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+<pre>
+  BIP: 21
+  Title: URI Scheme
+  Author: Nils Schneider <nils.schneider@gmail.com>
+          Matt Corallo <bip21@bluematt.me>
+  Status: Accepted
+  Type: Standards Track
+  Created: 29-01-2012
+</pre>
+
+This BIP is a modification of an earlier [[BIP 0020]] by Luke Dashjr. BIP 0020 was based off an earlier document by Nils Schneider. The alternative payment amounts in BIP 0020 have been removed.
+
+==Abstract==
+This BIP proposes a URI scheme for making Bitcoin payments.
+
+==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.
+
+==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.
+
+=== BNF grammar ===
+
+(See also [[#Simpler syntax|a simpler representation of syntax]])
+
+ bitcoinurn     = "bitcoin:" bitcoinaddress [ "?" bitcoinparams ]
+ bitcoinaddress = base58 *base58
+ bitcoinparams  = *bitcoinparam
+ standardparam  = amountparam | labelparam | messageparam | otherparam
+ bitcoinparam   = standardparam | reqparam
+ amountparam    = "amount=" *digit [ "." *digit ]
+ labelparam     = "label=" *pchar
+ messageparam   = "message=" *pchar
+ otherparam     = pchar *pchar "=" *pchar
+ reqparam       = "req-" standardparam
+
+=== Query Keys ===
+
+*label: Label for that address (e.g. name of receiver)
+*address: bitcoin address
+*message: message that describes the transaction to the user ([[#Examples|see examples below]])
+*size: amount of base bitcoin units ([[#Transfer amount/size|see below]])
+*(others): optional, for future extensions
+
+==== Transfer amount/size ====
+
+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.
+
+Bitcoin clients MAY display the amount in any format that is not intended to deceive the user.
+They SHOULD choose a format that is foremost least confusing, and only after that most reasonable given the amount requested.
+For example, so long as the majority of users work in BTC units, values should always be displayed in BTC by default, even if mBTC or TBC would otherwise be a more logical interpretation of the amount.
+== Rationale ==
+
+===Payment identifiers, not person identifiers===
+Current best practices are that a unique address should be used for every transaction.
+Therefore, a URI scheme should not represent an exchange of personal information, but a one-time payment.
+
+===Accessibility (URI scheme name)===
+Should someone from the outside happen to see such a URI, the URI scheme name already gives a description.
+A quick search should then do the rest to help them find the resources needed to make their payment.
+Other proposed names sound much more cryptic; the chance that someone googles that out of curiosity are much slimmer.
+Also, very likely, what he will find are mostly technical specifications - not the best introduction to bitcoin.
+
+==Forward compatibility==
+Variables which are prefixed with a req- are considered required.  If a client does not implement any variables which are prefixed with req-, it MUST consider the entire URI invalid.  Any other variables which are not implemented, but which are not prefixed with a req-, can be safely ignored.  
+
+==Backward compatibility==
+As this BIP is written, several clients already implement a bitcoin: URI scheme similar to this one, however usually without the additional "req-" prefix requirement.  Thus, it is recommended that additional variables prefixed with req- not be used in a mission-critical way until a grace period of 6 months from the finalization of this BIP has passed in order to allow client developers to release new versions, and users of old clients to upgrade.
+
+== Appendix ==
+
+=== Simpler syntax ===
+
+This section is non-normative and does not cover all possible syntax.
+Please see the [[#BNF grammar|BNF grammar]] above for the normative syntax.
+
+[foo] means optional, <bar> are placeholders
+
+ bitcoin:<address>[?amount=<amount>][?label=<label>][?message=<message>]
+
+=== Examples ===
+
+Just the address:
+ bitcoin:175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W
+
+Address with name:
+ 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
+
+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
+
+Characters must be URI encoded properly.
+
+== Reference Implementations ==
+=== Bitcoin clients ===
+* [[Bitcoin-Qt]] supports the old version of Bitcoin URIs (ie without the req- prefix), with Windows and KDE integration as of commit 70f55355e29c8e45b607e782c5d76609d23cc858.
+
+[[Category:Developer]]
+[[Category:Technical]]
+[[Category:BIP|D]]
+