Chapter 14 Protection Chapter 14 Protection n

Chapter 14: Protection

Chapter 14: Protection n Goals of Protection n Principles of Protection n Domain of Protection n Access Matrix n Implementation of Access Matrix n Access Control n Revocation of Access Rights n Capability-Based Systems n Language-Based Protection 2

Goals of Protection n Operating system consists of a collection of objects, hardware or software n Each object has a unique name and can be accessed through a well-defined set of operations. n Protection problem - ensure that each object is accessed correctly and only by those processes that are allowed to do so. 3

Principles of Protection n Guiding principle – principle of least privilege l Programs, users and systems should be given just enough privileges to perform their tasks 4

Domain Structure n Access-right = where rights-set is a subset of all valid operations that can be performed on the object. n Domain = set of access-rights 5

Domain Implementation (UNIX) n System consists of 2 domains: l User l Supervisor n UNIX l Domain = user-id l Domain switch accomplished via file system. 4 Each file has associated with it a domain bit (setuid bit). 4 When file is executed and setuid = on, then user-id is set to owner of the file being executed. When execution completes user-id is reset. 6

Domain Implementation (MULTICS) n Let Di and Dj be any two domain rings. n If j < I Di Dj 7

Access Matrix n View protection as a matrix (access matrix) n Rows represent domains n Columns represent objects n Access(i, j) is the set of operations that a process executing in Domaini can invoke on Objectj 8

Access Matrix 9

Use of Access Matrix n If a process in Domain Di tries to do “op” on object Oj, then “op” must be in the access matrix. n Can be expanded to dynamic protection. l Operations to add, delete access rights. l Special access rights: 4 owner 4 copy of Oi op from Oi to Oj 4 control – Di can modify Dj access rights 4 transfer – switch from domain Di to Dj 10

Use of Access Matrix (Cont. ) n Access matrix design separates mechanism from policy. l Mechanism 4 Operating system provides access-matrix + rules. 4 If ensures that the matrix is only manipulated by authorized agents and that rules are strictly enforced. l Policy 4 User dictates policy. 4 Who can access what object and in what mode. 11

Implementation of Access Matrix n Each column = Access-control list for one object Defines who can perform what operation. Domain 1 = Read, Write Domain 2 = Read Domain 3 = Read n Each Row = Capability List (like a key) Fore each domain, what operations allowed on what objects. Object 1 – Read Object 4 – Read, Write, Execute Object 5 – Read, Write, Delete, Copy 12

Access Matrix of Figure A With Domains as Objects Figure B 13

Access Matrix with Copy Rights 14

Access Matrix With Owner Rights 15

Modified Access Matrix of Figure B 16

Access Control n Protection can be applied to non-file resources n Solaris 10 provides role-based access control to implement least privilege l Privilege is right to execute system call or use an option within a system call l Can be assigned to processes l Users assigned roles granting access to privileges and programs 17

Role-based Access Control in Solaris 10 18

Revocation of Access Rights n Access List – Delete access rights from access list. l Simple l Immediate n Capability List – Scheme required to locate capability in the system before capability can be revoked. l Reacquisition l Back-pointers l Indirection l Keys 19

Capability-Based Systems n Hydra l Fixed set of access rights known to and interpreted by the system. l Interpretation of user-defined rights performed solely by user's program; system provides access protection for use of these rights. n Cambridge CAP System l Data capability - provides standard read, write, execute of individual storage segments associated with object. l Software capability -interpretation left to the subsystem, through its protected procedures. 20

Language-Based Protection n Specification of protection in a programming language allows the high-level description of policies for the allocation and use of resources. n Language implementation can provide software for protection enforcement when automatic hardware-supported checking is unavailable. n Interpret protection specifications to generate calls on whatever protection system is provided by the hardware and the operating system. 21

Protection in Java 2 n Protection is handled by the Java Virtual Machine (JVM) n A class is assigned a protection domain when it is loaded by the JVM. n The protection domain indicates what operations the class can (and cannot) perform. n If a library method is invoked that performs a privileged operation, the stack is inspected to ensure the operation can be performed by the library. 22

