code: func (w *keystoreWallet) SignDataWithPassphrase(account accounts.Account, passphrase, mimeType string, data []byte) ([]byte, error) {
  	// Make sure the requested account is contained within
  	if !w.Contains(account) {
  		return nil, accounts.ErrUnknownAccount
  	}
  	// Account seems valid, request the keystore to sign
  	return w.keystore.SignHashWithPassphrase(account, passphrase, crypto.Keccak256(data))
  }
docs: // SignDataWithPassphrase signs keccak256(data). The mimetype parameter describes the type of data being signed