Compression is also of interest. A well-compressed file should resemble noise: if you can predict the next bit, you could have avoided sending it. As we've moved to digital formats, there's an increasing opportunity to save bandwidth through compression, making the 'plaintext' much less readable. You could theoretically backwards-engineer the mp3 format by seeing enough examples, but it's a harder problem than, say, playing back a record that someone velcro'ed onto the side of a space probe.