What is signing a message with a bitcoin address?
A cool function of Bitcoin not mentioned in introductions is its message signing and verification feature. I'll use the Satoshi client on a Mac as the example but the same functions are available in the wallet at blockchain.info and in any decent Bitcoin client.
Great, says the vendor, but how do I know that's your coin?
In the Satoshi client, go to the "Receive Coins" tab. At the bottom, you'll see an option that says, "Sign Message" (you can also go to File > Sign Message). Enter the Bitcoin address that you own that initiated the transaction (that's the address to the left of the green arrow in the transaction record mentioned earlier) into the address form. Write your message and then hit the "Sign Message" button.
The signature generated is unique to the address and to the message. Send your address, the message and the signature to the vendor. They'll take your info and enter it into the "Verify Message" function of Bitcoin, usually located somewhere near the "Sign Message" button.
This could have other uses found with some creativity. ; )
Another use for this feature.
Someone says they're me and you want proof. With this feature, it's a kind of ID system for anons.
If you know that I have access to the address in the above example and you give me a message to sign with that address, you can verify that it's me at someothersite.com.
Videos and links.
Tutorials for how to sign messages with different bitcoin wallets.
if you are useing another wallet please google how to sign a message with that wallet
if you are using a wallet that doesn't sign transactions then you will have to import your private keys into one of the wallets above.