<pre>
  BIP: 321
  Layer: Applications
  Title: URI Scheme
  Authors: Matt Corallo <bip21@bluematt.me>
  Status: Complete
  Type: Specification
  Assigned: 2024-11-15
  License: BSD-2-Clause
  Replaces: 21
</pre>

== Copyright ==

This BIP is licensed under the BSD 2-clause license.

== Abstract ==

This BIP proposes a URI scheme for describing Bitcoin payment instructions.

== Motivation ==

The purpose of this URI scheme is to enable users to easily make payments by simply clicking links on webpages or scanning QR Codes.

This BIP is a modification and intended replacement of [[bip-0021.mediawiki|BIP 0021]] to add information about the modern usage of bitcoin: URIs (including standard query parameters and modern address types) as well as provide forward-looking guidance on how to incorporate new payment instructions. It further adds an optional extension to provide the payment initiator with proof of payment. BIP 21 was based on BIP 20, which was, in turn based on an earlier document by Nils Schneider.

== Specification ==

=== General rules for handling (important!) ===

Bitcoin clients MUST NOT act on URIs without getting the user's authorization.
They SHOULD require the user to manually approve each payment individually, though in some cases they MAY allow the user to automatically make this decision.

=== Operating system integration ===

Graphical bitcoin clients SHOULD register themselves as the handler for the "bitcoin:" URI scheme by default, if no other handler is already registered. If there is already a registered handler, they MAY prompt the user to change it once when they first run the client.

=== General Format ===

Bitcoin URIs follow the general format for URIs as set forth in RFC 3986. The path component consists of a bitcoin address, and the query component provides additional payment options.

Elements of the query component may contain characters outside the valid range. These must first be encoded according to UTF-8, and then each octet of the corresponding UTF-8 sequence must be percent-encoded as described in RFC 3986.

=== ABNF grammar ===

 bitcoinurn     = "bitcoin:" [ bitcoinaddress ] [ "?" bitcoinparams ]
 bitcoinaddress = *base58 / *bech32 / *bech32m
 bitcoinparams  = bitcoinparam [ "&" bitcoinparams ]
 bitcoinparam   = [ amountparam / labelparam / messageparam / responseparam / otherparam / reqparam ]
 amountparam    = "amount=" *digit [ "." *digit ]
 labelparam     = "label=" *qchar
 messageparam   = "message=" *qchar
 responseparam  = [ "req-" ] "pop=" *qchar
 otherparam     = qchar *qchar [ "=" *qchar ]
 reqparam       = "req-" qchar *qchar [ "=" *qchar ]

Here, "qchar" corresponds to valid characters of an RFC 3986 URI query component, excluding the "=" and "&" characters, which this BIP takes as separators.

The scheme component ("bitcoin:") is case-insensitive, and implementations must accept any combination of uppercase and lowercase letters. The query parameter keys are also case-insensitive. Query parameter values and bitcoin address fields may be case-sensitive depending on their content.

Multiple query parameters with the same key MAY be included for query parameters representing payment instructions. Multiple query parameters with the same key MUST NOT be included for keys "label", "message", or "pop". Multiple query parameters with the same key for other keys MUST be allowed for unknown query parameters. Future query parameter keys may or may not allow for duplicate parameters.

=== Bitcoin Address ===

The bitcoinaddress body MUST be either a base58 P2SH or P2PKH address, bech32 Segwit version 0 address, bech32m Segwit address, or empty. Future address formats SHOULD instead be placed in query keys as optional payment instructions to provide backwards compatibility during upgrade cycles. The bitcoinaddress part of the URI MAY be left empty if there is at least one payment instruction provided in a query parameter, allowing for recipients wishing to avoid a standard on-chain fallback.

=== Query Keys ===

The following keys are defined generally and apply to any URI regardless of payment instructions:

*label: Label for the recipient (e.g. name of receiver)
*message: message that describes the transaction to the user ([[#examples|see examples below]])
*pop: a URI which the Bitcoin Wallet may return to in order to provide the application which initiated the payment with proof that a payment was completed.

The following keys are currently defined for payment instructions of various forms:

*lightning: Lightning BOLT 11 invoices
*lno: Lightning BOLT12 offers
*pay: [[bip-0351.mediawiki|BIP 351 Private Payment addresses]]
*sp: [[bip-0352.mediawiki|BIP 352 Silent Payment addresses]]

New payment instructions using bech32 or bech32m encodings SHOULD reuse their address format's Human Readable Part as the parameter key.

