Folks who know just a little bit about cryptography often think of cryptography as methods of hiding data from prying eyes. While this function -- encryption -- is indeed an important part of cryptography, there are many other aspects that are equally important. Here are a few that relate more to proving things about a message than they do to hiding a message.

Authentication: Prove that a message actually originates with its claimed originator. Suppose Peggy wishes to prove she sent a message. Peggy may prove to Victor that the message comes from her by performing a transformation on the message that Victor knows only Peggy knows how to perform (that is, because only Peggy, and maybe Victor, knows the key). Peggy may send the transformation either instead of or in addition to M, depending on the protocol.

Integrity: Prove that a message has not been altered in unauthorized ways. Peggy might demonstrate the integrity of a message in a number of different ways. The most common means is by using a cryptographic hash (discussed later). Anyone may perform a cryptographic hash transformation, in the general case, but Peggy may take steps to publish the hash on a channel less prone to tampering than the message channel.

Non-repudiation: Prevent an originator from denying credit (or blame) for creating or sending a message. Protocols for accomplishing this goal are a bit complicated, but the traditional non-digital world has familiar means of accomplishing the same goal through signatures, notarization, and presentation of photo ID. Non-repudiation has many similarities to authentication, but there are also subtle differences.