
There are many types of Statements (such as SBOMs, malware scans,
audit reports, policy definitions) that Issuers may want to turn into
Claims. The Issuer must first decide on a suitable format to
serialize the Statement, such as:
* JSON-SPDX
* CBOR-SPDX
* SWID
Birkholz, et al. Expires 11 June 2023 [Page 20]
Internet-Draft SCITT Architecture December 2022
* CoSWID
* CycloneDX
* in-toto
* SLSA
Once the Statement is serialized with the correct content type, the
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 Claim may only be registered on
Transparency Services that implement the associated policy. For
instance, if a Claim 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, the Issuer MAY use a standard
COSE implementation to produce the serialized Claim (the SCITT tag of
COSE_Sign1_Tagged is outside the scope of COSE, and used to indicate
that a signed object is a Claim).
6.3. Standard registration policies
*Editor's note*
The technical design for signalling 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.
TS 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 claim can
be registered. Any unsupported claims MUST be indicated separately
and corresponding unknown policy entries in the map of a claim 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.
Birkholz, et al. Expires 11 June 2023 [Page 21]
Internet-Draft SCITT Architecture December 2022
+=============+==============+==================================+
| Policy Name | Required | Implementation |
| | attributes | |
+=============+==============+==================================+
| TimeLimited | register_by: | Returns true if now () < |
| | uint | register_by at registration |
| | | time. The ledger MUST store the |
| | | ledger time at registration |
| | | along with the claim, and SHOULD |
| | | indicate it in receipts |
+-------------+--------------+----------------------------------+
| Sequential | sequence_no: | First, lookup in the ledger for |
| | uint | claims with the same issuer and |
| | | feed. If at least one is found, |
| | | returns true if and only if the |
| | | sequence_no of the new claim is |
| | | the highest sequence_no in the |
| | | existing claims incremented by |
| | | one. Otherwise, returns true if |
| | | and only if sequence_no = 0. |
+-------------+--------------+----------------------------------+
| Temporal | issuance_ts: | Returns true if and only if |
| | uint | there is no claim in the ledger |
| | | with the same issuer and feed |
| | | with a greater issuance_ts |
+-------------+--------------+----------------------------------+
| NoReplay | None | Returns true if and only if the |
| | | claim doesn't already appear in |
| | | the ledger |
+-------------+--------------+----------------------------------+
Table 1: An Initial Set of Named Policies
6.4. Registering Signed Claims
The same Claim may be independently registered in multiple TS. To
register a Claim, the service performs the following steps:
1. Client authentication. This is implementation-specific, and MAY
be unrelated to the Issuer identity. Claims may be registered by
a different party than their Issuer.
2. Issuer identification. The TS 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 resolve the Issuer
DID and record the resulting document, or rely on a cache of
recent resolutions.
Birkholz, et al. Expires 11 June 2023 [Page 22]
Internet-Draft SCITT Architecture December 2022
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
has a payload and the protected headers listed above. The
service MAY additionally verify the payload format and content.
5. Apply Registration policy: for named policies, the TS should
check that the required Registration info attributes are present
in the Envelope and apply the check described in Table 1. A TS
MUST reject Claims that contain an attribute used for a named
policy that is not enforced by the service. Custom Claims are
evaluated given the current Registry state and the entire
Envelope, and MAY use information contained in the attributes of
named policies.
6. Commit the new Claim to the Registry
7. Sign and return the Receipt.
The last two steps MAY be shared between a batch of Claims recorded
in the Registry.
The service MUST ensure that the Claim is committed before releasing
its Receipt, so that it can always back up the Receipt by releasing
the corresponding entry in the Registry. Conversely, the service MAY
re-issue Receipts for the Registry content, for instance after a
transient fault during Claim Registration.
6.5. Validation of Transparent Claims
This section provides additional implementation considerations, the
high-level validation algorithm is described in Section 5.3, with the
Registry-specific details of checking Receipts are covered in
[I-D.birkholz-scitt-receipts].
Before checking a Claim, 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
Claim is registered on.
Birkholz, et al. Expires 11 June 2023 [Page 23]
Internet-Draft SCITT Architecture December 2022
In some scenarios, the Verifier already expects a specific Issuer and
Feed for the Claim, 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 TS during Registration.
Some Verifiers MAY decide to locally re-apply some or all of the
Registration policies if they have limited trust in the TS. 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 payload,
as well as any local state.
Verifiers SHOULD offer options to store or share Receipts in case
they are needed to audit the TS in case of a dispute.
7. Federation
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
SCITT instances may be governed and operated by different
organizations that do not trust one another.
This may involve registering the same Claims at different
transparency services, each with their own purpose and registration
policy. This may also involve attaching multiple Receipts to the
same Claims, each Receipt endorsing the Issuer signature and a subset
of prior Receipts, and each TS verifying prior Receipts as part of
their registration policy.