==== Transfer amount ====

If an amount is provided, it MUST be specified in decimal BTC.
All amounts MUST contain no commas and use a period (.) as the separating character to separate whole numbers and decimal fractions.
I.e. amount=50.00 or amount=50 is treated as 50 BTC, and amount=50,000.00 is invalid.

=== Proof of Payment ===

The URI MAY include a "pop" (or "req-pop") parameter whose value can be used to build a URI which the wallet application can, after payment completes, "open" to provide proof the payment was completed or other information about the payment.

The value of a "pop" (or "req-pop") parameter shall be a percent-encoded (per RFC 3986 section 2.1) URI prefix. The wallet application, if it supports providing payment information SHOULD percent-decode the provided URI once, append the query parameter key from which the payment instructions used were read, append a single =, and finally append the Payment Information to the resulting URI and open it with the default system handler for the URI. For payment instructions read from the body of the URI, "onchain" SHALL be used in place of the key.

A wallet MUST validate that the provided URI's scheme is not (case-insensitive) "http", "https", "file", "javascript", "mailto" or any other scheme which will open in a web browser prior to opening it.

If a wallet will not open the pop scheme (either because it does not support returning payment information for the selected payment method or because it uses a URI scheme which should not be opened) and the parameter was passed as a "req-pop" parameter, the wallet MUST NOT initiate payment.

For payments made using an on-chain transaction, the Payment Information shall be the full (including witness data) Bitcoin transaction as it was broadcasted to the Bitcoin network, encoded in hex.

For payments made using a BOLT 11 invoice (communicated via the `lightning` parameter), the Payment Information shall be the hex-encoded payment preimage.

Other payment schemes will define their own Payment Information format. This BIP may be updated from time to time with Payment Information formats for other payment schemes.

== Rationale ==

=== Payment identifiers, not person identifiers ===

Best practices are that a unique address should be used for every transaction on-chain.
Therefore, a URI which contains an on-chain payment address MUST NOT represent an exchange of personal information, but a one-time payment instruction. URIs which represent only reusable non-address-reusing payment instructions (like Lightning BOLT12 offers or Silent Payments) MAY be reused as a wallet sees fit.

=== Proof of Payment ===

On many mobile operating systems (especially, or any operating system more generally), applications may "open" a bitcoin: URI in order to initiate a payment with the user's default wallet application. These payment-initiating applications may wish to learn about the completed payment.

For payments completed on-chain, this is largely addressed by having the payment-initiating application monitor the blockchain for payment completion, however for other payment schemes (e.g. lightning), no such global ledger of transactions exists. In that case, proof of payment must be provided via some other mechanism.

In order to avoid inadvertently revealing the sender's IP address or other information to the recipient, proof URIs must only be opened when they will simply switch to another locally-installed application (i.e. the application which initiated the payment). When clicking a URI from a website, the website should already have plenty of logic on its backend to process payment completion and a proof-of-payment callback is unnecessary.

== Reference Implementation ==

Documentation: https://docs.rs/bitcoin-payment-instructions

Code repository: https://github.com/rust-bitcoin/bitcoin-payment-instructions

== Forward compatibility ==

Query parameter keys which are prefixed with a req- are considered required.  If a client does not implement handling a query parameter which has a key prefixed with req-, it MUST consider the entire URI invalid.  Any other query parameters which are not implemented, but which are not prefixed with a req-, can be safely ignored.

As future new address types should be added using query parameters rather than the `bitcoinaddress` field, URIs seamlessly support various payment instructions while senders only need to support legacy instructions. This allows old senders to pay newer recipients which offer more modern payment instruction formats.

== Backward compatibility ==

Compared to BIP 21, this document describes standard query parameters containing payment instructions, makes query parameters case-insensitive, allows bech32 and bech32m addresses in the `bitcoinaddress` field, and allows for URIs with an empty `bitcoinaddress` field. Use of bech32 and bech32m `bitcoinaddress` fields were long-since common practice in 2024, and the `lightning` query parameter storing BOLT 11 payment instructions became common practice in the year or three leading up to 2024. Inclusion of standard query parameters was added to provide guidance on query parameter usage going forward.

Additionally, this BIP describes the "pop" query parameter, which was unused and will be ignored by BIP 21 implementations.

Any existing BIP 21 implementation should automatically be fully compliant with this BIP, as the changes only describe existing practice or impact future address format inclusion, with the one possible exception of query parameters being made case-insensitive. Note, however, that treating query parameters as case-insensitive is already common practice due to the use of mostly-uppercase URIs in QR codes.

