# asymmetric ciphers

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ASYMMETRIC CIPHERS. Contents. INTRODUCTION TO NUMBER THEORY PUBLIC-KEYCRYPTOGRAPHY AND RSA OTHER PUBLIC-KEY CRYPTOSYSTEMS. 1. INTRODUCTION TO NUMBER THEORY. Prime Numbers Fermats and Eulers Theorems Testing for Primality The Chinese Remainder Theorem Discrete Logarithms. KEY POINTS. - PowerPoint PPT Presentation

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General Security Concepts

ASYMMETRIC CIPHERSContentsINTRODUCTION TO NUMBER THEORYPUBLIC-KEYCRYPTOGRAPHY AND RSAOTHER PUBLIC-KEY CRYPTOSYSTEMS21. INTRODUCTION TO NUMBER THEORYPrime NumbersFermats and Eulers TheoremsTesting for PrimalityThe Chinese Remainder TheoremDiscrete Logarithms3KEY POINTSA prime number is an integer that can only be divided without remainder by positive and negative values of itself and 1. Prime numbers play a critical role both in number theory and in cryptography.Two theorems that play important roles in public-key cryptography are Fermats theorem and Eulers theorem.4KEY POINTS (cont.)An important requirement in a number of cryptographic algorithms is the ability to choose a large prime number. An area of ongoing research is the development of efficient algorithms for determining if a randomly chosen large integer is a prime number.Discrete logarithms are fundamental to a number of public-key algorithms. Discrete logarithms are analogous to ordinary logarithms but are defined using modular arithmetic.52. PUBLIC-KEY CRYPTOGRAPHY AND RSAPrinciples Of Public-Key CryptosystemsThe RSA Algorithm

6KEY POINTSAsymmetric encryption is a form of cryptosystem in which encryption and decryption are performed using the different keysone a public key and one a private key. It is also known as public-key encryption.Asymmetric encryption transforms plaintext into ciphertext using a one of two keys and an encryption algorithm. Using the paired key and a decryption algorithm, the plaintext is recovered from the ciphertext.7KEY POINTS (cont.)Asymmetric encryption can be used for confidentiality, authentication, or both.The most widely used public-key cryptosystem is RSA. The difficulty of attacking RSA is based on the difficulty of finding the prime factors of a composite number.8Terminology Related to Asymmetric EncryptionAsymmetric KeysTwo related keys, a public key and a private key, that are used to perform complementary operations, such as encryption and decryption or signature generation and signature verification.Public Key CertificateA digital document issued and digitally signed by the private key of a Certification Authority that binds the name of a subscriber to a public key. The certificate indicates that the subscriber identified in the certificate has sole control and access to the corresponding private key.9Terminology (cont.)Public Key (Asymmetric) Cryptographic AlgorithmA cryptographic algorithm that uses two related keys, a public key and a private key. The two keys have the property that deriving the private key from the public key is computationally infeasible.Public Key Infrastructure (PKI)A set of policies, processes, server platforms, software and workstations used for the purpose of administering certificates and public-private key pairs, including the ability to issue, maintain, and revoke public key certificates.10Public-Key Cryptography11

Public-Key Cryptography (cont.)12

A public-key encryption scheme has six ingredients:PlaintextEncryption algorithmPublic keyPrivate keyCiphertextDecryption algorithm13Conventional and Public-Key Encryption14

Public-Key Cryptosystem: Secrecy15

Public-Key Cryptosystem: Authentication16

Public-Key Cryptosystem: Authentication and Secrecy17

Applications for Public-Key Cryptosystems18

Public-Key CryptanalysisBrute-force attackcompute the private key given the public keyprobable-message attack19THE RSA ALGORITHM20

The RSA Algorithm (cont.)21

RSA Processing of Multiple Blocks22

The Security of RSAFour possible approaches to attacking the RSA algorithm areBrute force: This involves trying all possible private keys.Mathematical attacks: There are several approaches, all equivalent in effort to factoring the product of two primes.Timing attacks: These depend on the running time of the decryption algorithm.Chosen ciphertext attacks: This type of attack exploits properties of the RSA algorithm.23THE FACTORING PROBLEM24

Progress in Factorization25

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3. OTHER PUBLIC-KEY CRYPTOSYSTEMSDiffie-Hellman Key ExchangeElgamal Cryptographic SystemElliptic Curve ArithmeticElliptic Curve CryptographyPseudorandom Number Generation Based on an Asymmetric Cipher29KEY POINTSA simple public-key algorithm is Diffie-Hellman key exchange. This protocol enables two users to establish a secret key using a public-key scheme based on discrete logarithms. The protocol is secure only if the authenticity of the two participants can be established.Elliptic curve arithmetic can be used to develop a variety of elliptic curve cryptography (ECC) schemes, including key exchange, encryption, and digital signature.For purposes of ECC, elliptic curve arithmetic involves the use of an elliptic curve equation defined over a finite field. The coefficients and variables in the equation are elements of a finite field. Schemes using Zp and GF(2^m) have been developed.30The Diffie-Hellman Key Exchange Algorithm31

Key Exchange Protocols32

The ElGamal Cryptosystem33

The ElGamal Cryptosystem (cont.)34