Cryptography is the art or science of decoding coded messages. Even if you intercept a note from one of your siblings to the other, it's going to require cryptography to decipher it.
When you practice cryptography, you're an expert at both composing and unraveling messages that are written in some kind of code to ensure their privacy. In the old days, cryptography meant translating text into illegible nonsense that could only be understood by the recipient, and then decoding it using a code key or instructions. Today, most cryptography involves supercomputers and math. The Greek roots are kryptos, "hidden," and graphia, "writing."