BIP 121: Proof of Payment URI scheme

In plain English: Defines a link format (URI) for requesting a Proof of Payment, as described in BIP 120. A service can present such a link or QR code, and the user's wallet can respond by proving it made the relevant payment.

Status

Closed

Type

Specification

Layer

Applications

Authors

Kalle Rosenbaum

Assigned

2015-07-27

Last changed

2025-10-08

Topics

Scripts

Source

bip-0121.mediawiki on GitHub · Markdown

Abstract

This is a proposal for a URI scheme to be used in the Proof of Payment process.

Motivation

To make a Proof of Payment, the party that wants the proof needs to transfer a Proof of Payment request to the wallet software of the other party. To facilitate that transfer, a new URI scheme representing the PoP request is proposed. This URI can then be encoded in QR images or be sent over NFC in order to transfer it to the wallet.

Specification

The specification is the same as BIP0021, with the following differences:

• The URI scheme is btcpop instead of bitcoin

• The path component, i.e. the address part, is always empty.

• A mandatory p parameter whose value contains the destination for the PoP. This could for example be a https: URL or a mailto: URI.

• A mandatory n parameter representing the nonce, base58 encoded.

• An optional txid parameter containing the Base58 encoded hash of the transaction to prove.

Just as in BIP0021, 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.

All parameters except p and n are hints to the wallet on which transaction to create a PoP for.

The extensibility of BIP0021 applies to this scheme as well. For example, a date parameter or a toaddr parameter might be useful. req-* parameters are also allowed and obey the same rules as in BIP0021, clients not supporting a req-* parameter must consider the URI invalid.

Keep URIs short

Implementations should keep the URIs as short as possible. This is because it makes QR decoding more stable. A camera with a scratched lens or low resolution may run into problems scanning huge QR codes. This is why the txid parameter is encoded in Base58 instead of the classic hex encoded string. We get away with 44 characters instead of 64. Also, the nonce parameter is Base58 encoded for the same reason.

Interpretation

Transaction hints

The wallet processing the URI must use the hints in the PoP request to filter its transaction set. The label, amount and message parameters must, if present in the URI, exactly match the data associated with the original payment according to the following table:

The txid parameter value must match the transaction hash of the payment.

After filtering, the resulting transaction set is displayed to the user who selects one of them to prove. An implementation could also automatically select a transaction in the filtered set, but there must still be a way for the user to select freely among the matching transactions. If the filtered set is empty, no transaction fits the hints and a message about that is presented to the user. If the filtered set contains exactly one transaction, which is preferable, that transaction can be automatically selected.

As a fallback, there must also be a way for the user to select any transaction from the wallet regardless of the transaction hints. This can be useful if the metadata of the wallet is lost, possibly due to a restore from backup.

PoP destination p

The p parameter value is the destination where to send the PoP to. This destination is typically a https: URL or a http: URL, but it could be any type of URI, for example mailto:. To keep btcpop: URIs short, users should not make their p parameter unnecessarily long.

http: and https: URLs

Wallet implementations must support the http: and https: schemes in which case POST method must be used. The PoP is sent as a binary serialized transaction. The content type of the POST request must be set to application/bitcoin-pop

Examples

Send PoP for a transaction with label "video 42923" to https://www.example.com/pop/352, using nonce 0x73 0xd5 0x1a 0xbb 0xd8 0x9c:

 btcpop:?p=https://www.example.com/pop/352&n=zgWTm8yH&label=video%2042923

Send PoP through mail using mailto:[email protected]?subject=pop444, amount is 13370000 satoshis, nonce is 0x6f 0xe 0xfb 0x68 0x92 0xf9. Note that the ? before subject is OK according to RFC3986, since the query part starts from the first ?:

 btcpop:?p=mailto:[email protected]?subject%3Dpop444&n=xJdKmEbr&amount=0.1337

Send PoP for transaction with id cca7507897abc89628f450e8b1e0c6fca4ec3f7b34cccf55f3f531c659ff4d79 to pizza place at http://pizza.example.com/pop/laszlo111 using nonce 0xfc 0xcc 0x2c 0x35 0xf0 0xb8

 btcpop:?p=http://pizza.example.com/pop/laszlo111&n=3AtNpVrPh&txid=Emt9MPvt1joznqHy5eEHkNtcuQuYWXzYJBQZN6BJm6NL

Reference implementation

PoP Demo server on GitHub

PoP-enabled Mycelium fork on GitHub

References

BIP0021: URI Scheme

BIP0120: Proof of Payment

RFC3986: Uniform Resource Identifier (URI): Generic Syntax