For example, a supplier TS may provide a complete, authoritative
Registry for some kind of Claims, whereas a consumer TS may collect
different kinds of Claims to ensure complete auditing for a specific
use case, and possibly require additional reviews before registering
some of these claims.
8. Transparency Service API
Editor's Note: This may be moved to appendix.
Birkholz, et al. Expires 11 June 2023 [Page 24]
Internet-Draft SCITT Architecture December 2022
8.1. Messages
8.1.1. Register Signed Claims
8.1.1.1. Request
POST /entries
Body: SCITT COSE_Sign1 message
8.1.1.2. Response
One of the following:
* HTTP Status 201 - Registration was tentatively successful pending
service consensus.
* HTTP Status 400 - Registration was unsuccessful.
- Error code AwaitingDIDResolutionTryLater
- Error code InvalidInput
[TODO] Use 5xx for AwaitingDIDResolutionTryLater
The 201 response contains the x-ms-ccf-transaction-id HTTP header
which can be used to retrieve the Registration Receipt with the given
transaction ID. [TODO] this has to be made generic
[TODO] probably a bad idea to define a new header, or is it ok? can
we register a new one? https://www.iana.org/assignments/http-fields/
http-fields.xhtml
The 400 response has a Content-Type: application/json header and a
body containing details about the error:
json { "error": { "code": "", "message": "" } }
AwaitingDIDResolutionTryLater means the service does not have an up-
to-date DID document of the DID referenced in the Signed Claims but
is performing or will perform a DID resolution after which the client
may retry the request. The response may contain the HTTP header
Retry-After to inform the client about the expected wait time.
InvalidInput means either the Signed Claims message is syntactically
malformed, violates the signing profile (e.g. signing algorithm), or
has an invalid signature relative to the currently resolved DID
document.
Birkholz, et al. Expires 11 June 2023 [Page 25]
Internet-Draft SCITT Architecture December 2022
8.1.2. Retrieve Registration Receipt
8.1.2.1. Request
GET /entries//receipt
8.1.2.2. Response
One of the following:
* HTTP Status 200 - Registration was successful and the Receipt is
returned.
* HTTP Status 400 - Transaction exists but does not correspond to a
Registration Request.
- Error code TransactionMismatch
* HTTP Status 404 - Transaction is pending, unknown, or invalid.
- Error code TransactionPendingOrUnknown
- Error code TransactionInvalid
The 200 response contains the SCITT_Receipt in the body.
The 400 and 404 responses return the error details as described
earlier.
The retrieved Receipt may be embedded in the corresponding COSE_Sign1
document in the unprotected header, see TBD.
[TODO] There's also the GET /entries/
endpoint which returns the submitted COSE_Sign1 with the Receipt
already embedded. Is this useful?
9. Privacy Considerations
Unless advertised by the TS, every Issuer should treat its Claims as
public. In particular, their Envelope and Statement should not carry
any private information in plaintext.
Birkholz, et al. Expires 11 June 2023 [Page 26]
Internet-Draft SCITT Architecture December 2022
10. Security Considerations
On its own, verifying a Transparent Claim 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 TS. If the
Verifier trusts the Issuer, it can infer that the Claim 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 TS, it can independently infer that the Claim
passed the TS Registration policy and that has been persisted in the
Registry. Unless advertised in the TS Registration policy, the
Verifier should not assume that the ordering of Transparent Claims in
the Registry matches the ordering of their issuance.
Similarly, the fact that an Issuer can be held accountable for its
Transparent Claims does not on its own provide any mitigation or
remediation mechanism in case one of these Claims turned out to be
misleading or malicious---just that signed evidence will be available
to support them.
Issuers SHOULD ensure that the Statements in their Claims are correct
and unambiguous, for example by avoiding ill-defined or ambiguous
formats that may cause Verifiers to interpret the Claim 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. In case key re-use is unavoidable,
they MUST NOT sign any other message that may be verified as an
Envelope.
10.1. Threat Model
We provide a generic threat model for SCITT, describing its residual
security properties when some of its actors (identity providers,
Issuers, TS, and Auditors) are corrupt or compromised.
This model may need to be refined to account for specific supply
chains and use cases.
10.1.1. Claim authentication and transparency.
SCITT primarily supports evidence of Claim integrity, both from the
Issuer (authentication) and from the TS (transparency). These
guarantees are meant to hold for the long term, possibly decades.
We conservatively suppose that some issuers and some TS will be
corrupt.
Birkholz, et al. Expires 11 June 2023 [Page 27]
Internet-Draft SCITT Architecture December 2022
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
signed Claim from a given Issuer, registered at a given TS (both
identified in the Verifier's local authorization policy) and would
not depend on any other Issuer or TS.
