docs: function keygen (alg) {
      var algs =
          { 'HS256': 256/8
          , 'HS512': 512/8
          , 'A128CBC+HS256': 256/8
          , 'A256CBC+HS512': 512/8
          };
      if (!algs[alg]) return null;
      return (b64url.encode(crypto.randomBytes(algs[alg])))
  }