Below SAS-code will let you decode BASE64-encoded text directly in SAS.
%let id_all ='<BASE64 ENCODED TEXT>';
%let session_id = %sysfunc(inputc(%scan(&id_all,1,':')===,$base64x80));
%put &=session_id;
SAS uses BASE64-encoding in PROC PWENCODE for SAS001-encoding.
See this blogpost for information about PROC PWENCODE.