# Bidirectionally Authenticated Derivation of Privacy Restricted Type 42 Keys Ty Everett () ## Abstract Sometimes, there's a need for a third party to validate the linkage and key derivation process between two transacting parties. Type-42 derivation enables the private use of child keys between the parties based on an invoice number, but doesn't allow for third-party servers to verify the linkage of the keys. Proposals like BRC-84 describe ways to enable third-party verification while keeping the sender's key linked to the derivation process, but lack a means of sender authentication. By their nature, privacy is lacking in all of thes proposals, but more can be done to preserve it. When Bitcoin is not exchanged in a peer-to-peer manner, this specification outlines the most robust approach to Bidirectionally Authenticated Derivation with Privacy Restrictions (BAD Privacy). ## Specification Senders use type-42 with the anyone key as counterparty and an invoice number in this format: `2-BAD Privacy- ` The format of the signed data is: ``` BRC-86 BAD Privacy ``` Verifiers (and servers) check: * The signature * The nonce * The child key is derivable with the invoice number and the anyone key The signature verification process for the signature embedded in the invoice number is: 1. Extract the identity key of the sender from the message 2. Invoice number is `2-BAD Signature-` 3. Verification counterparty is `anyone` (see BRC-3 for signature verification on public signatures) 4. Message to verify is specified above --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://bsv.brc.dev/key-derivation/0086.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.