Cryptography is the science of keeping information secure by transforming it into form that unintended recipients cannot understand. In cryptography, an original human readable message, referred to as plaintext, is changed by means of an algorithm, or series of mathematical operations, into something that to an uninformed observer would look like gibberish; this gibberish is called ciphertext.

Cryptographic systems require some method for the intended recipient to be able to make use of the encrypted message — usually, though not always, by transforming the ciphertext back into plaintext.

Cryptography vs. encryption

Encryption is what we call the process of turning plaintext into ciphertext. (Crypt may make you think of tombs, but it comes from a Greek word that means “hidden” or “secret.”) Encryption is an important part of cryptography, but doesn’t encompass the entire science. Its opposite is decryption.

One important aspect of the encryption process is that it almost always involves both an algorithm and a key. A key is just another piece of information, almost always a number, that specifies how the algorithm is applied to the plaintext in order to encrypt it. Even if you know the method by which some message is encrypted, it’s difficult or impossible to decrypt without that key.

History of cryptography

This is all very abstract, and a good way to understand the specifics of what we’re talking about is to look at one of the earliest known forms of cryptography. It’s known as the Caesar cipher, because Julius Caesar used it for his confidential correspondence; as his biographer Suetonius described it, “if he had anything confidential to say, he wrote it in cipher, that is, by so changing the order of the letters of the alphabet … If anyone wishes to decipher these, and get at their meaning, he must substitute the fourth letter of the alphabet, namely D, for A, and so with the others.”

Suetonius’s description can be broken down into the two cryptographic elements we’ve discussed, the algorithm and the key. The algorithm here is simple: each letter is replaced by another letter from later in the alphabet. The key is how many letters later in the alphabet you need to go to create your ciphertext. It’s three in the version of the cipher Suetonius describes, but obviously other variations are possible — with a key of four, A would become E, for instance.