Decode a base64 string in xquery

We are trying to decode the encoded string in xquery. I didnt know how to decode the string.

Following is the xquery. this basically converts xml to json and we should also need to do the decoding.

declare namespace output = "http://ift.tt/1pE2aV6";

declare option jsoniq-version "0.4.42"; declare option output:method "json";

  {  

                 "Data":{                     
                      "Text" : string(/Request/EncodedText),
                  }        


   }     

how to do the decoding in this xquery.