aesgcm.rb

#!/usr/bin/env ruby
# klg, Sep 2020
# https://xmpp.org/extensions/inbox/omemo-media-sharing.html
# frozen_string_literal: true

require 'openssl'
require 'net/http'

ARGV.each do |arg|
  uri = URI(arg)
  raise 'Wrong URI scheme' unless uri.scheme == 'aesgcm'

  frag = [uri.fragment].pack('H*')
  raise 'Bad key' unless frag.length == 44

  uri.scheme = 'https'
  uri.fragment = nil

  c = OpenSSL::Cipher::AES256.new(:GCM)
  c.decrypt
  c.key = frag[-32..]
  c.iv = frag[..-33]
  frag = nil

  dat = Net::HTTP.get(URI(uri.to_s))
  c.auth_tag = dat[-16..]

  dat = c.update(dat[..-17]) + c.final
  $stdout.write(dat)
end