class Gem::Package::TarWriter

def add_file_signed name, mode, signer
  digest_algorithms = [
    signer.digest_algorithm,
    Digest::SHA512,
  ].compact.uniq
  digests = add_file_digest name, mode, digest_algorithms do |io|
    yield io
  end
  signature_digest = digests.values.compact.find do |digest|
    digest_name =
      if digest.respond_to? :name then
        digest.name
      else
        /::([^:]+)$/ =~ digest.class.name
        $1
      end
    digest_name == signer.digest_name
  end
  if signer.key then
    signature = signer.sign signature_digest.digest
    add_file_simple "#{name}.sig", 0444, signature.length do |io|
      io.write signature
    end
  end
  digests
end