SecuritySecurity in the real worldSecurity in Computer SystemsSecurity ThreatsCryptography OverviewSecret-Key CryptographySymmetric Key CryptographyPublic Key CryptographyDigital SignaturesAuthenticationOnline passwords attacksOffline password attacksMore offline attacksSalting ExampleOne time passwordsLamport’s hash notesChallenge Response SchemeAuth. Using Physical ObjectsSmart CardsBiometrics: something you areSecurity: AttacksTrojan HorseLogin SpoofingLogic BombsTrap DoorsBuffer OverflowViruses and WormsDenial of ServiceOther Network AttacksProtection: ACLs & CapabilitiesEncoding SecurityAccess Control ListsAccess Control Lists ExamplesACLs DiscussionCapabilitiesSlide 36Protecting CapabilitiesSlide 38Slide 39Capability RevocationComparing ACLs & CapabilitiesTrusted SystemsTrusted Computing BaseReference MonitorAccess ControlBell-La Padula ModelBiba ModelCovert ChannelsSteganographyOrange BookSecuritySecurity in the real world•Security decisions based on:–Value, Locks, Police•Some observations:–Not all locks are the same–People pay for security they need–Police are critical to the picture–Security is only as good as the weakest linkSecurity in Computer Systems•In computer systems, this translates to:–Authorization–Authentication–Audit•This is the Gold Standard for Security (Lampson)•Some security goals:–Data confidentiality: secret data remains secret–Data integrity: no tampering of data–System availability: unable to make system unusable–Privacy: protecting from misuse of user’s informationSecurity ThreatsIdentified by Defense Science Board:•Incomplete, inquisitive and unintentional blunders. •Hackers driven by technical challenges. •Disgruntled employees or customers seeking revenge. •Criminals interested in personal financial gain or stealing services. •Organized crime with the intent of hiding something or financial gain. •Organized terrorist groups attempting to influence U.S. policy by isolated attacks. •Foreign espionage agents seeking to exploit information for economic, political, or military purposes. •Tactical countermeasures intended to disrupt specific weapons or command structures. •Multifaceted tactical information warfare applied in a broad orchestrated manner to disrupt a major U.S. military mission. •Large organized groups or nation-states intent on overthrowing the USCryptography Overview•Encrypt data so it only makes sense to authorized users–Input data is a message or file called plaintext–Encrypted data is called ciphertext•Encryption and decryption functions should be public–Security by obscurity is not a good idea!Secret-Key Cryptography•Also called symmetric cryptography–Encryption algorithm is publicly known–E(message, key) = ciphertext D(ciphertext, key) = message•Naïve scheme: monoalphabetic substitution–Plaintext : ABCDEFGHIJKLMNOPQRSTUVWXYZ–Ciphertext: QWERTYUIOPASDFGHJKLZXCVBNM–So, attack is encrypted to: qzzqea–26! possible keys ~ 4x1026 possibilities•1 µs per permutation 10 trillion years to break –easy to break this scheme! How?•‘e’ occurs 14%, ‘t’ 9.85%, ‘q’ 0.26%Symmetric Key Cryptography•Which encryption algorithm is good?–DES was proposed in the 1970s•Encrypts 64 bits of data with 56 bit key to give 64-bit ciphertext•Uses 16 rounds of substitution and permutation•EFF invested $250000 to break DES message in 56 hours•DES made powerful by encrypting message 3 times (DES3)–Current standard is AES•A result of 3-year competition with entries from 12 countries•Winning entry was from Belgium, called ‘Rijndael’–Strong algorithms, such as DES3, RC4 are used•WEP uses RC4Public Key Cryptography•Diffie and Hellman, 1976•All users get a public key and a private key–Public key is published–Private key is not known to anyone else•If Alice has a packet to send to Bob,–She encrypts the packet with Bob’s public key–Bob uses his private key to decrypt Alice’s packet•Private key linked mathematically to public key–Difficult to derive by making it computationally infeasible (RSA)•Pros: more security, convenient, digital signatures•Cons: slowerDigital Signatures•Hashing function hard to invert, e.g. MD5, SHA•Apply private key to hash (decrypt hash)–Called signature block•Receiver uses sender’s public key on signature block–E(D(x)) = x should work (works for RSA)Authentication•Establish the identity of user/machine by–Something you know (password, secret)–Something you have (credit card, smart card)–Something you are (retinal scan, fingerprint)•In the case of an OS this is done during login–OS wants to know who the user is•Passwords: secret known only to the subject–Simplest OS implementation keeps (login, password) pair–Authenticates user on login by checking the password–Try to make this scheme as secure as possible!•Display the password when being typed? (Windows, UNIX)Online passwords attacks•Online attacks: system used to verify the guesses–How someone broke into LBL–Thwart these attacks: •limit the number of guesses •better passwordsOffline password attacks•Depends on how passwords are stored•Approach 1: store username/password in a file–Attacker only needs to read the password file–Security of system now depends on protection of this file!•Approach 2: store username/encrypted password in file–Properties of the one-way hash function h:•h is not invertible: h(m) easy to compute, h-1(m) difficult •It is hard to find m and m’ s.t. h(m) = h(m’)–Should use standard functions, such as SHA, MD5, etc.Crypto hash ComparePassword FILEMore offline attacks•Previous scheme can be attacked: Dictionary Attack–Attacker builds dictionary of likely passwords offline–At leisure, builds hash of all the entries–Checks file to see if hash matches any entry in password file–There will be a match unless passwords are truly random–20-30% of passwords in UNIX are variants of common words•Morris, Thompson 1979, Klein 1990, Kabay 1997•Solutions:–Shadow files: move password file to /etc/shadow•This is accessible only to users with root permissions–Salt: store (user name, salt, E(password+salt))•Simple dictionary attack will not work. Search space is more.Salting Example•If the hacker guesses Dog, he has to try Dog0001, …•UNIX adds 12-bit of salt•Passwords should be made secure:–Length, case, digits, not from
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