code: func (a *announceData) sign(privKey *ecdsa.PrivateKey) {
  	rlp, _ := rlp.EncodeToBytes(announceBlock{a.Hash, a.Number, a.Td})
  	sig, _ := crypto.Sign(crypto.Keccak256(rlp), privKey)
  	a.Update = a.Update.add("sign", sig)
  }
docs: // sign adds a signature to the block announcement by the given privKey