class Net::SSH::Authentication::Methods::Publickey

def authenticate_with_alg(identity, next_service, username, alg, sig_alg = nil)
  debug { "trying publickey (#{identity.fingerprint}) alg #{alg}" }
  send_request(identity, username, next_service, alg)
  message = session.next_message
  case message.type
  when USERAUTH_PK_OK
    buffer = build_request(identity, username, next_service, alg,
                           true)
    sig_data = Net::SSH::Buffer.new
    sig_data.write_string(session_id)
    sig_data.append(buffer.to_s)
    sig_blob = key_manager.sign(identity, sig_data, sig_alg)
    send_request(identity, username, next_service, alg, sig_blob.to_s)
    message = session.next_message
    case message.type
    when USERAUTH_SUCCESS
      debug { "publickey succeeded (#{identity.fingerprint})" }
      return true
    when USERAUTH_FAILURE
      debug { "publickey failed (#{identity.fingerprint})" }
      raise Net::SSH::Authentication::DisallowedMethod unless
        message[:authentications].split(/,/).include? 'publickey'
      return false
    else
      raise Net::SSH::Exception,
            "unexpected server response to USERAUTH_REQUEST: #{message.type} (#{message.inspect})"
    end
  when USERAUTH_FAILURE
    return false
  when USERAUTH_SUCCESS
    return true
  else
    raise Net::SSH::Exception, "unexpected reply to USERAUTH_REQUEST: #{message.type} (#{message.inspect})"
  end
end