If you want to decipher an encrypted message, it’s helpful to plant some plaintext seeds.
When you’re trying to crack a code, any clues about the content of the message are helpful. If you know part of the original message (called the plaintext) and also the encrypted transmission (the ciphertext) you can begin to establish just how the encryption goes from one to the other.
Here’s an example from World War II. Say you intercept several messages from Nazi Germany. For some reason they always end the same way, repeating the same encrypted text. Making an educated guess, you could surmise that the hidden ending in each case is “Heil Hitler.” Knowing that, you get vital clues about the method of encryption, clues that could help you work out the rest of the message.
The technical term for this is a “known-plaintext attack.” If you had a list of words and phrases that you expected to appear in an encrypted message you could run through several attempts at decryption and wait for some of those words to appear. The known words become breadcrumbs leading you towards the solution. There’s only one problem with this technique: how do you work out which words are coming?
While deciphering messages encrypted by the German Enigma machine in World War II, massive word lists (“cribs”) were developed to account for the people, places, and phrases likely to appear in military communications. Apparently weather reports were especially useful, with their limited vocabulary and consistent structure. But sometimes this just wasn’t enough. Sometimes, rather than waiting for particular words to appear you could force them to appear.
Here’s a real life example, again from World War II. The German fleet had just cleared mines out of an area of the ocean. Cryptographers requested that new mines be dropped into the area, and then watched the resulting German transmissions for words like “minen” or the name of the newly-mined location. This process was known as gardening, after the practice of sewing mines like seeds in the water, and it was one of the methods by which the Engima codes were broken.