I have a base64 'cipher' text, I know that in clear text is a hidden xml document (I do not know nothing about its structure), but the base64 aplhabet was somehow shuffled. Is there any smart way, how to detect the modified alphabet better than checking all 64! permutations? I can reduce slightly the number of permutations by using the knowledge that xml document starts (without doctype, just root element) with opening tag (character '<') and ends with '>'. But there is still 62! permutations which is still not computable.
I have also tried to infer some other characters, but as the tags may be arbitraty long and the content may be any character, I don't see much space for frequence analysis.