Signature Creation Application (SCA).

The four ETSI actors

ETSI TS 119 432 identifies four cooperating actors in a cloud signature. Every remote signature flow in CodeB is a specific realization of these:

• Signer — the natural person whose intent binds the signature. Signs in as themselves via OIDC.
• SCA (Signature Creation Application) — composes the DTBS, gathers signer intent, orchestrates SAP, drives the RWSCD, finalises the envelope. = sign.html + csc.ashx.
• SIC (Signature Initiator Client) — where the signer starts (a document viewer, a PMS, a legal-review app). In the reference flow the SIC and SCA are collocated inside sign.html; in the API flow the SIC is your business app calling the CSC v2 API.
• RWSCD (Remote Wallet Secure Cryptographic Device) — holds the signing key, releases operations only on presentation of a valid SAD. = ICryptoModule boundary. Today: SoftwareCryptoModule.cs. Roadmap: HsmAzureKeyVaultCryptoModule.cs, HsmPkcs11CryptoModule.cs.

ETSI TS 119 432 operation → CodeB endpoint mapping

Every protocol operation in the ETSI spec has a concrete corresponding call in the CodeB stack:

Signature Activation Level (SCAL) trust anchors

Per ETSI EN 419 241-1:

ARF §5.7.5 — wallet as SAP authoriser

The July 2026 EU Digital Identity Wallet Architecture Reference Framework 3.0 defines a transaction_data parameter on OpenID4VP requests (Sec. 5.7.5, referencing OpenID4VP 1.0 Final §5.7.5). This is the mechanism that lets the EU Wallet substitute for the WebAuthn / PIN second factor in SCAL2 — making the wallet the authorised source of signer intent.

Wire format

The SCA appends ?transaction_data=<b64u> to /oidc.ashx?action=vp-start. The b64u value decodes to a JSON array of base64url-encoded transaction_data entries. Each entry decodes to a JSON object with:

{
  "type":           "qes_authorization",              // ETSI TS 119 432 SAP marker
  "credential_ids": [ "aloaha-eudi-pid" ],           // DCQL credential id(s) the entry binds to
  "documentDigests": [
    { "hash": "<b64 SHA-256 of DTBS>",
      "hashAlgorithmOID": "2.16.840.1.101.3.4.2.1",
      "label": "Contract v3.pdf" }
  ],
  "processID": "sca-txn-a1b2c3d4"
}

The server (see oidc.ashx vp-start) validates that:

• the outer array has 1–16 entries;
• each entry is a base64url string that decodes to a JSON object;
• each entry has type and a non-empty credential_ids array;
• the total value fits inside the 32 KiB body cap.

Any failure returns HTTP 400 invalid_request with an error_description that names the failing check (fail-closed per RASP policy).

What the wallet does

Per OpenID4VP 1.0 Final §5.7.5, when the wallet processes each entry it MUST:

• display the transaction context to the holder for explicit consent;
• compute hash(transaction_data_entry_string) using an agreed hash algorithm (SHA-256 baseline);
• include transaction_data_hashes and transaction_data_hashes_alg inside the KB-JWT of every SD-JWT VC in the returned VP token.

What the SCA verifies

The SCA re-computes the same hash locally, compares byte-for-byte with the wallet's echoed hash, and only then mints the SAD JWT with authorization_source=wallet. The SAD payload includes transaction_data_hashes as a signed claim so downstream RWSCD calls can verify the same binding.

What ships today (2026-07-27)

• Shipped: vp-start accepts transaction_data, validates strictly, stores on VpSession, and emits the array inside the signed JAR request object so wallets that implement §5.7.5 hash + echo correctly.
• Shipped: LOUD diagnostics ([ARF-TXDATA-DIAG]) + [METRIC] arf.txdata.accepted|rejected.
• Shipped: per-tenant persistence through SaveVpSessionToDisk so the binding survives app-pool recycles for the full 300 s TTL.
• Next iteration: KB-JWT transaction_data_hashes extraction inside vp-response + surface into the GET-poll body + SAD-issuance hook in credentials/authorize.

DTBS / DTBSR construction

Per ETSI TS 119 432 §5.3, the SCA constructs the Data To Be Signed (DTBS) and its representation (DTBSR = SHA-256 of the CMS signedAttributes). CodeB layout:

• DTBS = the bytes of the PDF covered by the ByteRange array (the whole file except the /Contents<...> hex window).
• signedAttributes = DER-sorted SET of five mandatory CMS attributes: contentType, messageDigest (SHA-256 of DTBS), signingTime, signingCertificateV2 (RFC 5035 ESSCertIDv2 with issuerSerial), id-aa-CMSAlgorithmProtection (RFC 6211).
• DTBSR = SHA-256 of the DER-encoded signedAttributes SET.
• SAD binding = the DTBSR base64 is the hash[0] value in credentials/authorize. The SAD JWT payload includes this same hash, and signHash refuses to sign any value that does not match.

This layout produces envelopes that Adobe Reader (built-in Adobe.PPKLite validator) verifies as PAdES basic profile, and that pyHanko / iText validate as PAdES-B-B / B-T / B-LT / B-LTA depending on the added unsigned attributes.

Observability + operation logging

Every protocol operation emits a diagnostic line and a metric so an auditor can reconstruct the full SCA flow from logs alone:

[CSC-SCAL2-DIAG] authorize enter tenant=<t> sub=<sub> scal=2 hashes=1
[CSC-SCAL2-DIAG] webauthn-options-minted tenant=<t> authId=<16hex> ...
[CSC-SCAL2-DIAG] confirm-ok tenant=<t> authId=<16hex> factor=webauthn+pin
[METRIC] csc.sad.issued tenant=<t> scal=2 factor=webauthn
[ARF-TXDATA-DIAG] vp-start tenant=<t> sid=<sid> tx_entries=1 raw_bytes=488
[METRIC] arf.txdata.accepted tenant=<t> entries=1
[ARF-TXDATA-DIAG] JAR-embed sid=<sid> entries=1

The metric tags are stable across releases so downstream SIEM pipelines can build dashboards.

Standards + citations

• ETSI TS 119 432 — Protocols for Remote Digital Signature Creation. SCA / SIC / RWSCD / Signer roles + protocol operation catalogue.
• ETSI EN 419 241-1 — Trustworthy Systems Supporting Server Signing. SCAL1 vs SCAL2 definitions.
• Cloud Signature Consortium API v2 — concrete protocol realization for most SCA verbs.
• OpenID4VP 1.0 Final §5.7.5 — transaction_data parameter + transaction_data_hashes return.
• EU Digital Identity Wallet Architecture Reference Framework 3.0 (21 July 2026) §5.7.5 — wallet-as-SAP profile.
• Regulation (EU) 910/2014 Articles 3(11), 26, 32 — AdES definition, validation requirements.