Operating System Security Andy Wang COP 5611 Advanced

Operating System Security Andy Wang COP 5611 Advanced Operating Systems

Outline n n n Introduction Threats Basic security principles Security on a single machine Distributed systems security q Data communications security

Introduction n n Security is an engineering problem Always a tradeoff between safety, cost, and inconvenience Not much solid theory in the field Hard to provide any real guarantees q q Because making mistakes is easy And the nature of the problem implies that mistakes are always exploited

History of Security Problem n n n Originally, there was no security problem Later, there was a problem, but nobody cared Now, there are increasing problems, and people are beginning to care q q q Automation Action at a distance Technique propagation

Constraints of Practical Computer Security n Security costs q n n n If too much, it won’t be used If it isn’t easy, it won’t be used Misuse often makes security measures useless Fit the stringency of the measure to the threat being countered

Security is as Strong as the Weakest Link n n n Opponents will attack the weakest point Putting an expensive lock on a cheap door doesn’t help much Must look on security problems as part of an integrated system q Not just a single component

Security Threats n n n Extremely wide range of threats From a wide variety of sources Requiring a wide variety of countermeasures Generally, countering any threat costs something So people counter as few as they can afford

Physical Security n n n Some threats involve access to the equipment itself Such as theft, destruction tampering Physical threats usually require physical prevention methods

Social Engineering and Security n Computer security easily subverted by bad human practices q n n E. g. , giving key out over the phone to anyone who asks Social engineering attacks tend to be cheap, easy, effective So all our work may be for naught

A Classification of Threats n n Viewed as types of attacks on normal service So what is normal service? Information Source Information Destination

Classification of Threat Types n n Secrecy Integrity Availability Exclusivity

Interruption Information Source Information Destination

Interruption Threats n n Denial of service Prevents source from sending information to receiver Or receiver from sending request to source A threat to availability

How Does an Interruption Threat Occur? n n n Destruction of HW/SW Interference with communications channel Overloading a shared resource

Interception Information Source Information Destination Unauthorized Third Party

Another Type of Interception Information Source Information Destination Unauthorized Third Party

Interception Threats n n Data or services provided to unauthorized party Either in conjunction with or independent of authorized access A threat to secrecy Also a threat to exclusivity

How Do Interception Threats Occur? n n Eavesdropping Masquerading Break-ins Illicit data copying

Modification Information Source Information Destination Unauthorized Third Party

Another Type of Modification Threat 3 Information Source 1 2 Information Destination Unauthorized Third Party

Modification Threats n n Unauthorized parties modify data Either on the way to the users (inject ads with legit looking links) Or permanently at the servers A threat to integrity

How Do Modification Threats Occur? n n n Interception of data requests Masquerading Illicit access to servers/services

Fabrication Information Source Information Destination Unauthorized Third Party

Fabrication Threats n n Unauthorized party inserts counterfeit objects into the system Causing improper changes in data Or improper use of system resources A threat of integrity

How Do Fabrication Threats Occur? n n n Masquerading Bypassing protection measures Duplication of legitimate requests

Active Threats vs. Passive Threats n Passive threats are forms of eavesdropping q n n n No modifications, injections of requests, etc. occur Active threats are more aggressive Passive threats are mostly to secrecy Active threats are to availability, integrity, exclusivity

What Are We Protecting n n n Hardware Software Data Communications lines and networks Economic values

Basic Security Principles n n Terms and concepts Mechanisms

Security and Protection n Security is a policy q n Protection is a mechanism q n E. g. , “no unauthorized user may access this file” E. g. , “the system checks user identity against access permissions” Protection mechanisms implement security policies

Design Principles for Secure Systems n n n n Economy Complete mediation Open design Least privilege Least common mechanism Acceptability Fail-safe defaults

Economy in Security Design n Economical to develop q n n n And to use Should add little of no overhead Should do only what needs to be done Generally, try to keep it simple and small

Complete Mediation n Apply security on every access to an object that a mechanism is meant to protect q n E. g. , each read of a file, not just the open Does not necessarily require actual checking on each access q Secure session

Open Design n n Don’t rely on “security through obscurity” Assume all potential intruders know everything about the design q And completely understand it

Separation of Privileges n n n Provide mechanisms that separate the privileges used for one purpose from those used for another To allow flexibility in the security system E. g. , separate access control on each file

Least Privilege n n n Give bare minimum access rights required to complete a task Require another request to perform another type of access E. g. , don’t give write permission if he only asked for read

Least Common Mechanism n Avoid sharing parts of the security mechanism among different users q n E. g. passwords Coupling users leads to possibilities for them to breach the system

Acceptability n n Mechanism must be simple to use Simple enough that people will use it automatically q Example n n n Cashier register sticker “If you don’t get a receipt, your meal is free” Must rarely or never prevent permissible accesses

Fail-Safe Designs n n n Default to lack of access So if something goes wrong/is forgotten/isn’t done, no security is lost If important mistakes are made, you’ll find out about them q Without loss of security

