What is my sending address? Also strange transaction behavior.

[Boxman90]

Dear all, I have sumbled upon something I do not understand about the way the Bitcoin network works.

So the situation is as follows, I have about 10 different receive addresses in my client, and I've just made a payment to someone. He requires me to send him the address from which I sent the BTC. That is the first problem - I do not know how I can see which address my client used to send the payment.

Secondly, I tried to use the blockchain explorer to see what the sending address was. This is here: http://blockexplorer.com/tx/8d5c000d91b22e3a3ae65373b57ea8196a15274c7e8a0d783cf32d6342dbb8bc

Now, that address at the "input", namely 1AuaHSbRCL8YABK4WgQcKScFkjQ1mX6c83, is not in my "receive coins" list.
When I click the address (http://blockexplorer.com/address/1AuaHSbRCL8YABK4WgQcKScFkjQ1mX6c83), I see that on an earlier payment I made this evening, the funds to the 1A address came from 1N72G2Go7h32FzKF8k9vGZUvMwytRVFQP3, an address that IS one of the addresses in my receive coins list. It says it sent to the 14so~.* address, which is the person I made the payment to, but also it says that it sent to the 15D~.* address, which is again an address that I do not know. When I click on that (http://blockexplorer.com/address/1AuaHSbRCL8YABK4WgQcKScFkjQ1mX6c83) it says that that address now has a balance of 8.something BTC.

Still my wallet shows correctly that I still have about 20 BTC, so I haven't lost anything. So apparently that 15D~.* address is mine, yet it is not in my address list.. 

I cannot make sense of this and I hope someone can explain what is going on.

[Abdussamad]

If you are using bitcoin-qt 1AuaHSbRCL8YABK4WgQcKScFkjQ1mX6c83 is probably a change address. A change address is one where the balance from a transaction is sent. It is normally NOT show in the list of receive addresses. Bitcoin-qt abstracts it away.

Tell your friend that 1AuaHSbRCL8YABK4WgQcKScFkjQ1mX6c83 is the address you sent from.

[Boxman90]

Right, I read up on change addresses and I somewhat get that now.

However, the one I sent it to requires that I can sign messages with my sending address. Now, I don't suppose I can do that with a change address? As it's not in my list, I can't use that feature in the client obviously.

[runlinux]

What you can do is send the coins to yourself. At that point, you know where the coins are. From there, you are able to sign a message with a known address.

[Abdussamad]

Right, I read up on change addresses and I somewhat get that now.

However, the one I sent it to requires that I can sign messages with my sending address. Now, I don't suppose I can do that with a change address? As it's not in my list, I can't use that feature in the client obviously.

Ah I was afraid of that. Did you try copy pasting 1Aua... into the sign message window:

http://imgur.com/a/nUmZ0

In the example above you would replace the contents of the first text field that 1J6.. with 1Aua..

Since your wallet contains the private key for 1Aua.. you should be able to sign a message with it.

[Boxman90]

Ah yes, that works! Alright so the data is stored in my wallet but the client just doesn't show the particular address.

Next time I'll send the coins to myself first indeed, so I know where they are at a given moment. Thanks for clearing this up!

[Abdussamad]

About sending the coins to yourself, you would have to send the entire balance of your wallet to one address in your wallet. Then the next transaction you make will use that address as the input. Frankly I think this method is more trouble than it's worth.

[dentldir]

There really isn't a sending address in Bitcoin.  The topic comes up on the developer sub-forum often and seems to degrade into a pedantic argument of how Bitcoin was designed versus how it operates today.

However, there is an agreement that any of the addresses that blockchain.info calls "From Addresses" are what you should include in your email for the group buy in question.  You wont find them in your client because you did not create them.  I believe the client created them as part of the transaction output script and used your private keys to do so.

Look a From Address up on blockchain.info, then sign and verify a message with it.  I had to check that I was able to do this, and I was.  If successful, use that as the sending address in the email.