If you already have a version of GnuPG installed, you can simply
   verify the supplied signature.  For example to verify the signature
   of the file gnupg-2.5.19.tar.bz2 you would use this command:

     gpg --verify gnupg-2.5.19.tar.bz2.sig gnupg-2.5.19.tar.bz2

   This checks whether the signature file matches the source file.
   You should see a message indicating that the signature is good and
   made by one or more of the release signing keys.  Make sure that
   this is a valid key, either by matching the shown fingerprint
   against a trustworthy list of valid release signing keys or by
   checking that the key has been signed by trustworthy other keys.
   See the end of this mail for information on the signing keys.

 * If you are not able to use an existing version of GnuPG, you have
   to verify the SHA-1 checksum.  On Unix systems the command to do
   this is either "sha1sum" or "shasum".  Assuming you downloaded the
   file gnupg-2.5.19.tar.bz2, you run the command like this: