Funny you say that, I remember when one of Elcomsoft's developers were so good, they made available a decrypter tool that did precisely what you asking for, the only difference is I do not think we had 128 bit encryption in those days. I remember reading that the poor guy got arrested when he visited the U.S.A. for some conference. I just wish I could remember his name but the tool bypassed all passwords and decrypted the document no matter what. He was a genius and I am sure he still us today.