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Subject: [Bitcoin-development] Payment Protocol Proposal:
	Invoices/Payments/Receipts
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This is the next big "lets all agree to do things the same way" thing
I think we should tackle. I'm particularly looking for feedback from
other bitcoin client developers, even if it is just a quick "looks
reasonable, if everybody else is going to do it then I will
(eventually) too..."

Thanks to Pieter Wuille and Mike Hearn for lots of feedback and
suggestions and brainstorming.

This document is online at https://gist.github.com/4120476

If you respond to this message, please be considerate of people who
subscribe to the digest version of this mailing list and trim your
response.


Invoices, Payments and Receipts for Bitcoin Transactions
========================================================

This document proposes protocol buffer-based formats for signed,
authenticated "invoices" and "receipts" -- requests for payment, and
proof-of-payment.

Separate documents propose an extension to the Bitcoin URI syntax and
new MIME types to support them.

Motivation
==========

The idea of a "payment protocol" to improve on Bitcoin addresses has
been around for over a year. Users have been asking for some features
in this proposal (like the ability to provide a refund address so
overpayments or refunds can be returned to customers without the need
to ask them for their address) for two or three years, and have
started to work around shortcomings in the Bitcoin payment process
with creative (but inefficient) uses of transactions.

The key features of this proposal are:

+ Requests for payment (Invoices) are tied to authenticated identities
using the only widely-deployed identity authentication system we have
right now (X.509 certificates signed by root certificate authorities)
+ Invoices include a user-friendly description of what the payment is for
+ Payments include where refunds should be sent
+ At the end of the payment process, the customer holds a
cryptographically signed Receipt that can be used as proof-of-payment
if there is any dispute with the merchant.


Specification
=============

Invoice/SignedInvoice
---------------------

An Invoice is a request for payment from a merchant to a customer:

::

    message Output {
	optional uint64 amount = 1;
	required bytes script = 2;
    }

amount: Number of satoshis (0.00000001 BTC) to be paid. If not given
or zero, then the customer will be asked how much to pay.

script: a "TxOut" script to which the customer should direct payment.
This will normally be one of the standard Bitcoin transaction script
(e.g. pubkey OP_CHECKSIG).

::

    message Invoice {
        repeated bytes x509chain = 1;
        repeated Output outputs = 2;
        required uint64 time = 3;
        optional uint64 expires = 4;
        optional bool single_use = 5 [default = true];
        optional string memo = 6;
        optional string receiptURI = 7;
        optional bytes merchant_data = 8;
    }

outputs: one or more outputs where Bitcoins are to be sent.

x509chain: one or more DER-encoded X.509 certificates that identifies
the merchant. See the "Certificates" section below for details.

time: Unix timestamp (seconds since 1-Jan-1970) when the Invoice was created.

expires: Unix timestamp after which the Invoice should be considered
invalid. If not given, the Invoice may be re-used until the earliest
certificate expiration date in the X509chain.

single_use: If true, this Invoice should be used for only one payment.
If false, it may be added to the user's address book and used
repeatedly until it expires (e.g. for donations or a recurring
payment).

memo: UTF-8 encoded, plain-text (no formatting) note that should be
displayed to the customer, explaining what this Invoice is for.

receiptURI: Secure (https) URI where a Payment message (see below) may
be sent to obtain a SignedReceipt as proof-of-payment.

merchant_data : Arbitrary data ignored by the client that may be used
by the merchant to identify the Invoice.

::

    message SignedInvoice {
        required Invoice invoice = 1;
        required bytes signature = 2;
    }

A SignedInvoice is an Invoice signed using the private key
corresponding to the public key in the first certificate in the
x509chain and the HMAC SHA-256 algorithm.

When a Bitcoin client receives a SignedInvoice, it must authorize
payment by doing the following:

1. Validate the x509chain certificate chain up to it's list of root
certificate authorities
2. Validate that the time on the customer's system is before Invoice.expires
3. Display the "Common Name" (CN) string from the first x509chain
certificate and ask the customer if they would like to submit payment

Payment
-------

::

    message Payment {
        required Invoice invoice = 1;
        repeated bytes transactions = 2;
        repeated Output refund_to = 3;
        optional string memo = 4;
    }

invoice : the invoice received from the merchant. A merchant must
validate the Invoice and may reject the Payment if the Invoice was
altered by the customer.

transactions : One or more valid, signed Bitcoin transactions that
fully pay the Invoice

refund_to : One or more outputs where the merchant may return funds,
if necessary.

memo : UTF-8 encoded, plain-text note from the customer to the merchant.