We cannot stop authorized supply chain actors from making false
claims (either by mistake or by corruption) but we can make them
accountable by ensuring their Claims are systematically registered at
a trustworthy TS.
Similarly, we aim to provide strong residual guarantees against a
faulty/corrupt TS. We cannot stop a TS from registering Claims that
do not meet its stated Registration Policy, or to issue Receipts that
are not consistent with their append-only Registry, but we can hold
it accountable and guarantee that it will be blamed by any Auditor
that replays their Registry against any contested Receipt. Note that
SCITT does not require trust in a single centralized TS: different
actors may rely on different TS, each registering a subset of claims
subject to their own policy.
In both cases, SCITT provides generic, universally-verifiable
cryptographic evidence to individually blame the Issuer or the TS.
This enables valid actors to detect and disambiguate malicious actors
who make contradictory Claims 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 for SCITT.
Verifiers and Auditors need not be trusted by other actors. In
particular, they cannot "frame" an Issuer or a TS for claims they did
not issue or register.
*Append-only log*
If a TS is honest, then a transparent signed Claim with a correct
Receipt of registration at a given position ensures that the signed
claim passed its Registration Policy and was recorded at that
position in its Registry.
Conversely, a corrupt TS may 1. refuse or delay the registration of
Claims; 2. register Claims that do not pass its Registration Policy
(e.g. Claims with Issuer identities and signatures that do not
verify.) 3. issue verifiable Receipts for Claims that do not match
its Registry; 4. refuse access to its Registry (e.g. to Auditors,
possibly after storage loss)
Birkholz, et al. Expires 11 June 2023 [Page 28]
Internet-Draft SCITT Architecture December 2022
An Auditor granted (partial) access to the 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 TS for them. This ensures any Verifier that trust at
least one such Auditor that (2,3) will be blamed to the TS.
Due to the operational challenge of maintaining a globally consistent
append-only Registry, some TS may provide limited support for
historical queries on the Claims 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.
*Availability of Transparent Signed Claims*
Networking and Storage are trusted only for availability.
Auditing may involve access to data beyond what is persisted in the
TS log. For example, the registered TS 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 Claims they
issue, endorse, verify, or audit.
10.1.2. Confidentiality and privacy.
The network is untrusted. All contents exchanged between actors is
protected using secure authenticated channels (TLS) but, as usual,
this may not exclude network traffic analysis.
*Claims and their registration*
The TS is trusted with the confidentiality of the claims presented
for registration. Some TS may publish every claim in their logs, to
facilitate their dissemination and auditing. Others may just return
receipts to the client that present claims for registration, and
disclose the ledger only to auditors trusted with the confidentiality
of its contents.
A collection of transparent Claims leaks no information about the
contents of other Claims registered at the TS.
Birkholz, et al. Expires 11 June 2023 [Page 29]
Internet-Draft SCITT Architecture December 2022
Nonetheless, Issuers should carefully review the inclusion of
private/confidential materials in their Claims; they may for instance
remove any PII, or include instead opaque cryptographic commitments,
such as hashes.
*Queries to the Registry*
The confidentiality of queries is implementation-specific, and
generally not guaranteed. For example, while offline Claim
verification is private, a TS may monitor which of its Claims are
being verified from lookups to ensure their freshness.
10.1.3. Cryptographic Assumptions
We rely on standard cryptographic security for signing schemes (EUF-
CMA: for a given key, given the public key and any number of signed
messages, the 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 claim in this log.)
SCITT 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. TS Clients
Trust in clients that submit Claims for registration is
implementation-specific. Hence, an attacker may attempt to register
any Claim it has obtained, at any TS that accepts them, possibly
multiple times and out of order. This may be mitigated by a TS 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. (The TS
and other parties may record identity-resolution evidence to
facilitate its auditing.)
If one of the credentials of an Issuer gets compromised, SCITT still
guarantee the authenticity of all claims signed with this credential
that have been registered on a TS before the compromise. It is up to
the Issuer to notify TS of credential revocation to stop Verifiers
from accepting Claims signed with compromised credentials. [See the
thread of revocation for additional details.]
Birkholz, et al. Expires 11 June 2023 [Page 30]
Internet-Draft SCITT Architecture December 2022
The confidentiality of any identity lookup during Claim Registration
or Claim Verification is out of scope.
11. IANA Considerations
See Body Section 4.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, .
[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, .
[RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate
Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
December 2021, .
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,
.
Birkholz, et al. Expires 11 June 2023 [Page 31]
Internet-Draft SCITT Architecture December 2022
[PBFT] Castro, M. and B. Liskov, "Practical byzantine fault
tolerance and proactive recovery", ACM Transactions on
Computer Systems, Volume 20, Issue 4 , November 2002,
.
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
Birkholz, et al. Expires 11 June 2023 [Page 32]