`MpcContext` with local share public to protocol Rounds(eg. gg18 and gg20) cause security issue

[joyoFeng]

We notice that gg18 and gg20 protocols, their sub-protocols refresh and sign are implemented by importing keys, then constructing MpcContext and providing it to multiple Rounds to perform cryptographic operations. The MpcContext in a Round is a public member, and furthermore its local share (SSS slice) is also public, exposing the local share publicly during the execution of each Round, which is a security risk for key management!

Note: The private key slice (local share) should be destroyed as soon as it is used up, with strict lifecycle control and no public access. (From PlatON Cryptography Team)