Depending on the encryption mechanism used, the same key might be used for both encryption and decryption, while for other mechanisms, the keys used for encryption and decryption might be different. But today s cryptography is more than secret writing, more than encryption and decryption. Authentication is as fundamental a part of our lives as privacy. We use authentication though out our everyday life, for instance when we sign our name to some document. As we move to a world where our decisions and agreements are communicated electronically, we need to replicate these procedures.
Cryptography provides mechanisms for such procedures.
A digital signature binds a document to the possessor of a particular key, while a digital timestamp binds a document to its creation at a particular time. But the field of cryptography contains even more when we include some of the things cryptography enables us to do. While modern cryptography is growing increasingly diverse, cryptography is fundamentally based on problems that are difficult to solve. A problem may be difficult because its solution requires some secret knowledge, such as decrypting an encrypted message or signing some digital document, or the problem may be hard because it is intrinsically difficult to complete, such as finding a message which produces a given hash value. So as the field of cryptography has advanced, the dividing lines for what is and what is not cryptography have become blurred.