SCITT Reference APIs

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.¶

2.1. Authenticating Clients

TBD (comments on OAuth / Client Attestation).¶

2.2. Discovering Federation

TBD (comments on GAIN / OIDC).¶

2.3. Discovering Feeds

TBD (comments on URLs / QR Codes).¶

3.1. Key Binding Confirmation

In cases where a signed statement is issued by one party and registered by another, there is a need to prove possession of key material and detect tampering while authenticating both parties.¶

Typically a nonce would be chosen by the transparency service and the second party would sign over the nonce, when registering the first issuer's signed statement.¶

In order to avoid interactivity and improve interoperability, document describes a non-exclusive, but mandatory to support, confirmation scheme¶

In this scheme the verifier's challenge is a recent Unix timestamp, the presenting party need not request this information from the transparency service.¶

Here is an example key binding token that can be paired with the confirmation claim in a signed statement:¶

{
  "iat": 1698077790,
  "aud": "https://transparency.example",
  "nonce": "1698077790"
}

When applying registration policies to signed statements with confirmation, the transparency service acts as a verifier, and performs the following checks:¶

1. verify the integrity of the issuer's signed statement¶
2. confirm the verified content meets the registration policy for the transparency service.¶
3. verify the key binding token, using the confirmation claim in the verified issuer signed statement¶
4. ensure the key binding token has a nonce that is a string representation of a recent Unix timestamp¶

The exact window of validity for proving possession is a configuration detail of the transparency service. Unix timestamps are used so that only a losely synchronised notion of time need be assumed and there is no requirement to account for timezones.¶

If the confirmation key is stolen, the attacker can produce key binding tokens from that point forward in time. In an interactive confirmation schema, the transparency service can force the confirmation key holder to produce a signature over a nonce that is not guessable, and this prevents certain attacks related to the duration of access to a signing capability and other timing details. However, the cost of coordinating with the transparency service, coupled with the purpose of registering with a transparency service (to obtain a receipt, proving a signed statement was acceptable at a point in time) justify specifying the recent timestamp nonce as a mandatory to implement context binding.¶

In the case that a SCITT transparency service wants to support challenges (nonces) that are context binding, the transparency service can expose a "challenge token endpoint".¶

This endpoint can process request parameters, and issuer a challenge token, that future registrations can use to bind to the original request. This interaction model works well for scenarios where requirements for a given registration might change over time, but it is important for the registering party to commit to acceptable values at the time that a signed statement is registered. These endpoints are optional to implement.¶

GET https://transparency.example/registration/challenge

POST https://transparency.example/registration

3.2. 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:".¶
• 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": "Signing algorithm 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.¶

POST https://transparency.example/entries

GET https://transparency.example/operations/{operation_id}

GET https://transparency.example/entries/{entry_id}

GET https://transparency.example/entries/{entry_id}/receipt

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

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

   Registry name:  scitt

   Specification:  [RFCthis]

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

   Index value:  No transformation needed.

6.2. Media Types

TODO: Register them from here.¶

6.3. Well Known URIs

For discovering scitt configuration.¶

TODO: Register them from here.¶

6.4. Media Type Registration

This section requests registration of the "application/receipt+cose" media type [RFC2046] in the "Media Types" registry in the manner described in [RFC6838].¶

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

To indicate that the content is a SCITT Receipt:¶

• Type name: application¶
• Subtype name: 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¶