Chapter 11 · Tabs

The escape hatch

No typed details class — a JSON payload in a general-purpose VDXF object, with chain-time alignment and bigint arithmetic instead of floats.

FIGURE 11.1 · The newest tab takes a different approach.

Why this tab is different

Every other tab has a purpose-built details class in the primitives library. Transaction signing does not: it sends a JSON payload describing a pre-built transaction, inside a general-purpose VDXF object keyed by a text key. That is the escape hatch the library provides for request types it does not model yet.

new GeneralTypeOrdinalVDXFObject({
  type: VDXF_OBJECT_RESERVED_BYTE_VDXF_ID_STRING,
  key: TX_SIGNING_TEMPLATE_VDXF_TEXT_KEY,
  data: Buffer.from(JSON.stringify({
    type: "tx-signing-template",
    chain: isTestnet ? "VRSCTEST" : "VRSC",
    signed, signingId,
    hextx,
    outputtotalsSats, feeSats
  }), "utf-8")
})

The details that show real care

Amounts never touch floats. parseFixedPointToSats rejects scientific notation, requires at most eight decimals, and does the arithmetic in bigint.
The hex is validated before it is embedded. Even length, hex characters only — a malformed transaction is a 400, not a QR that fails at the wallet.
createdAt is aligned to chain time. alignCreatedAtToChainTime asks for the current height, fetches the block, and uses the block's timestamp — so a request generated on a machine with a drifting clock is still inside the wallet's validity window.
The signature is applied conditionally. applyOptionalSignature and verifySignedRequestSignature let the tab return either an unsigned template or a verified signed request.

What the payload promises

Field	Meaning
`hextx`	The exact transaction to sign
`outputtotalsSats`	Per-destination totals the wallet should verify against the transaction
`feeSats`	Declared fee, so the wallet can refuse a mismatch
`chain`	Which network the transaction belongs to

General-purpose objects are how you ship a request type before the library models it. The cost is that nothing validates your JSON but your own code — which is why this file is 329 lines for one payload.

Reading the code

src/routes/requests/txSigning.ts
  parseFixedPointToSats / parseHexTransaction / parseOutputTotals
  buildTxSigningRequest / alignCreatedAtToChainTime
  verifySignedRequestSignature / applyOptionalSignature
  generateTxSigningQr