Chapter 15: Security

Chapter 15: Security n The Security Problem n Program Threats n System and Network Threats n Cryptography as a Security Tool n User Authentication n Implementing Security Defenses n Fire-walling to Protect Systems and Networks n Computer-Security Classifications n An Example: Windows XP 24

The Security Problem n Security must consider external environment of the system, and protect the system resources n Intruders (crackers) attempt to breach security n Threat is potential security violation n Attack is attempt to breach security n Attack can be accidental or malicious n Easier to protect against accidental than malicious misuse 25

Security Violations n Categories l Breach of confidentiality l Breach of integrity l Breach of availability l Theft of service l Denial of service n Methods l Masquerading (breach authentication) l Replay attack 4 Message modification l Man-in-the-middle attack l Session hijacking 26

Standard Security Attacks 27

Security Measure Levels n Security must occur at four levels to be effective: l Physical l Human 4 Avoid social engineering, phishing, dumpster diving l Operating System l Network n Security is as week as the weakest chain 28

Program Threats n Trojan Horse l l Exploits mechanisms for allowing programs written by users to be executed by other users l n Code segment that misuses its environment Spyware, pop-up browser windows, covert channels Trap Door l l n Specific user identifier or password that circumvents normal security procedures Could be included in a compiler Logic Bomb l n Program that initiates a security incident under certain circumstances Stack and Buffer Overflow l Exploits a bug in a program (overflow either the stack or memory buffers) 29

Hypothetical Stack Frame After attack Before attack 30

Program Threats (Cont. ) n Viruses l Code fragment embedded in legitimate program l Very specific to CPU architecture, operating system, applications l Usually borne via email or as a macro 4 Visual Basic Macro to reformat hard drive Sub Auto. Open() Dim o. FS Set o. FS = Create. Object(’’Scripting. File. System. Object’’) vs = Shell(’’c: command. com /k format c: ’’, vb. Hide) End Sub 31

Program Threats (Cont. ) n Virus dropper inserts virus onto the system n Many categories of viruses, literally many thousands of viruses l File l Boot l Macro l Source code l Polymorphic l Encrypted l Stealth l Tunneling l Multipartite l Armored 32

A Boot-sector Computer Virus 33

System and Network Threats n Worms – use spawn mechanism; standalone program n Internet worm l Exploited UNIX networking features (remote access) and bugs in finger and sendmail programs l Grappling hook program uploaded main worm program n Port scanning l Automated attempt to connect to a range of ports on one or a range of IP addresses n Denial of Service l Overload the targeted computer preventing it from doing any useful work l Distributed denial-of-service (DDOS) come from multiple sites at once 34

Cryptography as a Security Tool n Broadest security tool available l Source and destination of messages cannot be trusted without cryptography l Means to constrain potential senders (sources) and / or receivers (destinations) of messages n Based on secrets (keys) 35

Secure Communication over Insecure Medium 36

Encryption n Encryption algorithm consists of l Set of K keys Set of M Messages l Set of C ciphertexts (encrypted messages) l A function E : K → (M→C). That is, for each k K, E(k) is a function for generating ciphertexts from messages. 4 Both E and E(k) for any k should be efficiently computable functions. l A function D : K → (C → M). That is, for each k K, D(k) is a function for generating messages from ciphertexts. 4 Both D and D(k) for any k should be efficiently computable functions. 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). l 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. l Since ciphertexts are generally exposed (for example, sent on the network), it is important that it be infeasible to derive D(k) from the ciphertexts l n 37

Symmetric Encryption n Same key used to encrypt and decrypt l E(k) can be derived from D(k), and vice versa n DES is most commonly used symmetric block-encryption algorithm (created by US Govt) l Encrypts a block of data at a time n Triple-DES considered more secure n Advanced Encryption Standard (AES), twofish up and coming n RC 4 is most common symmetric stream cipher, but known to have vulnerabilities l Encrypts/decrypts a stream of bytes (i. e wireless transmission) l Key is a input to psuedo-random-bit generator 4 Generates an infinite keystream 38

