Public key encryption

In 1976, a new way of encrypting messages called public key cryptography was invented by Whitfield Diffie and Martin Hellman. Public key cryptography (also referred to as asymmetric cryptography) solves the problem of exchanging keys. In this method, two mathematically related digital keys are used: a public key and a private key. The private key is kept secret by the owner, and the public key is widely disseminated.

Both keys can be used to encrypt and decrypt a message. However, once the keys are used to encrypt a message, that same key cannot be used to unencrypt the message. The mathematical algorithms used to produce the keys are one-way functions. A one-way irreversible mathematical function is one in which, once the algorithm is applied, the input cannot be subsequently derived from the output. Most food recipes are like this: it is easy to make scrambled eggs, but impossible to retrieve whole eggs from the scrambled eggs.

Public key cryptography is based on the idea of irreversible mathematical functions. The keys are sufficiently long (128, 256, and 512 bits) that it would take enormous computing power to derive one key from the other using the largest and fastest computers available.

In the simplest use of public key cryptography, the sender encrypts a message using the recipient's public key, then sends it over the Internet. The only person who can decrypt this message is the recipient, using his or her private key. However, this simple case does not ensure integrity or authentication — for that, digital signatures and hash digests are required (the next topic).