How to confirm someone owns an address?

[dan123]

What would be the best way to prove someone owns an address?
I can ask a person to send me a very small amount of bitcoin and I can check my transactions to see what account it's coming from.

But then if they're using something like coinbase, won't many people send from that same address?  Or that address change over time.
Would a good check be having them send from the same address twice (maybe a couple weeks apart)?
Is there some kind of check to see if the address is a group address?

[ThePurplePlanet]

What would be the best way to prove someone owns an address?
I can ask a person to send me a very small amount of bitcoin and I can check my transactions to see what account it's coming from.

But then if they're using something like coinbase, won't many people send from that same address?  Or that address change over time.
Would a good check be having them send from the same address twice (maybe a couple weeks apart)?
Is there some kind of check to see if the address is a group address?

They can just sign a message no need to send a small amount

[dan123]

how easy is it for someone to sign their address though?  You can't do that on blockchain.info or anything like that can you?
Is there a good site to do that, or do you mostly sign stuff if you have a desktop client wallet.  There's no way for me to do it for them cus I'd need their private key.

[DeathAndTaxes]

how easy is it for someone to sign their address though?  You can't do that on blockchain.info or anything like that can you?
Is there a good site to do that, or do you mostly sign stuff if you have a desktop client wallet.  There's no way for me to do it for them cus I'd need their private key.

Blockchain.info supports signing messages.  Sites like coinbase don't.  The user doesn't own the address.  They don't have the private key thus they can't sign anything (nor should they be able to). 

[Foxpup]

I smell an X-Y problem. Why do you want to prove someone owns an address? If you think you need to, you're probably doing something wrong.

[ThePurplePlanet]

how easy is it for someone to sign their address though?  You can't do that on blockchain.info or anything like that can you?
Is there a good site to do that, or do you mostly sign stuff if you have a desktop client wallet.  There's no way for me to do it for them cus I'd need their private key.

Same as signing a transaction to sent bitcoins. Instead of signing a transaction message you sign any arbitrary message. Any client has an option to do that I think. Look for sign a message option.

[dan123]

Yeah, thanks. Signing would work fine and I see blockchain also supports signing.  There's also a coinbase API with oauth2 for a coinbase user account.

[DeathAndTaxes]

There's also a coinbase API with oauth2 for a coinbase user account.

Which has nothing to do with proving someone has control of an address.  I think foxpup got it right.

[Luke-Jr]

Bitcoin wallets have a Sign Message that proves who receives with an address.
Note that there is no functionality to prove who controls the private key associated to it, which often is a different person.