Converting from base64 to Raw

The below command converts raw value 'b562b3b63a1b4f41bf6a3a95770550d6' to base64 value 'AbVis7Y6G09Bv2o6lXcFUNY='

SQL> select utl_raw.cast_to_varchar2( utl_encode.base64_encode('01'||substr(mkeyid,1,4))) || utl_raw.cast_to_varchar2( utl_encode.base64_encode(substr(mkeyid,5,length(mkeyid)))) masterkeyid_base64 FROM (select 'b562b3b63a1b4f41bf6a3a95770550d6' mkeyid from dual);

MASTERKEYID_BASE64

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AbVis7Y6G09Bv2o6lXcFUNY=

I'm looking for reverse conversion from input 'AbVis7Y6G09Bv2o6lXcFUNY=' to output 'b562b3b63a1b4f41bf6a3a95770550d6'.