Cryptography
Authors: Piper Fred 
Published: February 2015

Cryptography is the study of the mathematical techniques related to aspects of information security such as confidentiality, data integrity, message authentication, and entity authentication. It is assuming an ever‐increasing importance in their provision.

The provision ofconfidentialitymeans keeping information secret from all except those who are authoriged to know it. The provision ofdata integritygives assurance that unauthoriged changes have not been made to the information.Message authentication, which is also frequently calleddata origin authentication, corroborates the source of the information, whileentity authenticationcorroborates the identity of an entity.

If someone wishes to disguise confidential information in such a way that it is unintelligible to an unauthorised person, then they are likely to use acipher system.

The information to be disguised is called thecleartextorplaintextor, simply, themessage, and the operation of disguising is known asencryption(orenciphering). The enciphered message is called theciphertextorcryptogram. The algorithm used for this operation also has a second input known as theenciphering key. Clearly this process has to be reversible to enable the genuine recipient to obtain the information. The process of obtaining the plaintext from the ciphertext is known asdecryption(ordeciphering), and, in addition to the ciphertext, the deciphering algorithm needs adeciphering key. The process will “work” in the sense that the receiver obtains the correct message, only if the correct deciphering key is used and, in general, the receiver will be the only person with knowledge of this key.

The usual concept of an “attack” on an algorithm is where the attacker attempts to determine the plaintext corresponding to some ciphertext without being given the deciphering key. (This process is calledcryptanalysis.) When this is achieved, the algorithm is said to have been broken. Of course, if the attacker were able to determine this key, then the attack would be successful and, in fact, many people regard breaking the algorithm and finding the key as being synonymous. There are three types of attack that might be launched against an encryption algorithm these are discussed.

Since the main aim of attacks is to determine the deciphering key, it is clear that these keys have immense value and, consequently, they need careful management.

An eavesdropper who attacks a communication system and tries to obtain the message from an intercepted cryptogram in fact wants to obtain the deciphering key. The enciphering key has no intrinsic value to the attacker unless it helps to determine the deciphering key. This simple observation leads to the following definitions. A cipher system, or equivalently an encryption scheme, is said to besymmetricif, for each corresponding pair of enciphering and deciphering keys, it is easy to determine the deciphering key from the enciphering key. Frequently they will be identical. In view of this, symmetric cipher systems are often calledone‐key systems. (Other names includesingle‐key,secret‐key, orconventionalsystems.) If, on the other hand, it is computationally impossible to determine the deciphering key from the enciphering key, then we have apublic key cryptosystemor, equivalently, anasymmetricsystem.

Symmetric and asymmetric cryptography are almost two different subjects; the algorithms are different, the key management problems are different, and most of the applications are different.