 <p>You have not Googled your question. If you do so you'll get a lot of content about it. The link below will teach you how to check the integrity of a downloaded file.</p> <p><a href="https://help.ubuntu.com/community/HowToMD5SUM" rel="nofollow">https://help.ubuntu.com/community/HowToMD5SUM</a></p> <p>You may also check the following link, which shows that it is very difficult to modify or replace the <code>.md5</code> and <code>.sh1</code> files.</p> <p><a href="http://books.google.co.in/books?id=AHzAcvHWbx4C&amp;pg=PA305&amp;lpg=PA305&amp;dq=can+a+hacker+modify+the+md5+hashes&amp;source=bl&amp;ots=c2kDjYXSXE&amp;sig=-ZIEzo8Q66Go0fLkn1HTyvho0sw&amp;hl=en&amp;ei=GiLPTu_UA5HxrQeCot2xDA&amp;sa=X&amp;oi=book_result&amp;ct=result&amp;resnum=9&amp;sqi=2&amp;ved=0CG0Q6AEwCA#v=onepage&amp;q=can%20a%20hacker%20modify%20the%20md5%20hashes&amp;f=false" rel="nofollow">Here's the link.</a></p>