Table of Contents

# What is Cryptology?

Cryptology is the practice and study of hiding information. In modern times cryptography is considered a branch of both mathematics and computer science, and is affiliated closely with information theory, computer science and engineering Cryptology is used in applications present in technologically advanced societies, examples include the security of ATM cards.com puter passwords and electronic commerce, which all depend on Cryptology.

**NOTE :-** (cryptology word is derived from Greek word Krypto hidden and the verb grafo “to write or beg in “to speak)

Until modern times Cryptology referred almost exclusively to encryption, the process of converting ordinary information (plain text) into unintelligible gibberish (ie..cipher text), Decryption is the reverse, moving from unintelligible ciphertext to plaintext. A cipher for cyphers is a pair of algorithms which creates the encryption and the reversing decryption.

The detailed operation of a cipher is controlled both by the algorithm and in each instance, by a key, This is a secret parameter (ideally, known to the communicants) for a specific message exchange context. Keys are important, as ciphers without variable keys are trivially breakable and therefore less than useful for most purposes

Historically, ciphers were often used directly for encryption or decryption, without addtional procedures such as authentication or integrity checks.

In colloquial use, the term “code” is often used to mean any method of encryption or concealment of meaning. However in Cryptology code has a more specific meaning, it means the replacement of a unit of plaintext (i. c., a meaningful word or phrase) with a code word (for example “apple pie” replaces “attack at dawn”). Codes are no longer used in serious Cryptology-except incidentally for such things as unit designations-since properly chosen ciphers are both, more practical and more secure than even the best codes, and better adapted to computers as well. Some use the terms Cryptology and cryptology interchangeably in English, while others use

Cryptology refers specifically to the use and practice of cryptographic techniques, and cryptology to refer to the combined study of Cryptology and Cryptanalysis.

The study of characteristics of languages that have some application in cryptology, i.e. frequency data, letter combinations, universal patterns, etc. is called Cryptolinguistics.

## History of Cryptology and cryptanalysis:

Before the modern era, Cryptology was concerned solely with confidentiality (ie.. encryption)-conversion of messages from a comprehensible form into an incomprehensible one, and back again at the other end, rendering it unreadable by interceptors or eavesdroppers without secret knowledge (namely, the key needed for decryption of that message).

In recent decades the field has expanded beyond confidentiality concerns to include techniques for message integrity checking. sender/receiver identity authentication, digital signatures, interactive proofs, and secure computation, amongst others.

The earliest forms of secret writing required little more than local pen and paper analogs as most people could not read. More literacy, or opponent literacy required actual Cryptology. Themain classical ciphers types are transposition ciphers, substitution ciphers and caeser ciphers.

Ciphertext produced by classical ciphers always reveal statistical information about the plaintext, which can often be used to break them. Essentially all ciphers remained valnerable toncryptanalysis using frequency analysis until the invention of the polyalphabetic ciphers by Leon Battista Alberti around the year 1467 Alberti’s innovation was to use different ciphers for various parts of a message. He also invented what was probably the first automatic cipher device,a wheel which implemented a partial realization of his invention.

The development of digital computers and electronics after would war -I made possible much more complex ciphers. Furthermore, computers allowed for the encryption of any kind of data represented by computers in any binary format, unlike classical ciphers which only encrypted writ ten language text, thus dissolving much of the utility of a linguistic approach to cryptanalysis. Exten sive open academic research into Cryptology is relatively recent. It began only in mid 1970’s with the public specification of DES (the Data Encryption Standard) the US Government’s National Bureau of Standards, the Diffie-Hellman paper, and the public release of the RSA Algorithm. Since then, Cryptology has become a widely used tool in communications, computer networks and com puter security generally.

Esentially, prior to the early 20th century, Cryptology was chiefly concerned with lingiuistic patterns. Since then the emphasis was shifted, and Cryptology now makes extensive use of math ematics, including aspects of information theory, computational complexity, statistics, combinatorics, abstract algebra and number theory: Cryptology is also a branch of engineering, but an unusual one as it deals with active, intelligent and malevolent opposition; most other kinds of engineering need deal only with neutral natural forces. There is also active research examining the relationship be tween cryptographic problems and quantum physics.

### Modern Cryptology: The modern field of Cryptology can be divided into several areas of study. The chief onesare discussed here

**Symmetric key Cryptology**

It refers to encryption methods in which both the sender and receiver share the same key This was the only kind of encryption publicly known until June 1976 The modern study of symmet ric-key ciphers relates mainly to the study of block ciphers and stream ciphers and to their applica tions. Block ciphers take as input a block of plain text and a key, and an output a block of ciphertext of the same size. Since messages are almost always longer than a single block, some method of knitting together successive block is required. The Data Encryption Standard (DES) and Advanced Encryption Standard (AES) are block cipher designs (discussed in later chapters) which have been designated Cryptology standards by the US government.

Stream ciphers, in contrast to the “block” type, create an arbitrarily long stream of key material, which is combined with the plain text bit-by-bit or character-by-character In a stream cipher, output stream is created based on an internal state which changes as the cipher operates RC4 is an example of well known, and widely used stream cipher.

Cryptographic hash functions (often called message digest functions) do not necessarily use keys, but are a related and important class of cryptographic algorithms. They take input data (often an entire message), and output a short, fixed, length hash, and do so as a one way function. For good ones, collisions (two plaintext which produce the same hrsh) are extremely difficult to find Message Authentication Codes (MACs) are much like cryptographic hash functions, except that a secret key is used to authenticate the hash value on receipt. These block an attack against plain hash functions.

**Public Key Cryptology**

In 1976.hitheld Dittie and Martin Hellman proposed the notion of public key (also known as asymmetric key) Cryptology in which two different but mathematically related keys are used-a public key and a private key. A public key system is so constructed that calculation of one key thic private key) is computationally infeasible from the other (the public key), even though they are necessarily related. In public-key Cryptology, the public key may be freely distributed, while its paired private key must remain secret.

The public key is typically used for encryption and private key is for decryption. In addition to encryption, public-key Cryptology can be used to implement digital signature schemes/A digital signature is reminiscent of an ordinary signature; they both have the characteristic that they are easy for a user to produce, but difficult for anyone else to forge. In digital signature schemes there are two algorithms, one for signing in which a secret key is used to process the message. (or a hash of the message, or both) and one for verification, in which the matching public key is used with the message to check the validity of the signature. RSA and DSA are two of the most popular digital signature schemes.

## 1 Comment