[Formatted version of this post is here:
https://gist.github.com/kiminuo/cc2f19a4c5319e439fc7be8cbe5a39f9]
Hi all,
BIP 21 [https://github.com/bitcoin/bips/blob/master/bip-0021.mediawiki] defines
a URI scheme for making Bitcoin payments and the purpose of the URI scheme is
to enable users to easily make payments by simply clicking links on webpages or
scanning QR Codes. An example of a BIP21 URI is:
bitcoin:bc1qd4fxq8y8c7qh76gfnvl7amuhag3z27uw0w9f8p?amount=0.004&label=Kiminuo&message=Donation
Now to make it easier, these URIs are typically clickable. Bitcoin wallets
register the "bitcoin" URI scheme so that a BIP21 URI is parsed and data are
pre-filled in a form to send your bitcoin to a recipient. Notably, wallets do
not send your bitcoin once you click a BIP21 URI, there is still a confirmation
step that requires user's attention. Very similar experience is with a QR code
that encodes a BIP21 URI where one just scans a QR code and data is, again,
pre-filled in a wallet's UI for your convenience.
While working on Wasabi's BIP21 implementation I noticed that based on the
BIP21 grammar
[https://github.com/bitcoin/bips/blob/master/bip-0021.mediawiki#abnf-grammar],
it is actually allowed to specify URI parameters multiple times. This means
that the following URI is actually valid:
bitcoin:bc1qd4fxq8y8c7qh76gfnvl7amuhag3z27uw0w9f8p?amount=0.004&label=Kiminuo&message=Donation&amount=1.004
(note that the 'amount' parameter is specified twice)
Bitcoin Core implements "the last value wins" behavior[^3] so amount=1.004 will
be taken into account and not "amount=0.004"[^4]. However, in general, the fact
that the same parameter can be specified multiple times can lead to a confusion
for users and developers[^1][^2]. In the worst case, it might be exploited by
some social engineering attempts by attempting to craft a 'clever' BIP21 URI
and exploting behavior of a particular wallet software. For the record, I'm not
aware that it actually happens, so this is rather a concern.
The main question of this post is: Is it useful to allow specifying BIP21
parameters multiple times or is it rather harmful?
Regards,
K.
[^1]: https://github.com/JoinMarket-Org/joinmarket-clientserver/pull/1510
[^2]:
https://github.com/MetacoSA/NBitcoin/blob/93ef4532b9f2ea52b2c910266eeb6684f3bd25de/NBitcoin/Payment/BitcoinUrlBuilder.cs#L74-L78
[^3]: I added a test to that effect in
https://github.com/bitcoin/bitcoin/pull/27928/files, see
https://github.com/bitcoin/bitcoin/blob/83719146047947e588aa0c7b5eee02f44884553d/src/qt/test/uritests.cpp#L68-L73.[^4]:
You can test your wallet's behavior by scanning the last image here
https://github.com/zkSNACKs/WalletWasabi/pull/10578#issue-1687564404 (or
directly
https://user-images.githubusercontent.com/58662979/265389405-16893ce8-7c19-4262-bb60-5fd711336685.png).
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