Introduction to Cryptography and Data Security - Understanding Cryptography - page 3

Fig. 1.3 Overview of the field of cryptology
that we notice is that the most general term is cryptology and not cryptography .
Cryptology splits into two main branches:
Cryptography is the science of secret writing with the goal of hiding the mean-
ing of a message.
Cryptanalysis is the science and sometimes art of breaking cryptosystems. You
might think that code breaking is for the intelligence community or perhaps or-
ganized crime, and should not be included in a serious classification of a scien-
tific discipline. However, most cryptanalysis is done by respectable researchers
in academia nowadays. Cryptanalysis is of central importance for modern cryp-
tosystems: without people who try to break our crypto methods, we will never
know whether they are really secure or not. See Sect. 1.3 for more discussion
about this issue.
Because cryptanalysis is the only way to assure that a cryptosystem is secure,
it is an integral part of cryptology. Nevertheless, the focus of this topic is on
crypto graphy : We introduce most important practical crypto algorithms in detail.
These are all crypto algorithms that have withstood cryptanalysis for a long time, in
most cases for several decades. In the case of crypt analysis we will mainly restrict
ourselves to providing state-of-the-art results with respect to breaking the crypto al-
gorithms that are introduced, e.g., the factoring record for breaking the RSA scheme.
Let's now go back to Fig. 1.3. Cryptography itself splits into three main branches:
Symmetric Algorithms are what many people assume cryptography is about:
two parties have an encryption and decryption method for which they share a
secret key. All cryptography from ancient times until 1976 was exclusively based
on symmetric methods. Symmetric ciphers are still in widespread use, especially
for data encryption and integrity check of messages.
Asymmetric (or Public-Key) Algorithms In 1976 an entirely different type of
cipher was introduced by Whitfield Diffie, Martin Hellman and Ralph Merkle. In
public-key cryptography, a user possesses a secret key as in symmetric cryptog-
raphy but also a public key. Asymmetric algorithms can be used for applications
such as digital signatures and key establishment, and also for classical data en-
cryption.
Cryptographic Protocols Roughly speaking, crypto protocols deal with the ap-
plication of cryptographic algorithms. Symmetric and asymmetric algorithms