Download decryption?
 

I've noticed that often when a file is made available for download on a popular website, the download links are occasionally accompanied by what I assume are decryption keys. For example, on the official Ruby website, each download link is followed by md5 and sha256 codes. What is the purpose of these codes? I can download any of the files without a problem and decompress them without entering any sort of code. :confused:

They are for security reasons. A hash is computed for the file, and it is posted. When you receive the file on your side you compute the hash yourself and compare. This way you can check if the file was tampered with on the way to you.

Can you check the hash yourself, or is it done automatically by your decompression app?

You generally check the hash yourself. Check out a utility called "md5sum" for checking md5 keys.