SCITT Reference APIs

Introduction The SCITT Architecture defines the core operations necessary to support supply chain transparency using COSE (CBOR Object Signing and Encryption).

Issuance of Signed Statements
Verification of Signed Statements
Registration of Signed Statements
Issuance of Receipts
Verification of Receipts
Production of Transparent Statements
Verification of Transparent Statements

In addition to defining concrete HTTP endpoints for these operations, this specification defines support for the following endpoints which support these operations:

Resolving Verification Keys for Issuers
Retrieving Receipts Asynchronously
Retrieving Signed Statements from an Artifact Repository
Retrieving Statements from an Artifact Repository

Terminology 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 when, and only when, they appear in all capitals, as shown here. This specification uses the terms "Signed Statement", "Receipt", "Transparent Statement", "Artifact Repositories", "Transparency Service", "Append-Only Log" and "Registration Policy" as defined in . This specification uses "payload" as defined in .

Endpoints Authentication is out of scope for this document. If Authentication is not implemented, rate limiting or other denial of service mitigations MUST be applied to enable anonymous access. NOTE: '' line wrapping per in HTTP examples. 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 () 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:".
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: 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 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.

Mandatory The following HTTP endpoints are mandatory to implement to enable conformance to this specification.

Transparency Configuration Authentication SHOULD NOT be implemented for this endpoint. This endpoint is used to discovery the capabilities of a transparency service implementing this specification. Request: Response: Additional fields may be present. Fields that are not understood MUST be ignored.

Signed Statement Registration Authentication MUST be implemented for this endpoint. The following is a non-normative example of a HTTP request to register a Signed Statement: Request: The Registration Policy for the Transparency Service MUST be applied to the payload bytes, before any additional processing is performed. If the payload is detached, the Transparency Service depends on the authentication context of the client in the Registration Policy. If the payload is attached, the Transparency Service depends on both the authentication context of the client, and the verification of the Signed Statement in the Registration Policy. The details of Registration Policy are out of scope for this document. If registration succeeds the following identifier MAY be used to refer to the Signed Statement that was accepted: urn:ietf:params:scitt:signed-statement:sha-256:base64url:5i6UeRzg1...qnGmr1o If the payload was attached, or otherwise communicated to the Transparency Service, the following identifier MAY be used to refer to the payload of the Signed Statement: urn:ietf:params:scitt:statement:sha-256:base64url:5i6UeRzg1...qnGmr1o Response: One of the following:

Status 201 - Registration is successful The response contains the Receipt for the Signed Statement. Fresh receipts may be requested through the resource identified in the Location header.

Status 202 - Registration is running { "receipt": "urn:ietf:params:scitt:receipt\ :sha-256:base64url:5i6UeRzg1...qnGmr1o", } ]]> The response contains a reference to the receipt which will eventually be available for the Signed Statement. If 202 is returned, then clients should wait until Registration succeeded or failed by polling the receipt endpoint using the receipt identifier returned in the response.

Status 400 - Invalid Client Request One of the following errors: TODO: other error codes

Optional Endpoints The following HTTP endpoints are optional to implement.

Issue Signed Statement Authentication MUST be implemented for this endpoint. This endpoint enables a Transparency Service to be an issuer of Signed Statements on behalf of authenticated clients. This supports cases where a client lacks the ability to perform complex cryptographic operations, but can be authenticated and report statements and measurements. Request: Response:

Resolve Statement Authentication SHOULD be implemented for this endpoint. This endpoint enables Transparency Service APIs to act like Artifact Repositories, and serve payload values directly, instead of indirectly through Receipts. Request: Response:

Resolve Signed Statement Authentication SHOULD be implemented for this endpoint. This endpoint enables Transparency Service APIs to act like Artifact Repositories, and serve Signed Statements directly, instead of indirectly through Receipts. Request: Response:

Resolve Receipt Authentication SHOULD be implemented for this endpoint. Request: Response: If the Signed Statement requested is already included in the Append-Only Log: If the Signed Statement requested is not yet included in the Append-Only Log: { "receipt": "urn:ietf:params:scitt:receipt\ :sha-256:base64url:5i6UeRzg1...qnGmr1o", } ]]> Additional eventually consistent operation details MAY be present. Support for eventually consistent Receipts is implementation specific, and out of scope for this specification.

