Introduce GPG operations

lib/cask/gpg.rb

@@ -0,0 +1,50 @@
+class Cask::GpgCheck
+  attr_reader :available
+
+  def initialize(cask, command=Cask::SystemCommand)
+    @command = command
+    @cask = cask
+    @available = @cask.gpg ? installed? : false
+  end
+
+  def installed?
+    cmd = @command.run('/usr/bin/type',
+                       :args => ['-p', 'gpg'])
+
+    # if `gpg` is found, return its absolute path
+    cmd.success? ? cmd.stdout : false
+  end
+
+
+  def fetch_sig(force=false)
+    unversioned_cask = @cask.version.is_a?(Symbol)
+    cached = @cask.metadata_subdir('gpg') unless unversioned_cask
+
+    meta_dir = cached || @cask.metadata_subdir('gpg', :now, true)
+    sig_path = meta_dir.join("signature.asc")
+
+    curl(@cask.gpg.signature, '-o', sig_path.to_s) unless cached || force
+
+    sig_path
+  end
+
+  def import_key
+    args = case
+           when @cask.gpg.key_id  then ['--recv-keys', @cask.gpg.key_id]
+           when @cask.gpg.key_url then ['--fetch-key', @cask.gpg.key_url.to_s]
+           end
+
+    @command.run!('gpg', :args => args)
+  end
+
+  def verify(file)
+    import_key
+    sig = fetch_sig
+
+    ohai "Verifying GPG signature for #{@cask}"
+
+    @command.run!('gpg',
+                  :args => ['--verify', sig, file],
+                  :print_stdout => true)
+  end
+end