May 28, 2018

Hi @neillm! Yes, we are leaning towards the "external" option as SBI BITS has chosen an older algorithm that would likely not be the first choice for a libbitcoin-provided algorithm. The first option also does not preclude libbitcoin from providing vetted threshold signature implementations, perhaps via a function bx sign-threshold [-h] EC_PRIVATE_KEY_SHARE KEY_SHARE_TYPE INTERMEDIATES_PRIV INTERMEDIATES_PUB [SIGHASH] which after several calls produces the EC signature to be concatenated with the sighash_type. Also, from my memory, no current threshold signature algorithm completes signing in less than 4 rounds so it is unlikely that anyone wants to use the command line for threshold signing. Users will likely prefer to wrap the library call instead, creating a communication network that fits their preferences and passing around the public intermediates until the signature is complete. So even if the second "internal" option is chosen, users are likely to externalize it anyway for ease-of-use and performance.