You can always use a secure multiparty computation algorithm to do it.

https://en.wikipedia.org/wiki/Secure_multi-party_computation

But those aren't the fastest algorithms in the world, and usually both
participants needs to be online at the same time. I guess most people would
prefer a two-step algorithm that can be performed asynchronously.

- Sent from my phone
Den 8 mar 2014 18:44 skrev "Adam Back" <adam@cypherspace.org>:

> Also the other limitation for ECDSA is that there is no known protocol to
> create a signture with a+b (where keys P=aG, Q=bG, R=P+Q=(a+b)G). without
> either a sending its private key to b or viceversa (or both to a third
> party).
>
> With Schnorr sigs you can do it, but the k^-1 term in ECDSA makes a
> (secure)
> direct multiparty signature quite difficult.
>
> ps probably only 1 party needs to hash their key
>
> P=aG
>             H(P) ->
>
>                 <- Q=bG
>
>            P ->
>
> Adam
>
> On Sat, Mar 08, 2014 at 12:37:30PM +0200, Joel Kaartinen wrote:
> >   If both parties insist on seeing a hash of the other party's public key
> >   before they'll show their own public key, they can be sure that the
> >   public key is not chosen based on the public key they themselves
> >   presented.
>
>
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