If the customer authorizes payment, then the Bitcoin client:

1. Creates and signs a transaction with one output sending the Invoice.script
2. If there is no Invoice.receiptURI, then the transaction is
broadcast on the Bitcoin p2p network.
3. Else POST a Payment message to Invoice.receiptURI and expect a
SignedReceipt in response.

Invoice.receiptURI must be secure against man-in-the-middle attacks
that might alter Payment.refund_to.

*Note: an alternative would be a SignedPayment message that ties the
signatures in Payment.transactions to a signature for the entire
Payment message. Spending multisig inputs that may be controlled by
more than one person or spending arbitrary non-standard transactions
makes that non-trivial.*

Receipt/SignedReceipt
---------------------

::

    message Receipt {
        required Payment payment = 1;
        required bool accepted = 2;
        optional string memo = 3;
    }

accepted : true if the Payment is accepted and will be broadcast on
the Bitcoin p2p network.

memo : UTF-8 encoded note that should be displayed to the customer
indicating that the transaction is complete.

::

    message SignedReceipt {
        required Receipt receipt = 1;
        required bytes signature = 3;
    }

A SignedReceipt is a Receipt signed using the private key
corresponding to the public key in the first certificate in the
Receipt->Payment->Invoice.x509chain and the HMAC SHA-256 algorithm.

Upon receiving a SignedReceipt, a Bitcoin client should validate the
signature and, if valid, display the Receipt.memo and store the
SignedReceipt as proof-of-payment.

If a SignedReceipt is not received for any reason (timeout, error) and
Payment.transactions has not been broadcast by the merchant on the
Bitcoin p2p network, then the Bitcoin client should assume that the
payment failed, inform the customer that the payment failed, and
return coins involved in the transaction to the customer's wallet.


Certificates
============

The Invoice.x509chain (X.509 Certificate Chain) field contains the
X.509 public key certificate or certificate chain [RFC5280]
corresponding to the key used to digitally sign the Invoice and
Receipt. The certificate or certificate chain is represented as an
array of DER [ITU.X690.1994] PKIX certificate value. The certificate
containing the public key of the entity that digitally signed the
Invoice MUST be the first certificate. This MAY be followed by
additional certificates, with each subsequent certificate being the
one used to certify the previous one. The recipient MUST verify the
certificate chain according to [RFC5280] and reject the payment
request if any validation failure occurs.

*What should we say about root certificates and certificate management
in general? Any requirements, or leave it up to each Bitcoin client to
determine which root CA's are trustworthy, as happens with web
browsers? Gavin suggests trusting only (say) ten of the Extended
Validation authorities:
http://en.wikipedia.org/wiki/Extended_Validation_Certificate#Extended_Validation_certificate_identification
*

*X.509 is widely criticised for doing too much. However, it is the
Public Key Infrastructure (PKI) system we're stuck with. Do web
browsers / certificate authorities support the full X.509 spec, or
only a subset? Should Bitcoin clients only support some well-defined
subset of X.509 ? More research needed here... *

Use Cases
=========

Merchant Payment Service
------------------------

A merchant payment service (like Paysius or bit-pay.com) would use
Invoices and Receipts as follows:

1. Merchant pays for a certificate from a certificate authority, and
then gives the payment service the certificate and their private key.
This could be the same certificate and private key as is used for the
merchant's web site, but best security practice would be to purchase a
separate certificate for authenticating Invoices. Very successful
merchant payment services might act as intermediate certificate
authorities, issuing certificates for their merchants.
2. Customer goes through the checkout process on either the merchant's
or payment service's web site.
3. At the end of the checkout process, a SignedInvoice is generated
and sent to the customer's Bitcoin client.
4. Customer's Bitcoin client displays the Invoice, showing that the
payment is for the merchant.
5. On customer approval, a Payment is sent to the payment service's
paymentURI. The merchant is notified of the payment, and the customer
receives a SignedReceipt as proof-of-payment.

SatoshiDice
-----------

SatoshiDice (www.satoshidice.com) is an extremely popular game that
uses tiny transactions for some customer/service communications. In
particular, customers can add an extra output to their transactions to
indicate where winnings should be sent. And SatoshiDice creates tiny
transactions to let their customers know that a bet was received, but
lost.

Assuming Bitcoin clients upgrade to support this proposal, a bet on
SatoshiDice would proceed as follows:

1. Customer clicks on a link on SatoshiDice.com and their Bitcoin
client receives a SignedInvoice.
2. Customer authorizes payment, and their Bitcoin client creates a
Payment message and submits it directly to
https://satoshidice.com/something
3. The SatoshiDice web server checks to make sure the transaction is
valid, broadcasts it, and determines whether the customer wins or
loses. It returns a SignedReceipt with either a "You win" or "You
lost" memo.
4. If the customer won, it broadcasts a transaction to pay them using
Payment.refund_to
5. Customer's Bitcoin client displays the win/lose memo, and if they
won the winnings appear in their wallet when received over the p2p
network.

