Security and CryptographyNetwork threats and attacksSecurity requirements for transmitting informationPowerPoint PresentationSlide 5Slide 6Classification of cryptosystemsShift cipher--exampleSecret cryptosystem--DESKey management and exchangeWhy public-key cryptographyPublic-key cryptosystemPublic-key system: how it worksRSA cryptosystemTwo party key managementDLP (Discrete Logarithm Problem)(Two-party) Diffie-Hellman (DH) key exchangecryptologyKerckhoff principle and attack levelsInternet security protocolsIPSec key agreementSSL positionSSL functionalitySSL handshake1Security and Cryptography•Security: all issues which make secure communication (information transmission, two (multiple) party interaction) over insecure channels.•Cryptography: the science and art of manipulating messages to make them secure.•Classical cryptographic techniques.•Along with the development of communication networks and their broad applications, network security is becoming a more serious problem.•Thus, call for modern cryptography.2Network threats and attacksPassive:Active:EavesdroppingTraffic analysisMasquerading Replay ModificationDenial of serviceClientServerAttacker ServerClientServermodifyClientclientServerMan-in-middleClientServerreplay3Security requirements for transmitting information•Privacy or confidentiality: the information should be readable only by the intended receiver. i.e., protect the information from eavesdropping. •Integrity: the receiver can confirm that a message has not been altered during transmission, i.e., protect the information from tampering. •Authentication: any party (sender or receiver) can verify that the other party is who he or she claims to be, i.e., validate the identity of the other party.•Nonrepudiation: the sender can not deny having sent a given message. i.e., if a transaction (e.g., a purchase) has occurred between two parties, the nonrepudiation service can prove that for any party, he/she really performed the transaction him/herself, not by any other person.4Approaches to implementing securityConfidentiality: By encryption (and decryption)Sender: encrypts the message using a key and sends the encrypted message.Receiver: decrypts the encrypted message using the same key as the sender’s key or a key derivable from the sender’s key. Integrity: By checksum or hash value/message digest.Sender: computes checksum/hash value/message digest from the message and sends the message along with the checksum/hash value/message digest.Receiver: re-computes checksum/hash value/message digest from received message and compares with the transmitted checksum/hash value/message digest.In some sense, it likes error-detection.Problem: the attacker, after intercepting the message, modifies the message, computes the checksum for modified message, and resends them.Solution: keyed checksum/hash value/message digest.message checksumBoth are transmittedmessage checksumkeyMessage + checksum are transmitted5Approaches to implementing security (cont.)Authentication:Traditional user ID and password. Modern cryptography based authentication.--Digital signature.Nonrepudiation: Undeniable signature, i.e., Digital signature + verification protocol + disavowal protocol6Security requirements and their implementationConfidentiality: encryption (and decryption)Integrity: checksum or hash value/message digest or MAC.Authentication: user ID and password or Digital signature.Nonrepudiation: Undeniable signatureAvailability: Intrusion detection and defenseAuthorization: Access controlAccountability: Log, record, trace, system administrationQ: how to defense Replay attack?Timestamps and/or sequence numbers.7Classification of cryptosystems•Secret key systems vs. public key systems•Classical vs. modern–Classical: secret key systems•Shift, Affine, Vigenere, Hill, Permutation (transposition) cipher, Stream cipher –Modern:•Secret key systems–DES, AES, PGM•Public key systems–RSA, ElGamal, Elliptic Curve8Shift cipher--example•Suppose a plaintext word: cryptography•Change each letter by shifting the letter three position rightward• The cipherword is: FUBSWRJUDSKBQuestion: if given the above cipherword, how to get original word?Change each letter by shifting the letter three position leftward.This kind of cryptosystem is called “Caesar Cipher”9Secret cryptosystem--DES•Data Encryption Standard (DES)•First version in 1975, developed by IBM.•A type of iterated cipher. •Plaintext block: 64 bits, key: 56 bits, ciphertext block:64 bits. •Steps:–Initial permutation (IP)–16 rounds of transformations–Inverse permutation (IP-1)10Key management and exchange•Key is the essential part in any cryptosystem, especially in secret key systems.•How to distribute/exchange key/keys between two users/any pair of multiple users.•Therefore key management and key exchange come into play.•Also public key systems appeared.11Why public-key cryptography1. The two communicants in secret key system require the prior communication of key, using a secure channel. it is very difficult to achieve in practice. Unless the two communicants meet together, phone call, post mail, email etc., are not secure.2. Suppose there are n users and every pair of users want to communicate. In secret-key system, it is necessary that the total number of keys is n(n-1)/2. Very difficult to management and quite insecure. However, in public-key system, every user selects his/her own private key and public key, and publicizes the public key but keep the private key secret. Quite easy and very secure. The main problem with public-key system is that it is very slow.12Public-key cryptosystem•Secret-key cryptosystem: –eK & dK: dK is the same as or derived from eK.–Called symmetric-key cryptosystem.–Problem: how to distribute eK & dK to Alice & Bob securely.•Public-key cryptosystem:–Computationally infeasible to compute dK from eK.–Called asymmetric-key cryptosystem.–eK is made public, called public key –But dK is kept secret, called private key.13Public-key system: how it works•Everybody selects its own public key P and private key S, and publicizes P.•Therefore Alice has (Pa , Sa), and Bob has (Pb , Sb). •Everybody knows Pa, Pb, …•Suppose Alice wants to send a message to Bob.–Alice encrypts the message with Bob’s public key Pb and sends out.–(only) Bob can decrypt the message using his