Everyone has secrets, and some have more than others. When it becomes necessary to transmit those secrets from one point to another, it's important to protect the information while it's in transit. Cryptography presents various methods for taking legible, readable data, and transforming it into unreadable data for the purpose of secure transmission, and then using a key to transform it back into readable data when it reaches its destination.
Predating computers by thousands of years, cryptography has its roots in basic transposition ciphers, which assign each letter of the alphabet a particular value. A simple example is to assign each letter a progressively higher number, where A=1, B=2, and so forth. Using this formula for example, the word "wiseGEEK", once encrypted, would read "23 9 19 5 7 5 5 11". During World War Two, machines were invented that made the ciphers more complicated and difficult to break, and today, computers have made them even stronger still.
The Secure Sockets Layer (SSL) is a common encryption protocol used in e-commerce. When someone makes a purchase over the Internet, this is the technology the merchant uses to make sure that the buyer can safely transmit his credit card information. Using this protocol, the computer and the online merchant's computer agree to create a type of private "tunnel" through the public Internet. This process is called the "handshake." When a URL in a Web browser starts with "https" instead of "http," it is a secure connection that is using SSL.
Some methods of cryptography use a "secret key" to allow the recipient to decrypt the message. The most common secret key cryptosystem is the Data Encryption Standard (DES), or the more secure Triple-DES, which encrypts the data three times.
More common are systems that use a public key system, such as the Diffie-Hellman key agreement protocol. This system uses two keys that work together: a public one, which anyone can access, and a private one, which is kept secret by the party receiving the data. When a person wants to send a secure message to someone else, the person encrypts that message using the recipient's public key. Once encrypted, the recipient must use his or her private key to decrypt it.
The goal of cryptography extends beyond merely making data unreadable; it also extends into user authentication, which is providing the recipient with assurance that the encrypted message originated from a trusted source. Hash functions are sometimes used in conjunction with private key or public key systems. This is a type of one-way encryption, which applies an algorithm to a message, such that the message itself cannot be recovered. Unlike key-based cryptography, the goal of the hash function is not to encrypt data for later decryption, but to create a sort of digital fingerprint of a message. The value derived from applying the hash function can be re-calculated at the receiving end, to ensure that the message has not been tampered with during transit. Then, a key-based system is applied to decipher the message.
The study of this field is advancing steadily, and scientists are rapidly creating mechanisms that are more difficult to break. The most secure type yet may be quantum cryptography, a method that has not yet been perfected. Instead of using a key, it relies on the basic laws of physics and the movement and orientation of photons to establish a connection that is absolutely secure and unbreakable.