Lecture Network security: Chapter 17 - Dr. Munam Ali Shah

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Network Security Lecture 17 Presented by: Dr. Munam Ali Shah Summary of the Previous Lecture  We discussed stream ciphers and its working  We explored how stream ciphers are efficient when compared to block ciphers in terms of performance  Some examples of stream ciphers such as RC4, RC5 and blowfish etc. were explored Summary of the previous Lecture  Stream Cipher Properties some design considerations are:  long period with no repetitions  statistically random  depends on large enough key  large linear complexity  use of highly non-linear boolean functions  Ci = Mi XOR StreamKeyi Stream Cipher Illustration Summary of the Previous Lecture (RC4)  a proprietary cipher owned by RSA another Ron Rivest design, simple but effective  variable key size (1-256 bytes)  byte-oriented stream cipher  widely used (web SSL/TLS, wireless WEP)  key forms random permutation of all 8-bit values  uses that permutation to scramble input info processed a byte at a time  Remained trade secret till 1994 Part 2 (d) Asymmetric Key Cryptography Outlines of today’s lecture  We will explore the need, features and characteristics of public key cryptography  The working/function of a public key cryptography scheme will be discussed in detail  RSA, as an example, will be explained Objectives  You would be able to present an understanding of the public key cryptography.  You would be able use and implement the RSA technique. Different names  Public key cryptography  Asymmetric key cryptography  2 key cryptography Presented by Diffie & Hallman (1976) New directions in cryptography Why Public-Key Cryptography?  Key distribution under symmetric encryption requires Two communicants already share a key  The use of Key Distribution Center (KDC)  Whitfield Diffie & Martin Hellman reasoned  2nd requirement neglected the essence of cryptography, i.e. the ability to maintain total secrecy over your own communication  how to verify a message comes intact from the claimed sender?  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  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 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 Essential steps  Each user  generates its pair of keys  Places public key in public folder  Bob encrypt the message using Alice’s public key for secure communication  Alice decrypts it using her private key Asymmetric Key Cryptography  In symmetric cryptography: 1. If Alice and Bob are physically apart and communicate, they have to agree on a key  Meet  Use 2. personally, or trusted couriers Alice needs one secret key for Bob, one for Carol, one for Dave and so on  Storage of so many secret keys is not feasible Asymmetric Key Cryptography  In Asymmetric Key Cryptography:  2 people who never met can communicate securely  Alice can securely communicate with all her friends by storing just a single private key  2 keys are used  Public: known to everyone (for encryption or signature verification)  Private: known to receiver only (for decryption or signature generation) Public-Key Cryptography Public-Key Cryptography 1. Plaintext 2. Encryption algorithm 3. Public and private keys 4. Ciphertext 5. Decryption algorithm Public-Key Cryptography Confidentiality  Y = E(PUb, X )  X = D(PRb, Y )  Adversary can access PUb and Y, attempt to recover X or PRb 22 Integrity  Impossible to alter the message without access to A’s private key  Authenticate the source  Ensure data integrity 23 Authentication and Confidentiality  Z = E(PUb, E(PRa, X))  X = D(PUa, E(PRb, Z))  Overhead: public key algorithm executed four times Public-Key Applications  can classify uses into 3 categories:  encryption/decryption (provide secrecy)  digital signatures (provide authentication)  key exchange (of session keys) Algorithm En/decryption Digital signature Key exchange RSA Yes Yes Yes Elliptic curve Yes Yes Yes Diffie Hellman No No Yes DSS No Yes No Requirements for Public key cryptography  Computationally easy  for B to generate a pair of key (public and private)  for sender A, knowing the public key and the message M to generate the ciphertext C = E(PUb, M)  for receiver B, to decrypt the ciphertext using its private key to recover M M = D(PRb, C) = D(PRb, E(PUb, M) )  Computationally infeasible for an adversary  knowing the PUb to determine the private key PRb  knowing the PUb and ciphertext C to recover M Security of Public Key Schemes  like private key schemes brute force exhaustive search     attack is always theoretically possible keys used are too large (>512bits) security relies on a large enough difference in difficulty between easy (en/decrypt) and hard (cryptanalyse) problems requires the use of very large numbers hence is slow compared to private/symmetric key schemes The RSA Algorithm  by Rivest, Shamir & Adleman of MIT in 1977  best known & widely used public-key scheme  Block cipher scheme: plaintext and ciphertext are integer between 0 to n-1 for some n  Use large integers e.g. n = 1024 bits RSA Key Setup  each user generates a public/private key pair by:  selecting two large primes at random - p, q  Computing n=p.q  ø(n)=(p-1)(q-1)   selecting at random the encryption key e  where 1
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