Key cryptography is one of the most important elements of any system used to encrypt computer data. It aims to tackle the problem that any well-established encryption system will be widely known and understood. To counter this, key cryptography uses a second layer of protection which is created from scratch with each set of encrypted data.
Encryption aims to deal with the fact that data sent over the Internet passes through many locations between the sender and recipient. Even with data which needs to be kept secret, there will still be points where it could be read by other people. For this reason, the data needs to be sent in a coded form.
To understand key cryptography, you can think back to when you were a child and used the secret code by which the letter A was written as B, the letter B as C, and so on. Of course, other people reading your coded material could figure out the principle of the encryption: that each real letter was replaced by another. You could then confuse them by using a different system, such as A being written as Z, B written as Y and so on. The reader would know the principle of encryption you had used, but would not be able to decipher the message unless they knew the precise replacements you had used for each letter.
While the actual system used in most modern computer encryption systems is more complicated, the principle isn't that different. Put simply, all the data sent through a particular system will be encrypted using the same techniques. However, every time a batch of data is sent, there is also a separate "key" which determines the precise details of the encryption, and which is needed to decipher the data.
One obvious problem with this set-up is that if the sender transmits the key, anyone who intercepts the data can also intercept the key. To get around this, a dual-key system named public key cryptography is used. In this system, the encryption key is provided by the recipient, not the sender.
Each computer or user that wants to receive encrypted data produces two keys, known as public and private. It makes the public key available to anyone who wants to send it confidential information, and this key is used to encrypt the data. However, this encryption is set up so that it can only be unlocked using the private key, which the recipient keeps to themselves.