Sharing Security Spectrum n n n No protection Isolation Share all or nothing Share with access limitations Share with dynamic capabilities

Important Security Mechanisms n n n Encryption Authentication Passwords Other authentication mechanisms Access control mechanisms

Encryption n n Various algorithms can be used to make data unreadable to intruders This process is called encryption q q Typically, encryption uses a secret key known only to legitimate users of the data Without the key, decrypting the data is computationally infeasible

Encryption Example n n M is the plaintext (text to be encrypted) E is the encryption algorithm Ke is the key C is the ciphertext (encrypted text) C = E(M, Ke)

Decrypting the Ciphertext n n n C is the ciphertext D is the decryption algorithm Kd is the decryption key M = D(C, Kd)

Symmetrical Encryption n Many common encryption algorithms are symmetrical q n I. e. : E = D and Ke = Kd Some important encryption algorithms are not symmetrical, however

Encryption Security Assumptions n Assume that someone trying to break the encryption knows: q q n The algorithms E and D Arbitrary amounts of matching plaintext and ciphertext M and C But does not know the keys Ke and Kd

Evaluating Security of Encryption n Given these assumptions, and a new piece of ciphertext Cn, how hard is it to discover Mn? Either by figuring out Kd or some other method What if Mn matches one of the known pieces of plaintext?

Practical Security of Encryption n Most encryption algorithms can be broken Goal is to make breaking them too expensive to bother How do we protect our encryption?

Key Issues in Encryption n Security often depends on length of key q q n Long keys give better security But slows down encryption The more data sent with a given key, the greater the chance of compromise q The more data sent with a given key, the greater the value of deducing it

Encryptions not Enough n n Limited possibilities: E(“Buy”, K), E(“Sell”, K) Reordering of encrypted blocks q Alice sends Bob some encrypted blocks n q q Eve intercepts and rearranges blocks Bob deciphers it n n E(“L”, K), E(“I”, K), E(“V”, K), E(“E”, K) EVIL Statistical regularities q If plaintext repeats, cipher text may too

Encryption is not Enough n Incorporated with file system q Keys should not be a function of block numbers n n q q q Risks key reuse (e. g. , archival, flash, content shifting due to insertions, etc. ) C 1 = K xor P 1 C 2 = K xor P 2 C 1 xor C 2 = P 1 xor P 2 (with much lower entropy) Random access issues Key storage issues Padding issues

Encryption is not Enough n Incorporated with file system q q n Caching issues (plaintext and ciphertext) Swapping and hibernation areas More info q q The Code Book See when cryptography meets storage

Stream, Block Ciphers n n M = B 1 B 2…with Bi of fixed length Block cipher q q E(M, K) = E(B 1, K)E(B 2, K)… DES n n Bi = 64 bits, K = 56 bits Stream cipher q q K = K 1 K 2… E(M, K) = E(B 1, K 1)E(B 2, K 2)…

One-Time Pads n n n Theoretically unbreakable security A symmetrical encryption system Use one bit of key for each bit of plaintext Never reuse any key bits Generate key bits truly randomly

Advantages of One-Time Pads n n Proved secure (in information theoretic sense) Encryption is computationally cheap q n XOR message with key Required procedures for proper use well understood

Problems with One-Time Pads n n They burn keys like crazy Need to keep key usage in sync If the keys aren’t truly random, patterns can be deduced in the bits Distribution of pads

Authentication n If a system supports more than one user, it must be able to tell who’s doing what q n I. e. : all requests to the system must be tagged with user identity Authentication is required to assure system that the tags are valid q Based on what one knows, what one has, and what one is

Passwords n n A fundamental authentication mechanism A user proves his identity by supplying a secret q n Either at login or other critical time The secret is the password

Password Security n n n Password selection Password storage and handling Password aging

Selecting a Password n Desirable characteristics include: q q Unguessable Easy to remember (and type) Not in a dictionary Too long to search exhaustively

Password Storage and Handling n n Passwords are secrets, so their security depends on careful handling But seemingly the system must store the password q n To compare when users log in If system storage is compromised, so is all authentication

Securely Storing Passwords n n Store only in encrypted form To check a password, encrypt it and compare to the encrypted version Encrypted version can be stored in a file But there are tricky issues

Tricky Issues in Storing Encrypted Passwords n What do I encrypt them with? q q n If I use single key to encrypt them all, what if the key is compromised? That key must be stored in the system What if two people choose the same password?

