Merge pull request bitcoin#377 from OpenBitcoinPrivacyProject/bip47

BIP-0047: version 2 payment codes

Author: luke-jr

bip-0047.mediawiki

@@ -1,7 +1,7 @@
 RECENT CHANGES:
+* (19 Apr 2016) Define version 2 payment codes
 * (17 Apr 2016) Clarify usage of outpoints in notification transactions
 * (18 Dec 2015) Update explanations to resolve FAQs
-* (12 Oct 2015) Revise blinding method for notification transactions
 
 <pre>
   BIP: 47
@@ -84,7 +84,27 @@ Hardened derivation is used at this level.
 
 Except where noted, all keys derived from a payment code use the public derivation method.
 
-==Standard Payment Codes (v1)==
+==Versions==
+
+Payment codes contain a version byte which identifies a specific set of behavior.
+
+Unless otherwise specified, payment codes of different versions are interoperable. If Alice uses a version x payment code, and Bob uses a version y payment code, they can still send and receive transactions between each other.
+
+Currently specified versions:
+
+* Version 1
+** Address type: P2PKH
+** Notification type: address
+* Version 2
+** Address type: P2PKH
+** Notification type: bloom-multisig
+
+===Recommended Versions===
+
+* Wallets which have bloom filtering capabilities SHOULD create version 2 payment codes instead of version 1 payment codes.
+* Version 1 payment codes are only recommended for wallets which lack access to bloom filtering capability.
+
+==Version 1==
 
 ===Representation===
 
@@ -127,6 +147,8 @@ It is assumed that Alice can easily obtain Bob's payment code via a suitable met
 
 Prior to the first time Alice initiates a transaction to Bob, Alice MUST inform Bob of her payment code via the following procedure:
 
+Note: this procedure is used if Bob uses a version 1 payment code (regardless of the the version of Alice's payment code). If Bob's payment code is not version 1, see the appropriate section in this specification.
+
 # Alice constructs a transaction which sends a small quantity of bitcoins to Bob's notification address (notification transaction)
 ## The inputs selected for this transaction MUST NOT be easily associated with Alice's notification address
 # Alice derives a unique shared secret using ECDH:
@@ -180,6 +202,17 @@ Alice SHOULD use an input script in one of the following standard forms to expos
 
 Compatible wallets MAY provide a method for a user to manually specify the public key associated with a notification transaction in order to recover payment codes sent via non-standard notification transactions.
 
+=====Post-Notification Privacy Considerations=====
+
+Incautious handling of change outputs from notification transactions may cause unintended loss of privacy.
+
+The recipient of a transaction which spends a change output from a prior notification transaction will learn about the potential connection between the sender and the recipient of the notification transaction.
+
+The following actions are recommended to reduce this risk:
+
+* Wallets which support mixing SHOULD mix change outputs from notification transactions prior to spending them
+* Wallets which do not support mixing MAY simulate mixing by creating a transaction which spends the change output to the next external BIP44 address
+
 ====Sending====
 
 # Each time Alice wants to initiate a transaction to Bob, Alice derives a unique P2PKH address for the transaction using ECDH follows:
@@ -294,6 +327,41 @@ The sender transmits their payment code in base58 form to the calculated Bitmess
 
 In order to use Bitmessage notification, the recipient must have a Bitmessage client which listens at the address which the senders will derive and is capable of relaying received payment codes to the Bitcoin wallet.
 
+==Version 2==
+
+Version 2 payment codes behave identifically to version 1 payment codes, except as modified below.
+
+===Representation===
+
+====Binary Serialization====
+
+* Byte 0: version. required value: 0x02
+
+===Protocol===
+
+====Definitions====
+
+* Notification change output: the change output from a notification transaction which which resides in the sender's wallet, but can be automatically located by the intended recipient
+* Payment code identifier: a 33 byte representation of a payment code constructed by prepending 0x02 to the SHA256 hash of the binary serialization of the payment code
+
+====Notification Transaction====
+
+Note: this procedure is used if Bob uses a version 2 payment code (regardless of the the version of Alice's payment code). If Bob's payment code is not version 2, see the appropriate section in this specification.
+
+# Construct a notification transaction as per the version 1 instructions, except do not create the output to Bob's notification address
+# Create a notification change address as follows:
+## Obtain the pubkey corresponding to the next change address
+## Construct a multisig output in the form:
+<pre>OP_1 <Bob's payment code identifier> <change address pubkey> OP_2 OP_CHECKMULTISIG</pre>
+
+The relative ordering of the payment code identifier and change address pubkey in the above script MAY be randomized
+
+Bob detects notification transactions by adding his payment code identifier to his bloom filter.
+
+# When the filter returns a notification transaction, the sender's payment code is unblinded using the same procedure as for version 1 notification transactions.
+
+Alice's wallet should spend the notification change output at the next appropriate opportunity.
+
 ==Test Vectors==
 
 * [[https://gist.github.com/SamouraiDev/6aad669604c5930864bd|BIP47 Reusable Payment Codes Test Vectors]]