# [curves] Threshold ECDSA for Bitcoin

Trevor Perrin trevp at trevp.net
Sun Mar 30 23:19:57 PDT 2014

```On Fri, Mar 28, 2014 at 3:59 PM, Michael Hamburg <mike at shiftleft.org> wrote:
> Out of curiosity, what's wrong with the following "obvious" protocol for threshold Schnorr?
>
> The signers have a polynomial share x_i of x.  All the signers in the signing group know who is signing right now, and they know that x = sum a_i x_i, and they know the a_i.  If weeding out bad participants is desired, then each signer's share [x_i]G of the public key is known to the other group members.
>
> Each signer computes R_i = [k_i]G for a random nonce k_i.  They broadcast commitments to these choices, then broadcast revelations.
>
> Each signer computes R = sum [a_i] R_i, so that effectively r = sum a_i k_i; and c = Hash(R,m).
>
> Each signer creates and broadcasts a mini-sig s_i = c x_i + k_i.  The signature is (R, s = sum a_i s_i).  Since k = sum a_i k_i and x = sum a_i x_i, we have s = cx + k as desired.

Are you describing this, or something close to it?

http://cacr.uwaterloo.ca/techreports/2001/corr2001-13.ps

Comparing against the threshold ECDSA of:

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.67.9913
http://www.cs.princeton.edu/~stevenag/bitcoin_threshold_signatures.pdf

The ECDSA protocol seems similar, except with extra steps to deal with
the inversion.

So is it accurate to say that both threshold ECDSA and threshold
Schnorr can be done "well" (efficiently, robustly, good security
proofs), but the Schnorr version is simpler / less communication?

Trevor
```