# Cryptography

Joshi, Manisha and Banerjee, Pradipto (2018) Cryptography. Masters thesis, Indian Institute of Technology Hyderabad.

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## Abstract

The methodology of concealing the content of messsages ,comes from the greeek root word Kryptos ,meaning hidden and graphikos, meaning writing . It is the practice and study of techniques for secure communication in the presence of third parties called adversaries(enemy) who prevent the users of cryptosystem from achieving their goal.The modern scienti�c study of cryptography is sometimes referred to as cryptology. SymmetricCipher A symmetric key cipher (also called a secret-key cipher, or a one-key cipher, or a private-key cipher, or a shared-key cipher) one that uses the same (necessarily secret) key to encrypt messages as it does to decrypt messages. This is the simplest kind of encryption that involves only one secret key to cipher and decipher information. Symmetrical encryption is an old and best-known technique. A�ne Cipher, Hill cipher,Substitution Cipher are examples of symmetric encryption. The main disadvantage of the symmetric key encryption is that all parties involved have to exchange the key used to encrypt the data before they can decrypt it. Asymmetric Cipher Asymmetric cryptography, also known as public key cryptography (PKC), uses public and private keys to encrypt and decrypt data respectively. The keys are simply large numbers that have been paired together but are not identical (asymmetric). One key in the pair can be shared with everyone; it is called the public key. The other key in the pair is kept secret; it is called the private key. Either of the keys can be used to encrypt a message; the opposite key from the one used to encrypt the message is used for decryption. In contrast to the symmetric ciphers, there are only three known asymetric cipher techniques- Di�e-Hellman Key exchange method, RSA Exchange Method and Digital Signatures.

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