1CSC 4103 - Operating SystemsFall 2009Tevfik Ko!arLouisiana State UniversityNovember 19th, 2009Lecture - XXIIIProtection and SecurityThe Security Problem• Protecting your system resources, your files, identity, confidentiality, or privacy• Intruders (crackers) attempt to breach security• Threat is potential security violation• Attack is attempt to breach security• Attack can be accidental or malicious• Easier to protect against accidental than malicious misuseSecurity Violations• Categories– Breach of confidentiality • information theft, identity theft– Breach of integrity • unauthorized modification of data– Breach of availability • unauthorized destruction of data – Theft of service • unauthorized use of resources– Denial of service • crashing web serversSecurity Violation Methods– Masquerading (breach authentication)• Pretending to be somebody else– Replay attack (message modification)• Repeating a valid data transmission (eg. Money transfer)• May include message modification– Session hijacking• The act of intercepting an active communication session– Man-in-the-middle attack• Masquerading both sender and receiver by intercepting messagesProgram Threats • Trojan Horse– Code segment that misuses its environment– Exploits mechanisms for allowing programs written by users to be executed by other users– Spyware, pop-up browser windows, covert channels• Trap Door–A hole in the security of a system deliberately left in place by designers or maintainers– Specific user identifier or password that circumvents normal security procedures• Logic Bomb– Program that initiates a security incident under certain circumstances• Stack and Buffer Overflow– Exploits a bug in a program (overflow either the stack or memory buffers)Program Threats (Cont.)• Viruses– Code fragment embedded in legitimate program– Very specific to CPU architecture, operating system, applications– Usually borne via email or as a macro• Visual Basic Macro to reformat hard driveSub AutoOpen()Dim oFS Set oFS = CreateObject(’’Scripting.FileSystemObject’’) vs = Shell(’’c:command.com /k format c:’’,vbHide)End SubProgram Threats (Cont.)• Virus dropper inserts virus onto the system• Many categories of viruses, literally many thousands of viruses:– File (appends itself to a file, changes start pointer, returns to original code)– Boot (writes to the boot sector, gets exec before OS)– Macro (runs as soon as document containing macro is opened)– Source code (modifies existing source codes to spread)– Polymorphic (changes each time to prevent detection)– Encrypted (first decrypts, then executes)– Stealth (modify parts of the system to prevent detection, eg read system call)– Tunneling (installs itself as interrupt handler or device driver)– Multipartite (can infect multiple pars of the system, eg. Memory, bootsector, files)– Armored (hidden and compressed virus files)System and Network Threats• Worms – use spawn mechanism; standalone program• Internet worm (Robert Morris, 1998, Cornell)– Exploited UNIX networking features (remote access) and bugs in finger and sendmail programs– Grappling hook program uploaded main worm program• Port scanning– Automated attempt to connect to a range of ports on one or a range of IP addresses• Denial of Service– Overload the targeted computer preventing it from doing any useful work– Distributed denial-of-service (DDOS) come from multiple sites at onceCryptography as a Security Tool• Broadest security tool available– Source and destination of messages cannot be trusted without cryptography– Means to constrain potential senders (sources) and / or receivers (destinations) of messages• Based on secrets (keys)Secure Communication over Insecure MediumEncryption• Encryption algorithm consists of– Set of K keys– Set of M Messages– Set of C ciphertexts (encrypted messages)–A function E : K ! (M!C). That is, for each k ! K, E(k) is a function for generating ciphertexts from messages. –A function D : K ! (C ! M). That is, for each k ! K, D(k) is a function for generating messages from ciphertexts. –Encryption• An encryption algorithm must provide this essential property: Given a ciphertext c ! C, a computer can compute m such that E(k)(m) = c only if it possesses D(k). – Thus, a computer holding D(k) can decrypt ciphertexts to the plaintexts used to produce them, but a computer not holding D(k) cannot decrypt ciphertexts.– Since ciphertexts are generally exposed (for example, sent on the network), it is important that it be infeasible to derive D(k) from the ciphertextsSymmetric Encryption• Same key used to encrypt and decrypt– E(k) can be derived from D(k), and vice versa• DES is most commonly used symmetric block-encryption algorithm (created by US Govt)– Encrypts a block of data at a time (64 bit messages, with 56 bit key)• Triple-DES considered more secure (repeat DES three times with three different keys)• Advanced Encryption Standard (AES) replaces DES– Key length upto 256 bits, working on 128 bit blocks• RC4 is most common symmetric stream cipher (works on bits, not blocks), but known to have vulnerabilities– Encrypts/decrypts a stream of bytes (i.e wireless transmission, web browsers)– Key is a input to psuedo-random-bit generator• Generates an infinite keystreamSecure Communication over Insecure MediumAsymmetric Encryption• Encryption and decryption keys are different• Public-key encryption based on each user having two keys:– public key – published key used to encrypt data– private key – key known only to individual user used to decrypt data• Must be an encryption scheme that can be made public without making it easy to figure out the decryption scheme– Most common is RSA (Rivest, Shamir, Adleman) block cipherEncryption and Decryption using RSA Asymmetric CryptographyAsymmetric Encryption (Cont.)• Formally, it is computationally infeasible to derive D(kd , N) from E(ke , N), and so E(ke , N) need not be kept secret and can be widely disseminated–E(ke , N) (or just ke) is the public key–D(kd , N) (or just kd) is the private key– N is the product of two large, randomly chosen prime numbers p and q (for example, p and q are 512 bits each)–Select ke and kd, where ke satisfies kekd mod (p"1)(q "1) = 1–Encryption algorithm is E(ke , N)(m) = mke mod N, –Decryption algorithm is then