Asymmetric Encryption n Public-key encryption based on each user having two keys: l public key – published key used to encrypt data l private key – key known only to individual user used to decrypt data n Must be an encryption scheme that can be made public without making it easy to figure out the decryption scheme l Most common is RSA block cipher l Efficient algorithm for testing whether or not a number is prime l No efficient algorithm is know for finding the prime factors of a number 39

Asymmetric Encryption (Cont. ) n 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 l E(ke , N) (or just ke) is the public key l D(kd , N) (or just kd) is the private key l N is the product of two large, randomly chosen prime numbers p and q (for example, p and q are 512 bits each) l Encryption algorithm is E(ke , N)(m) = mke mod N, where ke satisfies kekd mod (p− 1)(q − 1) = 1 l The decryption algorithm is then D(kd , N)(c) = ckd mod N 40

Asymmetric Encryption Example n For example. make p = 7 and q = 13 n We then calculate N = 7∗ 13 = 91 and (p− 1)(q− 1) = 72 n We next select ke relatively prime to 72 and< 72, yielding 5 n Finally, we calculate kd such that kekd mod 72 = 1, yielding 29 n We how have our keys l l n Public key, ke, N = 5, 91 Private key, kd , N = 29, 91 Encrypting the message 69 with the public key results in the cyphertext 62 n Cyphertext can be decoded with the private key l Public key can be distributed in cleartext to anyone who wants to communicate with holder of public key 41

Encryption and Decryption using RSA Asymmetric Cryptography 42

Cryptography (Cont. ) n Note symmetric cryptography based on transformations, asymmetric based on mathematical functions l Asymmetric much more compute intensive l Typically not used for bulk data encryption 43

Authentication n Constraining set of potential senders of a message l l n Complementary and sometimes redundant to encryption Also can prove message unmodified Algorithm components l A set K of keys l A set M of messages l A set A of authenticators l A function S : K → (M→ A) 4 4 l That is, for each k K, S(k) is a function for generating authenticators from messages Both S and S(k) for any k should be efficiently computable functions A function V : K → (M× A→ {true, false}). That is, for each k K, V(k) is a function for verifying authenticators on messages 4 Both V and V(k) for any k should be efficiently computable functions 44

Authentication (Cont. ) n For a message m, a computer can generate an authenticator a A such that V(k)(m, a) = true only if it possesses S(k) n Thus, computer holding S(k) can generate authenticators on messages so that any other computer possessing V(k) can verify them n Computer not holding S(k) cannot generate authenticators on messages that can be verified using V(k) n Since authenticators are generally exposed (for example, they are sent on the network with the messages themselves), it must not be feasible to derive S(k) from the authenticators 45

Authentication – Hash Functions n Basis of authentication n Creates small, fixed-size block of data (message digest, hash value) from m n Hash Function H must be collision resistant on m l Must be infeasible to find an m’ ≠ m such that H(m) = H(m’) n If H(m) = H(m’), then m = m’ l The message has not been modified n Common message-digest functions include MD 5, which produces a 128 -bit hash, and SHA-1, which outputs a 160 -bit hash 46

Authentication - MAC n Symmetric encryption used in message-authentication code (MAC) authentication algorithm n Simple example: l MAC defines S(k)(m) = f (k, H(m)) 4 Where – f is a function that is one-way on its first argument k cannot be derived from f (k, H(m)) 4 Because of the collision resistance in the hash function, reasonably assured no other message could create the same MAC 4 A suitable verification algorithm is V(k)(m, a) ≡ ( f (k, m) = a) 4 Note that k is needed to compute both S(k) and V(k), so anyone able to compute one can compute the other 47

Authentication – Digital Signature n Based on asymmetric keys and digital signature algorithm n Authenticators produced are digital signatures n In a digital-signature algorithm, computationally infeasible to derive S(ks ) from V(kv) l l n V is a one-way function Thus, kv is the public key and ks is the private key Consider the RSA digital-signature algorithm l Similar to the RSA encryption algorithm, but the key use is reversed l Digital signature of message S(ks )(m) = H(m)ks mod N l The key ks again is a pair d, N, where N is the product of two large, randomly chosen prime numbers p and q l Verification algorithm is V(kv)(m, a) ≡ (akv mod N = H(m)) 4 Where kv satisfies kvks mod (p − 1)(q − 1) = 1 48

