TDDB63:
Concurrent programming
and operating systems
Protection
[SGG7] Chapter 14 and 15
Protection and Security
Provide goals and principles of protection
Introduce security threats and attacks
Explain the fundamentals of encryption and
authentication
Examine the uses of cryptography in computing
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Goals of Protection
Domain of Protection
Access Matrix
Implementation of Access Matrix
Revocation of Access Rights
Capability-Based Systems
Language-Based Protection
Copyright Notice: The lecture notes are mainly based on Silberschatz’s, Galvin’s and Gagne’s book (“Operating System
Concepts”, 7th ed., Wiley, 2005). No part of the lecture notes may be reproduced in any form, due to the copyrights
reserved by Addison-Wesley. These lecture notes should only be used for internal teaching purposes at the Linköping
University.
Andrzej Bednarski, IDA
Linköpings universitet, 2005
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Protection
Principle of Protection
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Operating system consists of a collection of objects, hardware or
software
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Need-to-know principle
+ At any time a process should only be able to access only
resources that it currently requires to complete its task
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Each object has a unique name and can be accessed through a
well-defined set of operations.
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Principle of least privilege
+ Programs, users and systems should be given just enough
privileges to perform their tasks
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Protection problem – ensure that each object is accessed correctly
and only by those processes that are allowed to do so.
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Domain Structure
Domain Implementation
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Access-right = <object-name, rights-set>
Rights-set is a subset of all valid operations
that can be performed on the object.
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System consists of two domains:
+ User
+ Supervisor
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Domain = set of access-rights
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UNIX
+ Domain = user-id
+ Domain switch accomplished via file system.
̶ Each file has associated with it a domain bit (setuid bit).
̶ 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.
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Domain = user, process, or procedure
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Access Matrix
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Use of Access Matrix
View protection as a matrix (access matrix)
Rows represent domains
Columns represent objects
Access(i, j) is the set of operations that a process executing in
Domaini can invoke on Objectj
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If a process in Domain Di tries to do “op” on object Oj, then “op”
must be in the access matrix.
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Can be expanded to dynamic protection.
+ Operations to add, delete access rights.
+ Special access rights:
̶ owner of Oi
̶ copy op from Oi to Oj
̶ control – Di can modify Dj access rights
̶ transfer – switch from domain Di to Dj
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Use of Access Matrix (Cont.)
Access Control
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Protection can be applied to non-file
resources
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Role-based access control implements
least privilege principle
+ Privilege is right to execute system
call or use an option within a
system call
+ Can be assigned to processes
+ Users assigned roles granting
access to privileges and programs
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Access matrix design separates mechanism from policy.
+ Mechanism
̶ Operating system provides access-matrix + rules.
̶ If ensures that the matrix is only manipulated by
authorized agents and that rules are strictly enforced.
+ Policy
̶ User dictates policy.
̶ Who can access what object and in what mode.
Implementation
+ Sparse
+ Access lists: associated with each object
+ Capability lists: associated with each domain
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Example of RBAC in Solaris
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Revocation of Access Rights
Language-Based Protection
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Specification of protection in a programming language allows the
high-level description of policies for the allocation and use of
resources.
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Language implementation can provide software for protection
enforcement when automatic hardware-supported checking is
unavailable.
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Interpret protection specifications to generate calls on whatever
protection system is provided by the hardware and the operating
system.
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Immediate vs delayed
Selective vs general
+ For some users or for all users
Partial vs total
+ Subset of rights associated with an object
Temporary vs permanent
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Reference Monitor
Reference Monitor (Cont.)
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Module: secure, always used and “fully” testable that controls all
software accesses to data object and devices [Wikipedia]
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Checks the nature of the request against a table of allowable
access types for each process on the system [Wikipedia]
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Portion of security kernel that controls access to objects
+ Tamperproof
+ Always invoked
+ Small enough to be subject to analysis and testing
Collection of access controls for devices, files, memory, IPC, etc.
