Threshold ECDSA from ECDSA Assumptions: The Multiparty Case

Cryptocurrency applications have spurred a resurgence of interest in the computation of ECDSA signatures using threshold protocols---that is, protocols in which the signing key is secret-shared among n parties, of which any subset of size t must interact in order to compute a signature. Among the resulting works to date, that of Doerner et al. requires the most natural assumptions while also achieving the best practical signing speed. It is, however, limited to the setting in which the threshold is two. We propose an extension of their scheme to arbitrary thresholds, and prove it secure against a malicious adversary corrupting up to one party less than the threshold under only the Computational Diffie-Hellman assumption in the Random Oracle model, an assumption strictly weaker than those under which ECDSA is proven. Whereas the best current schemes for threshold-two ECDSA signing use a Diffie-Hellman Key Exchange to calculate each signature's nonce, a direct adaptation of this technique to a larger threshold t would incur a round count linear in t; thus we abandon it in favor of a new mechanism that yields a protocol requiring log(t)+6 rounds in total. We design a new consistency check, similar in spirit to that of Doerner et al., but suitable for an arbitrary number of participants, and we optimize the underlying two-party multiplication protocol on which our scheme is based, reducing its concrete communication and computation costs. We implement our scheme and evaluate it among groups of up to 256 of co-located and 128 geographically-distributed parties, and among small groups of embedded devices. We find that in the LAN setting, our scheme outperforms all prior works by orders of magnitude, and that it is efficient enough for use even on smartphones or hardware tokens. In the WAN setting we find that, despite its logarithmic round count, our protocol outperforms the best constant-round protocols in realistic scenarios.