Resolve Issuer This endpoint is inspired by . Authentication SHOULD NOT be implemented for this endpoint. This endpoint is used to discover verification keys, which is the reason that authentication is not required. The following is a non-normative example of a HTTP request for the Issuer Metadata configuration when iss is set to https://transparency.example/tenant/1234: Request: Response:

Request Nonce This endpoint in inspired by . Authentication SHOULD NOT be implemented for this endpoint. This endpoint is used to demonstrate proof of possession, which is the reason that authentication is not required. Client holding signed statements that require demonstrating proof of possession MUST use this endpoint to obtain a nonce. Request: Response:

Resolve Issuer DID This endpoint enables the use of the DID Web Decentralized Identifier Method, as an alternative expression of the Issuer Metadata endpoint. This endpoint is DEPRECATED. The following is a non-normative example of a HTTP request for the Issuer Metadata configuration when iss is set to did:web:transparency.example:tenant:1234: Request: Response:

Privacy Considerations TODO

Security Considerations

General scope This document describes the interoperable API for client calls to, and implementations of, a Transparency Service as specified in . As such the security considerations in this section are concerned only with security considerations that are relevant at that implementation layer. All questions of security of the related COSE formats, algorithm choices, cryptographic envelopes,verifiable data structures and the like are handled elsewhere and out of scope of this document.

Applicable Environment SCITT is concerned with issues of cross-boundary supply-chain-wide data integrity and as such must assume a very wide range of deployment environments. Thus, no assumptions can be made about the security of the computing environment in which any client implementation of this specification runs.

User-host authentication defines 2 distinct roles that require authentication: Issuers who sign Statements, and clients that submit API calls on behalf of Issuers. While Issuer authentication and signing of Statements is very important for the trustworthiness of systems implementing the SCITT building blocks, it is out of scope of this document. This document is only concerned with authentication of API clients. For those endpoints that require client authentication, Transparency Services MUST support at least one of the following options:

HTTP Authorization header with a bearer JWT
domain-bound API key
TLS client authentication

Transparency Services MUST provide a configuration surface that allows Issuers to specify which authorized clients can submit Statements on their behalf. Where authentication methods rely on long term secrets, both clients and Transparency Services implementing this specification MUST allow for the revocation and rolling of authentication secrets.

Primary threats

In scope The most serious threats to implementations on Transparency Services are ones that would cause the failure of their main promises, to wit:

Threats to strong identification, for example representing the Statements from one issuer as those of another
Threats to payload integrity, for example changing the contents of a Signed Statement before making it transparent
Threats to non-equivocation, for example attacks that would enable the presentation or verification of divergent proofs for the same Statement payload

Denial of service attacks While denial of service attacks are very hard to defend against completely, and Transparency Services are unlikely to be in the critical path of any safety-liable operation, any attack which could cause the silent failure of Signed Statement registration, for example, should be considered in scope. In principle DoS attacks are easily mitigated by the client checking that the Transparency Service has registered any submitted Signed Statement and returned a Receipt. Since verification of Receipts does not require the involvement of the Transparency Service DoS attacks are not a major issue. Clients to Transparency Services SHOULD ensure that Receipts are available for their registered Statements, either on a periodic or needs-must basis, depending on the use case. Beyond this, implementers of Transparency Services SHOULD implement general good practice around network attacks, flooding, rate limiting etc.

