Decode a base64 string in xquery

Question

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://www.w3.org/2010/xslt-xquery-serialization";
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.