This volume of The Zcash Arboretum constructs threshold spend
authorisation for Orchard-protocol notes: FROST, the two-round threshold
Schnorr protocol of RFC 9591, with key generation by a trusted dealer and by a
distributed protocol, and its re-randomised variant specified in ZIP 312 over
the ciphersuite FROST(Pallas, BLAKE2b-512), whose aggregate is an ordinary
RedPallas spend-authorisation signature. Any threshold number of participants
signs, and no signing run reconstructs the signing key. The volume proves
correctness of each protocol and states its security with the assumptions and
the status of each result, marking open problems as such. It assumes the
Math Guide and the Crypto Guide, and cites the Ironwood
Guide for RedPallas, randomised validating keys, the signature digest and the
construction of a transaction. It is non-normative: the protocol
specification, the ZIPs and RFC 9591 are authoritative.