If I know
H = md5( SECRET_KEY | DATA )
then I can calculate
H' = md5( SECRET_KEY | DATA | DATA' )
That's length-extension attack. But is the opposite possible? E.g. if I know
H = md5( SECRET_KEY | DATA )
can I find
H'' = md5( SECRET_KEY )
where DATA is known and lenght of DATA is known too???
Basically, can I find md5 of the KEY without DATA (once again, DATA is known, H is known, but SECRET_KEY is unknown)?