Authentication (Cont. ) n Why authentication if a subset of encryption? l Fewer computations (except for RSA digital signatures) l Authenticator usually shorter than message l Sometimes want authentication but not confidentiality 4 Signed l patches et al Can be basis for non-repudiation 49

Key Distribution n Delivery of symmetric key is huge challenge l Sometimes done out-of-band n Asymmetric keys can proliferate – stored on key ring l Even asymmetric key distribution needs care – man-in-themiddle attack 50

Man-in-the-middle Attack on Asymmetric Cryptography 51

Digital Certificates n Proof of who or what owns a public key n Public key digitally signed a trusted party n Trusted party receives proof of identification from entity and certifies that public key belongs to entity n Certificate authority are trusted party – their public keys included with web browser distributions l They vouch for other authorities via digitally signing their keys, and so on 52

Encryption Example - SSL n Insertion of cryptography at one layer of the ISO network model n n n (the transport layer) SSL – Secure Socket Layer (also called TLS) Cryptographic protocol that limits two computers to only exchange messages with each other l Very complicated, with many variations Used between web servers and browsers for secure communication (credit card numbers) The server is verified with a certificate assuring client is talking to correct server Asymmetric cryptography used to establish a secure session key (symmetric encryption) for bulk of communication during session Communication between each computer theb uses symmetric key cryptography 53

User Authentication n Crucial to identify user correctly, as protection systems depend on user ID n User identity most often established through passwords, can be considered a special case of either keys or capabilities l Also can include something user has and /or a user attribute n Passwords must be kept secret l Frequent change of passwords l Use of “non-guessable” passwords l Log all invalid access attempts n Passwords may also either be encrypted or allowed to be used only once 54

Implementing Security Defenses n Defense in depth is most common security theory – multiple layers of security n Security policy describes what is being secured n Vulnerability assessment compares real state of system / network compared to security policy n Intrusion detection endeavors to detect attempted or successful intrusions l Signature-based detection spots known bad patterns l Anomaly detection spots differences from normal behavior 4 Can detect zero-day attacks False-positives and false-negatives a problem n Virus protection n Auditing, accounting, and logging of all or specific system or network activities l 55

Fire-walling to Protect Systems and Networks n A network firewall is placed between trusted and untrusted hosts The firewall limits network access between these two security domains n Can be tunneled or spoofed l Tunneling allows disallowed protocol to travel within allowed protocol (i. e. telnet inside of HTTP) l Firewall rules typically based on host name or IP address which can be spoofed n Personal firewall is software layer on given host l Can monitor / limit traffic to and from the host l n Application proxy firewall understands application protocol and can control them (i. e. SMTP) n System-call firewall monitors all important system calls and apply rules to them (i. e. this program can execute that system call) 56

Network Security Through Domain Separation Via Firewall 57

Computer Security Classifications n U. S. Department of Defense outlines four divisions of computer security: A, B, C, and D. n D – Minimal security. n C – Provides discretionary protection through auditing. Divided into C 1 and C 2. C 1 identifies cooperating users with the same level of protection. C 2 allows user-level access control. n B – All the properties of C, however each object may have unique sensitivity labels. Divided into B 1, B 2, and B 3. n A – Uses formal design and verification techniques to ensure security. 58

Example: Windows XP n Security is based on user accounts l Each user has unique security ID l Login to ID creates security access token 4 Includes security ID for user, for user’s groups, and special privileges 4 Every process gets copy of token 4 System checks token to determine if access allowed or denied n Uses a subject model to ensure access security. A subject tracks and manages permissions for each program that a user runs n Each object in Windows XP has a security attribute defined by a security descriptor l For example, a file has a security descriptor that indicates the access permissions for all users 59

End of Chapter 15