ECDSASigner constructor
If _digest
is not null it is used to hash the message before signing and verifying, otherwise, the message needs to be
hashed by the user of this ECDSASigner object.
If _kMac
is not null, RFC 6979 is used for k calculation with the given Mac. Keep in mind that, to comply with
RFC 69679, _kMac
must be HMAC with the same digest used to hash the message.
Implementation
ECDSASigner([this._digest, this._kMac]);