Objective To provide a practical technique for confirming that files have not been corrupted. Specifically, to learn the mechanics of using the MD5 message digest function to compute message digests of downloaded files and compare them with posted message digests.
Exercise 1 Visit the web site link at 'Example of File Authentication' in the resources section of the syllabus for today. Choose a file and download the file, then click the MD5 link and save the page showing its MD5 message digest. Then go the 'Example of File Authentication' link and download the file xsum.zip. Unzip this file, read the readme file (note that there is a manual provided as well, but you needn't read me in this), then use the mdx program in a DOS box to compute the MD5 digest of the file you downloaded. Compare it with the downloaded message digest to confirm that the two are the same.
Exercise 2 Create (or find handy on your hard disk) a text file of any size. Compute the MD5 message digest using mdx.exe. Bring up the file in TextPad and change a single letter in the file (change an 'a' to a 'b', for example), save the changed file, and compute the MD% message digest for the changed file. Compare the two digests. What do you see?
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