Cryptography and Network Security PDF

Cryptography and Network Security Chapter 9 Fourth Edition by William Stallings Lecture slides by Lawrie Brown  Private-Key Cryptography traditional private/secret/single key cryptography uses one key shared by both sender and receiver if this k...

Cryptography and Network Security Chapter 9 Fourth Edition by William Stallings Lecture slides by Lawrie Brown  Private-Key Cryptography traditional private/secret/single key cryptography uses one key shared by both sender and receiver if this key is disclosed communications are compromised also is symmetric, parties are equal hence does not protect sender from receiver forging a message & claiming is sent by sender  Public-Key Cryptography probably most significant advance in the 3000 year history of cryptography uses two keys – a public & a private key asymmetric since parties are not equal uses clever application of number theoretic concepts to function complements rather than replaces private key crypto  Public-Key Cryptography public-key/two-key/asymmetric cryptography involves the use of two keys: – a public-key, which may be known by anybody, and can be used to encrypt messages, and verify signatures – a private-key, known only to the recipient, used to decrypt messages, and sign (create) signatures is asymmetric because – those who encrypt messages or verify signatures cannot decrypt messages or create signatures Public-Key Cryptography  Public-Key Characteristics Public-Key algorithms rely on two keys where: – it is computationally infeasible to find decryption key knowing only algorithm & encryption key – it is computationally easy to en/decrypt messages when the relevant (en/decrypt) key is known – either of the two related keys can be used for encryption, with the other used for decryption (for some algorithms) Public-Key Cryptosystems Secrecy and Authentication  RSA Encryption Algorithm RSA encryption algorithm is a type of public-key encryption algorithm. Public Key encryption algorithm is also called the Asymmetric algorithm. Asymmetric algorithms are those algorithms in which sender and receiver use different keys for encryption and decryption. Each sender is assigned a pair of keys:  RSA Encryption Algorithm The Public key is used for encryption, and the Private Key is used for decryption. Decryption cannot be done using a public key. The two keys are linked, but the private key cannot be derived from the public key. The public key is well known, but the private key is secret and it is known only to the user who owns the key. It means that everybody can send a message to the user using user's public key. But only the user can decrypt the message using his private key.  RSA Encryption Algorithm The data to be sent is encrypted by sender A using the public key of the intended receiver B decrypts the received ciphertext using its private key, which is known only to B. B replies to A encrypting its message using A's public key. A decrypts the received ciphertext using its private key, which is known only to him. RSA Encryption Algorithm Publi d c  RSA algorithm uses the following procedure to generate public and private keys: Select two large prime numbers, p and q. Multiply these numbers to find n = p x q, where n is called the modulus for encryption and decryption. Choose a number e less than φ (n), such that e is relatively prime to (p - 1) x (q -1). It means that e and (p - 1) x (q - 1) have no common factor except 1. Choose "e" such that 1