Example: The UNIX Password File n n Each password has an associated salt UNIX encrypts a block of zeros q q n n Key built from password plus 12 -bit salt Encryption done with DES Stored information = E(zero, salt + password) To check password, repeat operations

How Does This Help the Problems? n No single key for encryption q q n So can’t crack that key And needn’t ever store it Each encryption (probably) performed with a different key q So two people with the same password have different encrypted versions

Does this solve the problem? n n n Not entirely Passwords exist in plaintext in process checking them Passwords may be transmitted in plaintext q q n Especially for remote logins Bluetooth keyboards Shoulder surfing q See Cashtags

Problems with Passwords n n People choose bad ones People forget them People reuse them People rarely change them

How to Deal with Bad Passwords n n n Educate users so they choose good ones Automatic password generation Check when changed Periodically run automated cracker Any solution must balance user needs, password security, and resources

Other Authentication Mechanisms n n n Challenge/response Smartcards Other special hardware Detection of personal characteristics All have some drawbacks Some are combined with passwords q Two-factor authentication n Something you know + something you own (card)

Zero-knowledge Proof (Authentication) n n n Alice and Bob know prime numbers p and g Alice knows secret x, Bob knows y = gx % p To authenticate Alice q For n rounds n Alice generates a random number r q n Then sends Bob C = gr % p Bob either asks for r and computes C to match q q Or asks for z = (x + r) % (p – 1), and computes gz % p to see if it matches with Cy mod p Note that (p – 1) is a relative prime of p

Zero-knowledge Proof n n n Alice knows the 3 -coloring solution to a graph Bob wants to prove that Alice knows the solution In N rounds q q n Alice shuffles the colors Bob asks for two adjacent nodes, Alice reveals the colors of the two nodes With enough N, Bob can prove Alice’s knowledge, without learning the solution

Data Access Control Mechanisms n n Methods of specifying who can access what in which ways when Based on assumption that the system has authenticated the user

Access Matrix n n n Describes permissible accesses for the system Subjects access objects with particular access rights A theoretical concept, never kept in practice

Access Matrix Example File 1 File 2 Server X Segment 57 User A Read, Write None Query Read User B Read Write Update None User C None Read Start, Stop None User D None Query None

Types of Access Control n Discretionary access control (DAC) q n Mandatory access control (MAC) q n System mechanism controls access to object Originator controlled access control (ORCON) q n Individual user sets ACL mechanism Creator of information control ACL Role-based access control (RBAC) q Bookkeeper has access to financial records

Methods for Implementing Access Matrix n Access control lists q n Decomposition by columns Capabilities q Decomposition by rows

Access Control Lists n Each object controls who can access it q Using an access control list Add subjects by adding entries n Remove subjects by removing entries + Easy to determine who can access object + Easy to change who can access object - Hard to tell what someone can access n

Access Control List Example n File 1’s ACL q q User A: Read, Write User B: Read n Segment 57’s ACL q User A: Read

Capabilities Each subject keeps track of what it can access n By keeping a capability for each object n Capabilities are like admission tickets + Easy to tell what a subject can access - Hard to tell who can access an object - Hard to revoke/control access (someone can keep an extra copy around) n

Capability Example n User A’s Capabilities q q q File 1: Read, Write Server X: Query Segment 57: Read n User B’s Capabilities q q q File 1: Read File 2: Write Server A: Update

Other Models of Access Control n n n Military model Information flow models Lattice model of information flow

Bell-La. Padula Model n n An example of confidentiality policy Clearance categories q n Top secret, confidential, unclassified Users can only create and write top secret and secret documents q q Users cannot read documents > their clearance Users cannot write documents < their clearance

Bell-La. Padula Model n Rationale q Cannot copy a top secret document over a unclassified one n q n And email the unclassified one away Information flows up Problems q q q Blind writes Classifications cannot change Interacts with capability-based systems (passing a capability from high clearance to low clearance)

Biba’s Model n n An example of integrity policy The higher the level, the more confidence q q n n That a program will execute correctly That data is accurate and/or reliable Note integrity levels ≠ security levels Assumption q Integrity and trustworthiness

Biba’s Model n Requirements q q Users use only existing programs Programmers will develop and test programs Program installations are controlled and audited Managers and auditors have access to both the system state and the system logs

Biba’s Model n Goal: q Prevent untrusted software from altering data or other software n n Credibility rating based on estimate of software’s trustworthiness Trusted file systems contain software with a single credibility level Process has risk level or highest credibility level at which process can execute Must use run-untrusted command to run software at lower credibility level

Chinese Wall Model n Problem q q n Tony advises American Bank about investments He is asked to advise Toyland Bank about investments Conflict of interest q His advice for either bank would affect his advice to the other bank

Chinese Wall Model n n Organize entities into conflict of interest classes Control subject access to each class Control writing to all classes to ensure info is not passed along in violation of rules Allow sanitized data to be viewed by everyone

Writing n n Anthony, Susan work in the same trading house Anthony can read Bank 1’s CD, Gas’s CD Susan can read Bank 2’s CD, Gas’s CD If Anthony could write to Gas’s CD, Susan can read it q Hence, indirectly, she can read information from Bank 1’s CD, a clear conflict of interest

Compare to Bell-La. Padula n Bell-La. Padula cannot track changes over time q Susan becomes ill, Anna needs to take over