Cryptography, to most people, is concerned with keeping communications private. Indeed, the protection of sensitive communications has been the emphasis of cryptography throughout much of its history. As we will see, however, this is only one part of today's cryptography.
Encryption is the transformation of data into some unreadable form. Its purpose is to ensure privacy by keeping the information hidden from anyone for whom it is not intended, even those who can see the encrypted data. Decryption is the reverse of encryption ; it is the transformation of encrypted data back into some intelligible form.
Encryption and decryption require the use of some secret information, usually referred to as a key. 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 (see Question 3).
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, when we sign our name to some document for instance, and 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 (see Question 3) binds a document to the possessor of a particular key, while a digital timestamp (see Question 108) binds a document to its creation at a particular time. These cryptographic mechanisms can be used to control access to a shared disk drive, a high security installation or to a pay-per-view TV channel.
But the field of cryptography contains even more when we include some of the things cryptography enables us to do. With just a few basic tools it is possible to build elaborate schemes and protocols which allow us to pay using electronic money (see Question 138), to prove we know certain information without revealing the information itself (see Question 107), and to share a secret quantity in such a way that no fewer than three from a pool of five people (for instance) can reconstruct the secret (see Question 103).
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. Cryptography today might be summed up as the study of techniques and applications that depend on the existence of difficult problems. A cryptanalyst attempts to compromise cryptographic mechanisms, and cryptology (from the Greek kryptós lógos, meaning "hidden word") is the discipline of cryptography and cryptanalysis combined.
A comprehensive review of modern cryptography can also be found in Applied Cryptography [Sch95b]; Ford [For94] provides detailed coverage of issues such as cryptography standards and secure communication.
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