Single point though which all accesses requests must pass
Correctness: likelihood of correctness decreases as
complexity/size of programs increases
Small, simple understandable RM
subjects
Audit
file
Reference
Monitor
(policy)
Security
kernel
base
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Trusted Computing Base
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Utilities
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User applications
TCB
Subject: security clearance
Object: security classification
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TCB Monitoring Functions
Everything in the trusted OS system necessary to enforce the
security policy
Handles correct enforcement of the policy
Part of the OS on which our trust in the security of the whole
system depends
Non-TCB
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objects
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User process coordination, synchronization
User I/O
Procedures, user processes
Directories
Segmentation, paging, memory management
Process activation
+ requires security-sensitive information
Domain switching
+ protection against access right escalation
Memory protection
+ code/data secrecy and integrity of domains
I/O operations
+ control of crossing domains
Primitive I/O
Basic operations
Clocks, timing
Interrupt handling
Hardware: registers, memory
Capabilities
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Security
The Security Problem
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Security must consider external environment of the system,
and protect it from:
+ Unauthorized access
+ Malicious modification or destruction
+ Accidental introduction of inconsistency
+ Legitimate use of the system (denial of service)
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Easier to protect against accidental than malicious misuse
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Definitions
+ Intruders/crackers: attempt to breach security
+ Attack: attempt to break security
+ Threat: potential for security violation (vulnerability)
The Security Problem
Program Threats
System and Network Threats
Cryptography as a Security Tool
User Authentication
Implementing Security Defenses
Firewalling to Protect Systems and Networks
Computer-Security Classifications
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Security Violations
Security Measure Levels
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Categories
+ Breach of confidentiality
+ Breach of integrity
+ Breach of availability
+ Theft of service
+ Denial of service
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Methods
+ Masquerading (breach authentication)
+ Replay attack
̶ Message modification
+ Man-in-the-middle attack
+ Session hijacking
Security at four different levels:
1) Physical
2) Human
3) Operating System
4) Network
Security is as week as the weakest chain
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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
+ Specific user id/password that circumvents normal security procedures
+ Could be included in a compiler
Logic Bomb
+ Program that initiates a security incident under certain circumstances
Stack and Buffer Overflow
+ Exploits programs’ bugs (overflow either the stack or memory buffers)
Viruses
+ Code fragment embedded in legitimate program
+ Very specific to CPU architecture, operating system, applications
+ Usually borne via email or as a macro
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Example:
C Program with Buffer-overflow Condition
Program Threats
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#include <stdio.h>
#define BUFFER SIZE 256
int main(int argc, char*argv[]) {
char buffer[BUFFER SIZE];
if (argc < 2)
return -1;
else {
strcpy(buffer,argv[1]);
return 0;
}
}
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Layout of stack frame
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Modified Shell Code
System and Network Threats
#include <stdio.h>
int main(int argc, char *argv[])
{
execlp(“/bin/sh”, “/bin/sh”, 0);
return 0;
}
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Worms – use spawn mechanism; standalone program
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Internet worm
+ Exploited UNIX networking features (remote access)
and bugs in finger and sendmail programs
+ Grappling hook program uploaded main worm program
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Port scanning
+ Automated attempt to connect to a range of ports
on one or a range of IP addresses
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Denial of Service (DOS)
+ Overload the targeted computer
preventing it from doing any useful work
+ Distributed denial-of-service (DDOS)
come from multiple sites at once
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Implementing Security Defenses
Anti-Intrusion Approaches
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1. Prevention
+Precludes/handicaps the
likelihood of intrusion’s
success
2. Preemption
+Strikes offensively against
likely threat agents
(prior to an intrusion)
3. Deterrence
+Dissuade potential gains of intruder
4. Deflection
+Direct an intruder into a “honey pot” + monitoring
5. Detection
+Abnormal activities observation → reaction (authorities)
6. Countermeasures
+Actively and autonomously counter an intrusion attempt
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Defense in depth is most common security theory
+ multiple layers of security
Security policy describes what is being secured
Vulnerability assessment compares real state of system/network
compared to security policy
Intrusion detection endeavors to detect attempted or
successful intrusions
+ Signature-based detection spots known bad patterns
+ Anomaly detection spots differences from normal behavior
̶ Can detect zero-day attacks
+ False-positives and false-negatives a problem
Virus protection
Auditing, accounting, and logging of all or specific system or
network activities
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Cryptography as a Security Tool
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+ Means to constrain potential
senders (sources) and/or
receivers (destinations) of
messages
Based on secrets (keys)
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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.
̶ Both E and E(k) for any k should be efficiently computable
functions.
+ A function D : K → (C → M). That is, for each k ∈ K, D(k) is a
function for generating messages from ciphertexts.
̶ Both D and D(k) for any k should be efficiently computable
functions.
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Encryption (Cont.)
Symmetric Encryption
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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 ciphertexts
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Encryption
Broadest security tool available
+ Source and destination of
messages cannot be trusted
without cryptography
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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 Government)
+ Encrypts a block of data at a time
Triple-DES considered more secure
Advanced Encryption Standard (AES), twofish up and coming
RC4 is most common symmetric stream cipher,
but known to have vulnerabilities
+ Encrypts/decrypts a stream of bytes (i.e. wireless transmission)
+ Key is a input to pseudo-random-bit generator
̶ Generates an infinite key-stream
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Asymmetric Encryption
Asymmetric Encryption (Cont.)