== Appendix ==

=== Examples ===

Note: The addresses used in these examples are intentionally invalid to prevent accidental transactions.

==== URIs ====

Just the address:
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W

Address with recipient's name as label:
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W?label=Luke-Jr

Request 20.30 BTC to "Luke-Jr":
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W?amount=20.3&label=Luke-Jr

Request 50 BTC with message:
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W?amount=50&label=Luke-Jr&message=Donation%20for%20project%20xyz

Request funds to be paid over lightning to a BOLT 11 invoice with a fallback to on-chain payments:
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W?lightning=lnbc420bogusinvoice

Request funds to be paid over lightning to a BOLT 11 invoice with no fallback:
 bitcoin:?lightning=lnbc420bogusinvoice

Request funds to be paid over lightning to a BOLT 12 offer with no fallback:
 bitcoin:?lno=lno1bogusoffer

Request funds to be paid over lightning to a BOLT 12 offer or silent payments address with no fallback:
 bitcoin:?lno=lno1bogusoffer&sp=sp1qsilentpayment

Request funds to be paid to a silent payments address with no fallback:
 bitcoin:?sp=sp1qsilentpayment

Request funds to be paid to a silent payments address with a fallback:
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W?sp=sp1qsilentpayment

Some future version that has variables which are (currently) not understood and required and thus invalid:
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W?req-somethingyoudontunderstand=50&req-somethingelseyoudontget=999

Some future version that has variables which are (currently) not understood but not required and thus valid:
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W?somethingyoudontunderstand=50&somethingelseyoudontget=999

Multiple segwit addresses may be included for various versions of segwit, note that the human-readable part for all of them is `bc`
 bitcoin:?bc=bc1qufgy354j3kmvuch987xe4s40836x3h0lg8f5nq&bc=bc1p5swkugezn97763tl0yty6556856uug0q6jflljvep9m4p7339x5qzyrh4q

Many QR codes utilize all-uppercase URIs, which should be handled fine
 BITCOIN:BC1QUFGY354J3KMVUCH987XE4S40836X3H0LG8F5NQ?BC=BC1P5SWKUGEZN97763TL0YTY6556856UUG0Q6JFLLJVEP9M4P7339X5QZYRH4Q
 BITCOIN:?BC=BC1QUFGY354J3KMVUCH987XE4S40836X3H0LG8F5NQ&BC=BC1P5SWKUGEZN97763TL0YTY6556856UUG0Q6JFLLJVEP9M4P7339X5QZYRH4Q

A testnet segwit addresses must be included in the `tb` parameter
 bitcoin:?tb=tb1qghfhmd4zh7ncpmxl3qzhmq566jk8ckq4gafnmq

Characters must be URI encoded properly.

==== Invalid URIs ====

Labels must not appear twice:
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W?label=Luke-Jr&label=Matt

Amounts must not appear twice:
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W?amount=42&amount=10

Amounts must not appear twice even if they are the same:
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W?amount=42&amount=42

Multiple proof of payment URIs must not appear, even if they are sometimes prefixed with req-:
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W?pop=callback%3a&req-pop=callback%3a

A testnet segwit addresses must be included in the `tb` parameter, not the `bc` parameter.
 bitcoin:?bc=tb1qghfhmd4zh7ncpmxl3qzhmq566jk8ckq4gafnmq

==== Proof of Payment ====

If the original URI is
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W?pop=initiatingapp%3a
the wallet should perform the payment information callback by opening
 initiatingapp:onchain=$HEX_ENCODED_TRANSACTION

If the original URI is
 bitcoin:?lightning=lnbc420bogusinvoice&pop=callbackuri%3abody%3fpop=
the wallet should perform the proof-of-payment callback by opening
 callbackuri:body?pop=lightning=$HEX_ENCODED_PAYMENT_PREIMAGE

If the original URI is
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W?lightning=lnbc420bogusinvoice&pop=app%3a%3f
and the wallet pays on-chain, it should perform the payment information callback by opening
 app:?onchain=$HEX_ENCODED_TRANSACTION
but if the app pays using lightning, it should perform the proof-of-payment callback by opening
 app:?lightning=$HEX_ENCODED_PAYMENT_PREIMAGE

If the original URI is
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W?pop=https%3aiwantyouripaddress.com
the wallet should make a payment as it normally would but MUST NOT interact with iwantyouripaddress.com

If the original URI is
 bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W?req-pop=https%3aevilwebsite.com
the wallet MUST NOT make a payment
