Bruteforcing will take a *very* long time. For all passwords composed of upper and lower-case letters 20 characters in length, there are 20,896,178,655,943,101,411,324,274,803,736,600 of them. At 1 million per second, it'll still take more than 1 billion times the current age of the universe to test them all. However, if you know certain characteristics of the password, e.g. where the capital letters are, you know part of it, etc. it can decrease the keyspace significantly. If you know part of the contents of the text file then known-plaintext attack may work.