Hi,
I realized that there isn't any way to ask for a signature (or to verify a message) as easily you can do when requesting a payment using a bitcoin URI scheme (BIP0021).
I think a URI scheme to use the signing tools in bitcoin core might be useful, and with a proper consensus it could become available in most bitcoin clients who already support message signing/verifying and payment url (or QRCode) and enable new uses of bitcoin signatures.
A way to gain proper consensus is going through a BIP, so that's why I'm here: to present my idea publicly before going any further (draft BIP and reference implementation).
Some thoughts
 - like BIP0021: "Bitcoin clients MUST NOT act on URIs without getting the user's authorization." so signing requires the user to manually approve the process
 - it could use the same URI scheme than BIP0021 with an additional parameter (ex: signaction=<verify/sign>) or use another one like BIP121 (ex: btcsig:)
PS : I'll also post a topic in "Development & Technical Discussion" section on Bitcointalk
 
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Arthur Bouquet