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Public-key encryption based on each user having two keys:
+ public key – used to encrypt data
+ private key – used to decrypt data
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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 cipher
+ Efficient algorithm for testing whether or not a number is prime
+ Relies on lack of efficient algorithm for finding
the prime factors of a number
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Asymmetric Encryption Example
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yielding 5
Calculate kd such that kekd mod 72 = 1,
yielding 29
We how have our keys
+ Public key, ke, N = 5, 91
+ Private key, kd , N = 29, 91
Encrypting the message 69
with the public key: cyphertext 62
Cyphertext can be decoded with
the private key
+ Public key can be distributed in clear-text
to anyone who wants to communicate
with holder of public key
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Cryptography (Cont.)
p = 7 and q = 13
N = 7∗13 = 91 and (p−1)(q−1) = 72
Select ke relatively prime to 72 and< 72,
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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 distributed
+ 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)
+ Encryption algorithm is E(ke , N)(m) = mke mod N, where ke
satisfies kekd mod (p−1)(q −1) = 1
+ The decryption algorithm is then D(kd , N)(c) = ckd mod N
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Note symmetric cryptography based on transformations,
asymmetric based on mathematical functions
+ Asymmetric much more compute intensive
+ Typically not used for bulk data encryption
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Authentication
Authentication (Cont.)
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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)
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Thus, computer holding S(k) can generate authenticators on
messages so that any other computer possessing V(k) can verify
them
Computer not holding S(k) cannot generate authenticators on
messages that can be verified using V(k)
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
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Constraining set of potential senders of a message
+ Complementary and sometimes redundant to encryption
+ Also can prove message unmodified
Algorithm components
+ A set K of keys
+ A set M of messages
+ A set A of authenticators
+ A function S : K → (M→ A)
̶ 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
̶ Both V and V(k) for any k should be efficiently computable
functions
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Authentication – Hash Functions
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Authentication - MAC
Basis of authentication
Creates small, fixed-size block of data (message digest, hash value)
from m
Hash Function H must be collision resistant on m
+ Must be infeasible to find an m’ ≠ m such that H(m) = H(m’)
If H(m) = H(m’), then m = m’
+ The message has not been modified
Common message-digest functions include MD5, which produces a
128-bit hash, and SHA-1, which outputs a 160-bit hash
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Symmetric encryption used in message-authentication code (MAC)
authentication algorithm
Simple example:
+ MAC defines S(k)(m) = f (k, H(m))
̶ Where f is a function that is one-way on its first argument
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k cannot be derived from f (k, H(m))
Because of the collision resistance in the hash function,
reasonably assured no other message could create the same
MAC
A suitable verification algorithm is V(k)(m, a) ≡ (f(k,m) = a)
Note that k is needed to compute both S(k) and V(k), so
anyone able to compute one can compute the other
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Authentication – Digital Signature
Authentication (Cont.)
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Based on asymmetric keys and digital signature algorithm
Authenticators produced are digital signatures
In a digital-signature algorithm,
computationally infeasible to derive S(ks) from V (kv)
+ V is a one-way function
+ Thus, kv is the public key and ks is the private key
Consider the RSA digital-signature algorithm
+ Similar to the RSA encryption algorithm, but the key use is reversed
+ Digital signature of message S(ks )(m) = H(m)ks mod N
+ The key ks again is a pair d, N, where N is the product of two large,
randomly chosen prime numbers p and q
+ Verification algorithm is V(kv)(m, a) ≡ (akv mod N = H(m))
̶ Where kv satisfies kvks mod (p − 1)(q − 1) = 1
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Why authentication if a subset of encryption?
+ Fewer computations (except for RSA digital signatures)
+ Authenticator usually shorter than message
+ Sometimes want authentication but not confidentiality
̶ Signed patches et al
+ Can be basis for non-repudiation
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Key Distribution
Digital Certificates
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Delivery of symmetric key is huge challenge
+ Sometimes done out-of-band
Asymmetric keys can proliferate – stored on key ring
+ Even asymmetric key distribution needs care
(man-in-the-middle attack)
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Proof of who or what owns a public key
Public key digitally signed a trusted party
Trusted party receives proof of identification from entity and
certifies that public key belongs to entity
Certificate authority are trusted party – their public keys included
with web browser distributions
+ They vouch for other authorities via digitally signing their
keys, and so on
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User Authentication
Recommended Reading
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Reading
+ [SGG7] Chapters 14 and 15
+ Chapters 18 and 19 (sixth edition)
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References:
+ Ravi S. Sandhu, Edward J. Coyne, Hal L. Feinstein and
Charles E. Youman. “Role-Based Access Control Models”
+ Lawrence R. Halme and R. Kenneth Bauer
“AINT Misbehaving: A Taxonomy of Anti-Intrusion Techniques”
+ Lecture notes: Brett D. Fleish
“Design of Trusted Operating Systems”
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Crucial to identify user correctly,
as protection systems depend on user ID
User identity most often established through passwords, can be
considered a special case of either keys or capabilities
+ Also can include something user has and /or a user attribute
Passwords must be kept secret
+ Frequent change of passwords
+ Use of “non-guessable” passwords
+ Log all invalid access attempts
Passwords may also either be encrypted or
allowed to be used only once
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