Public-Private Keys

Cryptography takes the original message, and produces an encoded version by using a special piece of information known to the sender and receiver. The original message is called the plaintext, the special information is called the key, and the resulting message is called the ciphertext. Cryptosystems work by taking the digital representation of the plaintext and manipulating it mathematically under the control of the digital key to produce the ciphertext.

Public-key cryptography was invented by Whitfield Diffie and Martin Hellman. It takes a message encrypted in one shared secret and decrypts it in another. The keys are mathematically related in such a way that a knowledge of one key does not make it possible to figure out the other key. This permits the one key, the public key, to be made widely known, while the corresponding private key is known only to a single user. The two keys together are called a key pair.

The Diffie-Hellman key produces shared secret keys directly from private and public components that are not in themselves keys. The advantage of a public-key system is that the secret components do not have to be shared to exchange information securely. The private portion is never given out to anyone, and it cannot feasibly be calculated from the public portion.