Eavesdropping Since the purpose of this API is to ultimately put the message payloads on a Transparency Log there is limited risk to eavesdropping. Nonetheless transparency may mean 'within a limited community' rather than 'in full public', so implementers MUST add protections against man-in-the-middle and network eavesdropping, such as TLS.

Message modification attacks While most relevant modification attacks are mitigated by the use of the Issuer signature on the Signed Statement, the Issue Statement endpoint presents an opportunity for manipulation of messages and misrepresentation of Issuer intent that could mislead later Verifiers. Transparency Services offering the Issue Statement endpoint MUST require authentication and transport-level security for that endpoint, MUST NOT modify anything in the message to be signed, and MUST take steps to ensure that the party calling the endpoint is authorized to register statements on behalf of the specified Issuer.

Message insertion attacks While most relevant insertion attacks are mitigated by the use of the Issuer signature on the Signed Statement, the Issue Statement endpoint presents an opportunity for insertion of messages and misrepresentation of Issuer intent that could mislead later Verifiers. There are 2 most likely avenues to this attack:

Stolen client endpoint authentication credentials
Stolen or misused Issuer keys held in the Transparency Service on behalf of clients

Clients relying on the Issue Statement endpoint SHOULD take steps to ensure their endpoint authentication credentials are securely stored and can be rotated and/or revoked in the case of a breach. Transparency Services offering the Issue Statement endpoint MUST require authentication and transport-level security for that endpoint, and MUST enable the rotation and revocation of those credentials. Transparency Services offering the Issue Statement endpoint MUST take careful steps in both design and operation of their software stack to prevent the theft or inappropriate use of the Issuer keys they use to sign Statements on behalf of Issuers, such as HSMs for storage and least-privilege, regularly refreshed access controls for use. Transparency Services MAY also implement additional protections such as anomaly detection or rate limiting in order to mitigate the impact of any breach.

Out of scope

Replay attacks Replay attacks are not particularly concerning for SCITT or SCRAPI: once a statement is made, it is intended to be immutable and non-repudiable, so making it twice should not lead to any particular issues. There could be issues at the payload level (for instance, the statement "it is raining" may true when first submitted but not when replayed), but being payload-agnostic implementations of SCITT services cannot be required to worry about that. If the semantic content of the payload are time dependent and susceptible to replay attacks in this way then timestamps MAY be added to the payload signed by the Issuer.

Message deletion attacks Once registered with a Transparency Service, Registered Signed Statements cannot be deleted. Thus, any message deletion attack must occur prior to registration else it is indistinguishable from a man-in-the-middle or denial-of-service attack on this interface.

TODO TODO: Consider negotiation for receipt as "JSON" or "YAML". TODO: Consider impact of media type on "Data URIs" and QR Codes.

IANA Considerations

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 , following the template in :

Well-Known URI for Issuers The following value is requested to be registered in the "Well-Known URIs" registry (using the template from ): URI suffix: issuer Change controller: IETF Specification document(s): RFCthis. Related information: N/A

Well-Known URI for Transparency Configuration The following value is requested to be registered in the "Well-Known URIs" registry (using the template from ): URI suffix: transparency-configuration Change controller: IETF Specification document(s): RFCthis. Related information: N/A TODO: Register them from here.

Media Type Registration This section requests registration of the "application/scitt.receipt+cose" media type in the "Media Types" registry in the manner described in . To indicate that the content is a SCITT Receipt:

Type name: application
Subtype name: scitt.receipt+cose
Required parameters: n/a
Optional parameters: n/a
Encoding considerations: TODO
Security considerations: TODO
Interoperability considerations: n/a
Published specification: this specification
Applications that use this media type: TBD
Fragment identifier considerations: n/a
Additional information:
- Magic number(s): n/a
- File extension(s): n/a
- Macintosh file type code(s): n/a
Person & email address to contact for further information: TODO
Intended usage: COMMON
Restrictions on usage: none
Author: TODO
Change Controller: IESG
Provisional registration? No