Multiperson Wallet
------------------

This use case starts with a multi-signature Bitcoin address or wallet,
with keys held by two different people (Alice and Bob). Payments from
that address/wallet must be authorized by both Alice and Bob, and both
are running multi-signature-capable Bitcoin clients.

Alice begins the payment process by getting a SignedInvoice from a
merchant that needs to be paid. She authorizes payment and her Bitcoin
client creates a Payment message with a partially-signed transaction,
which is then sent to Bob any way that is convenient (email
attachment, smoke signals...).

Bob's Bitcoin client validates the SignedInvoice and asks Bob to
authorize the transaction. He says OK, his Bitcoin client completes
the transaction by providing his signature, submits the payment to the
merchant, and then sends a message to Alice with the SignedReceipt he
received from the merchant, completing the payment process.


Design Notes
============

Why X.509 Certificates?
-----------------------

This proposal uses X.509 certificates as the identity system for
merchants because most of them will have already purchased a
certificate to secure their website and will be familiar with the
process of proving their identity to a certificate issuing authority.

Implementing a better global PKI is outside the scope of this
proposal. If a better PKI is adopted, the only change to this proposal
would be to replace the Invoice.x509chain with whatever that better
infrastructure uses to identify entities.


Why not JSON?
-------------

Invoice, Payment and Receipt messages could all be JSON-encoded. And
the Javascript Object Signing and Encryption (JOSE) working group at
the IETF has a draft specification for signing JSON data.

But the spec is non-trivial. Signing JSON data is troublesome because
JSON can encode the same data in multiple ways (whitespace is
insignificant, characters in strings can be represented escaped or
un-escaped, etc.), and the standards committee identified at least one
security-related issue that will require special JSON parsers for
handling JSON-Web-Signed (JWS) data (duplicate keys must be rejected
by the parser, which is more strict than the JSON spec requires).

A binary message format has none of those complicating issues. Which
encoding format to pick is largely a matter of taste, but Protocol
Buffers is a simple, robust, multi-programming-language,
well-documented, easy-to-work-with, extensible format.

What about a merchant-pays-fee feature?
---------------------------------------

It is desireable to allow a merchant to pay the cost of any Bitcoin
network transaction processing fees, so if a customer is paying for a
1 BTC item they pay exactly 1 BTC.

One way of accomplishing that is to add a 'maxfee' field to the
Invoice, and have the Bitcoin client construct a transaction that pays
the merchant (amount-maxfee).

Another way of accomplishing that is to change the transaction
selection code used by Bitcoin miners, so that dependent transactions
are considered as a group. Then a merchant with several unconfirmed
zero-fee transaction from customers can create a pay-to-self
transaction with a large enough fee to pay for the set of transactions
to be confirmed.

A third way of accomplishing that is for the Bitcoin client to sign
Payment.transactions[0] using the SIGHASH_ANYONECANPAY flag, and for
the merchant to add an additional, small-BTC-value input to the
transaction before broadcasting it. That additional input would go
directly to miners as a fee. *Note: Gavin is not sure if he loves or
hates this idea.*

Checking for revoked certificates
---------------------------------

The Online Certificate Checking Protocol (OCSP) is supposed to be a
quick and easy way for applications to check for revoked certificates.

In practice, it doesn't work very well. Certificate Authorities have
no financial incentive to support a robust infrastructure that can
handle millions of OCSP validation requests quickly.

Ideally, Bitcoin clients would use OCSP to check certificate statuses
every time they received or re-used an Invoice. But if that results in
long pauses or lots of false-positive rejections (because an OCSP
endpoint is offline or overwhelmed, perhaps) then merchants and
customers might revert to just using "never fails" Bitcoin addresses.



References
==========

Public-Key Infrastructure (X.509) working group :
http://datatracker.ietf.org/wg/pkix/charter/

RFC 2560, X.509 Internet Public Key Infrastructure Online Certificate
Status Protocol - OCSP : http://tools.ietf.org/html/rfc2560

Protocol Buffers : https://developers.google.com/protocol-buffers/

See Also
========

Javascript Object Signing and Encryption working group :
http://datatracker.ietf.org/wg/jose/

sipa's payment protocol proposal: https://gist.github.com/1237788

ThomasV's "Signed Aliases" proposal : http://ecdsa.org/bitcoin_URIs.html