So, for example, if you read an entire paragraph and notice that the letter D appears more often than any other, odds are that it used a Caesar cipher with a shift of 1 (making E a D in the code).

It's easy to decrypt if you take some time to figure it out, but it's plenty to keep non-motivated people from spying on your stuff. This is just one useful personal encryption method of many.

For machines, it might not matter. Without any prior knowledge, artificial intelligence algorithms have cracked two classic forms of encryption: the Caesar cipher and Vigenère cipher.