<pre>
  BIP: 121
  Title: Proof of Payment URI scheme
  Author: Kalle Rosenbaum <kalle@rosenbaum.se>
  Status: Draft
  Type: Standards Track
  Created: 2015-07-27
</pre>

== 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 <tt>btcpop</tt> instead of <tt>bitcoin</tt>
* The path component, i.e. the address part, is always empty.
* A mandatory <tt>p</tt> parameter whose value contains the destination for the PoP. This could for example be a <tt>https:</tt> URL or a <tt>mailto:</tt> URI.
* A mandatory <tt>n</tt> parameter representing the nonce, base58 encoded.
* An optional <tt>txid</tt> 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 <tt>p</tt> and <tt>n</tt> 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 <tt>date</tt> parameter or a <tt>toaddr</tt> parameter
might be useful. <tt>req-*</tt> parameters are also allowed and obey
the same rules as in BIP0021, clients not supporting a <tt>req-*</tt>
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 <tt>txid</tt> parameter is encoded in Base58
instead of the classic hex encoded string. We get away with 44
characters instead of 64. Also, the <tt>nonce</tt> 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 <tt>label</tt>, <tt>amount</tt> and
<tt>message</tt> parameters must, if present in the URI, exactly match
the data associated with the original payment according to the
following table:

{|
| <tt>btcpop:</tt> URI parameter || <tt>bitcoin:</tt> URI parameter || BIP70 PaymentDetails data
|-
| <tt>label</tt>                 || <tt>label</tt>                  || <tt>memo</tt>
|-
| <tt>amount</tt>                || <tt>amount</tt>                 || <tt>sum of outputs.amount</tt>
|-
| <tt>message</tt>               || <tt>message</tt>                || <tt>-</tt>
|}

The <tt>txid</tt> 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 <tt>p</tt> ===

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

==== <tt>http:</tt> and <tt>https:</tt> URLs ====

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

== Examples ==

Send PoP for a transaction with label "video 42923" to
<nowiki>https://www.example.com/pop/352</nowiki>, using nonce
<tt>0x73 0xd5 0x1a 0xbb 0xd8 0x9c</tt>:
<pre>
 btcpop:?p=https://www.example.com/pop/352&n=zgWTm8yH&label=video%2042923
</pre>
Send PoP through mail using
<nowiki>mailto:pop@example.com?subject=pop444</nowiki>, amount
is 13370000 satoshis, nonce is <tt>0x6f 0xe 0xfb 0x68 0x92 0xf9</tt>.
Note that the <tt>?</tt> before <tt>subject</tt> is OK according to RFC3986,
since the query part starts from the first <tt>?</tt>:
<pre>
 btcpop:?p=mailto:pop@example.com?subject%3Dpop444&n=xJdKmEbr&amount=0.1337
</pre>
Send PoP for transaction with id
<tt>cca7507897abc89628f450e8b1e0c6fca4ec3f7b34cccf55f3f531c659ff4d79</tt>
to pizza place at <nowiki>http://pizza.example.com/pop/laszlo111</nowiki> using nonce <tt>0xfc 0xcc 0x2c 0x35 0xf0 0xb8</tt>
<pre>
 btcpop:?p=http://pizza.example.com/pop/laszlo111&n=3AtNpVrPh&txid=Emt9MPvt1joznqHy5eEHkNtcuQuYWXzYJBQZN6BJm6NL
</pre>
== Reference implementation ==

[https://github.com/kallerosenbaum/poppoc PoP Demo server on GitHub]

[https://github.com/kallerosenbaum/wallet PoP-enabled Mycelium fork on GitHub]

== References ==

[https://github.com/bitcoin/bips/blob/master/bip-0021.mediawiki BIP0021]: URI Scheme

[https://github.com/bitcoin/bips/blob/master/bip-0120.mediawiki BIP0120]: Proof of Payment

[https://www.ietf.org/rfc/rfc3986.txt RFC3986]: Uniform Resource Identifier (URI): Generic Syntax
