It's nowhere to be found actually.
So how does it work exactly?
Well, during the process of purchase, your username and transaction ID are given to the retailer you are using. As the song loads for download, software embeds information that carries both the account name and transaction into the music file. After the download, your MP3 then hides away this information deep in its nether regions, never to be seen again...unless you know where to look.
The picture below shows my username (highlighted in blue) engrained in the entrails of an MP3 purchased on iTunes.
So what's so important about a song carrying your username and transaction ID?
While the data can't be exploited by others to purchase more music or hack into your account, it can be used against you—the original purchaser—if your MP3 is somehow leaked onto torrent sites and uploaded by others.
Once a song is released into the wild, record companies can dissect the MP3 and gather the necessary information to press charges against the person who originally purchased the song and allowed it be bounced around on the internet for free downloads.
While the idea is a smart one on the behalf of the record companies, what happens if your MP3 player is stolen and all of your music uploaded online to be downloaded by others? Well let me put it in layman's terms for ya—
You'll get screwed.
If you want to prevent this from happening, you can use a utility that could strip the personal information from the file, and thanks to Randy G, iDesiccate (Windows only) does just that.
The application comes with several options:
- If you want to remove Amazon info, check the PRIV and COMM boxes
- If you want to remove iTunes info, check COMM and UFID
- Checking COMM will remove all comments from your tags (for other MP3 download apps used)
So far, Randy G has cleaned over 2,000 files without any problems, but states "I can't guarantee that if you have a corrupted file that iDesiccate won't corrupt it further. So back up your files first and run small batches through it."
Once your files run through the program, you should be okay if your files are stolen or purposely released into the wild internet. Without the embedded information, your MP3s should now be squeaky clean!
Well, that's not exactly what I meant...but close enough.
Just be happy we don't have the same laws as Japan does.