BIP21

BIP21, or Bitcoin Improvement Proposal 21, defines the Bitcoin URI Scheme. It standardizes a simple, URL-like syntax for representing payment requests to a Bitcoin address. The core format is bitcoin:<address>, with an optional query string that can include parameters such as amount, label, and message. Implementations parse these URIs to pre-fill payment fields in wallets or to display a payment request to the user.

The amount parameter specifies the value in bitcoins (BTC) and is typically presented in decimal form, while label provides a human-friendly description and message can convey a note to the payer. Parameters are URL-encoded, and the exact set of supported fields can vary among wallets, but the basic structure is widely recognized.

Usage and interoperability: BIP21 URIs are commonly embedded in web pages, emails, invoices, and QR codes. Wallets that recognize the scheme can extract the recipient address and optional fields to speed up the payment process, reducing friction for users who wish to pay a given invoice or request.

Security and context: BIP21 is a lightweight standard that focuses on representation and interoperability rather than authentication or verification of the payment. It complements other approaches to payment workflows, such as the more feature-rich BIP70 Payment Protocol, which provides additional security and verification mechanisms for merchant invoices.

History and impact: Introduced as part of the Bitcoin Improvement Proposals process, BIP21 became one of the most widely supported formats for encoding simple payment requests, contributing to smoother user experience across wallets and services.