# File lib/sup/crypto.rb, line 30
  def sign from, to, payload
    payload_fn = Tempfile.new "redwood.payload"
    payload_fn.write format_payload(payload)
    payload_fn.close

    sig_fn = Tempfile.new "redwood.signature"; sig_fn.close

    message = run_gpg "--output #{sig_fn.path} --yes --armor --detach-sign --textmode --local-user '#{from}' #{payload_fn.path}", :interactive => true
    unless $?.success?
      info "Error while running gpg: #{message}"
      raise Error, "GPG command failed. See log for details."
    end

    envelope = RMail::Message.new
    envelope.header["Content-Type"] = 'multipart/signed; protocol=application/pgp-signature; micalg=pgp-sha1'

    envelope.add_part payload
    signature = RMail::Message.make_attachment IO.read(sig_fn.path), "application/pgp-signature", nil, "signature.asc"
    envelope.add_part signature
    envelope
  end