MI-04 Undocumented Cryptographic Key Derivation

[xoloki]

During the private shares step, all signers will compute a private share representing the evaluation of their polynomial f(X) at a scalar i and then encrypt it for another signer. This is done by first computing a shared secret using elliptic curve Diffie-Hellman and then using a custom key derivation scheme to derive a key to be used by the symmetric encryption scheme. However, this protocol is not documented and appears to be a custom creation. Since this key-exchange, key derivation and encryption protocol has not been analyzed, and other protocols using very similar elements have been analyzed and provide security guarantees, there is an unnecessary risk being taken that could be eliminated.