Quote from: davideak on November 20, 2013, 11:46:26 PM

Hi all,

I've been reading a lot about how the sending address is determined

https://bitcointalk.org/index.php?topic=189894.msg1972257#msg1972257

https://bitcointalk.org/index.php?topic=198048.0

But there is one thing I was not able to figure out:
If my client (Bitcoin-Qt) uses more than one address to send the appropriate funds like in this transaction

Which address is the "Sending" one?

do I sign a message with "..7hE" or "..PrL"? or can I choose and sign a message with any one of them to prove I'm the sender?

Thanks in advance.

You can't.
There are probably people that will tell you otherwise but, unfortunately for you (although perhaps fortunately for those of us that receive large amounts of email and sometimes can't give everybody a response as immediate as the one which they feel they deserve -- and frankly I don't see why I should respond "in the next two hours" to an email sent at 8pm on a Friday evening as some of my correspondents expect) there is no reliable way of doing this.

The email system is rather loose-knit and wasn't even really intended to provide an immediate service in the first instance (indeed, it's still possible for it to take a matter of hours or days for an email to propagate from sender to recipient although it generally requires some sort of infrastructure failure for that to occur). As such, facilities for doing this sort of thing have never been reliably incorporated.

There is a partial solution in the form of "receipts". Most modern email clients allow the sender to request a receipt (which is essentially a return email generated automatically by the email client used by the recipient) under one of two circumstances:

email received
email read

The first of these corresponds to the email being downloaded by the mail client (or in some instances the server so this can be pretty far removed from what you want to know). The second corresponds to the particular message being opened in the mail client (note that there's no way for the software to know that it's been read; it could just have been the user pressing the wrong key or skipping through all of the mail in their inbox).

The second of those is probably sounding okay to you, and you're probably now wondering why my summary line is "you can't". The answer is that the software the person you're emailing uses to check their email works for them; not for you. In short, there's no way you can trust what these receipts say as most modern email programs allow you to disable the sending of receipts entirely or on a case by case basis. In my case, my email software tells me if a receipt was requested and asks me if I want to send one: if I say no then it simply doesn't send it.

So, the short answer is that you can't. The long answer is that if you don't trust the person you're contacting to respond in a timely manner and to be honest about having received the communication then email may not be the ideal medium. If you can live with its vagaries then email is extremely convenient. If you require proof of delivery then traditional methods can provide much more rigorous certification (even if this comes with its own difficulties) and should certainly be used for anything with legal significance.