allow signing messages with 'change' addresses #3105

I just asked someone to sign a message with the address they sent a transaction from to prove he sent it. He was unable to, since the address he sent from wasn't in his 'receive' tab.

It turned out the transaction was funded by one of his 'change' addresses which don't show up on the 'receive' tab.

In the end I had to talk him through using the debug console to run 'walletpassphrase' and 'signmessage' but it's kind of ugly, and results in his wallet passphrase appearing on the screen.

It would be better if there was some way of signing a message with a change address from within the 'sign message' gui. Maybe a checkbox in 'receive' (defaulting to off) to show all addresses in the wallet including change addresses.

Agreed. Also, I would like to see Bitcoin-QT (and bitcoind?) support the simple wrapper that multibit employs.

I'm not familiar with multibit. What does its simple wrapper do?

That's a good suggestion, but I'd like to keep the distinction between the wallet abstraction (where addresses are things to receive transactions with, transactions don't have a sender, coins belong to a wallet) and the internal representation (where coins are assigned to addresses). Coin control breaks this abstraction (and I like it because it gives insight in how the system works, but only inside an advanced view). I guess it makes sense to move message signing to this advanced view as well, when coin control is merged?

It looks like the ASCII armoring provided by PGP: ---BEGIN BITCOIN SIGNED MESSAGE--- etc.

Makes life a bit easier when sending and receiving via email attachment, easier to detect truncation and other attributes. In Bitcoin-QT, a user could select a file with a file dialog, verifying (or signing) with a click.

@dooglus I am confused by this issue.

The sign message dialog in Bitcoin-qt lets you freely type in or paste the address you want to sign with already. There should have been no need to use the console.

@gmaxwell That's a good point. I was thinking I had to use one of the addresses in the 'receive' tab, because that tab's 'sign message' is inactive until I select an address. But now I see I can also pick 'sign message' from the file menu.

Already possible, so closing

I'd argue that it shouldn't be possible. Signed messages only work for receiving, not for sending. It doesn't make sense to sign with a change address.

@luke-jr Signing a message with a change address proves that the signer controls the change address that sent a transaction. Why would you argue for removing useful functionality?

What does it even mean to say "Signed messages only work for receiving, not for sending"? Signing a message with an address is unrelated to sending or receiving a transaction.

@dooglus Signing proves you receive at the address. Just because you receive at a given address, does not mean you sent the coins in a later transaction consuming the outputs (and indeed, the contrary is often true). Addresses are only used for receiving, not for sending.

If people need the ability to prove they sent a transaction, some new standard (whereby they sign with a transaction instead of an address) is needed.