The Security Problem
Security must consider external environment of the system, and protect the
system resources
Intruders (crackers) attempt to breach security
Chapter 15: 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 misuse
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Chapter 15: Security
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 An Example: Windows XP Categories
Security Violations
Breach of confidentiality Breach of integrity Breach of availability Theft of service Denial of service Masquerading (breach authentication) Replay attack
Message
Methods
modification
Man-in-the-middle attack Session hijacking
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Objectives
To discuss security threats and attacks To explain the fundamentals of encryption, authentication, and hashing To examine the uses of cryptography in computing To describe the various countermeasures to security attacks
Standard Security Attacks
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Security Measure Levels
Security must occur at four levels to be effective:
Layout of Typical Stack Frame
Physical Human
Avoid
social engineering, phishing, dumpster diving
Operating System Network
Security is as week as the weakest chain
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Program Threats
Modified Shell Code
#include <stdio.h> int main(int argc, char *argv[]) { execvp(\bin\sh,\bin \sh, NULL); return 0; }
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 Specific user identifier or password that circumvents normal security procedures Could be included in a compiler Program that initiates a security incident under certain circumstances Exploits a bug in a program (overflow either the stack or memory buffers)
Trap Door
Logic Bomb
Stack and Buffer Overflow
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C Program with Buffer-overflow Condition
#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; } }
Hypothetical Stack Frame
Before attack
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After attack
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Program Threats (Cont.)
Viruses
System and Network Threats
Worms use spawn mechanism; standalone program Internet worm
Code fragment embedded in legitimate program Very specific to CPU architecture, operating system, applications Usually borne via email or as a macro
Visual
Exploited UNIX networking features (remote access) and bugs in finger and sendmail programs Grappling hook program uploaded main worm program Automated attempt to connect to a range of ports on one or a range of IP addresses Overload the targeted computer preventing it from doing any useful work Distributed denial-of-service (DDOS) come from multiple sites at once
Basic Macro to reformat hard drive
Sub AutoOpen() Dim oFS Set oFS = CreateObject(Scripting.FileSystemObject) vs = Shell(c:command.com /k format End Sub c:,vbHide)
Port scanning
Denial of Service
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Program Threats (Cont.)
Virus dropper inserts virus onto the system Many categories of viruses, literally many thousands of viruses
The Morris Internet Worm
File Boot Macro Source code Polymorphic Encrypted Stealth Tunneling Multipartite Armored
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A Boot-sector Computer Virus
Cryptography 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)
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Secure Communication over Insecure Medium
Asymmetric Encryption
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 block cipher Efficient algorithm for testing whether or not a number is prime No efficient algorithm is know for finding the prime factors of a number
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Encryption
Asymmetric Encryption (Cont.)
Formally, it is computationally infeasible to derive D(kd , N) from E(ke
Encryption algorithm consists of
Set of K keys Set of M Messages Set of C ciphertexts (encrypted messages) A function E : K (MC). That is, for each k K, E(k) is a function for generating ciphertexts from messages.
, 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) Encryption algorithm is E(ke , N)(m) = mke mod N, where ke satisfies kekd mod (p1)(q 1) = 1 The decryption algorithm is then D(kd , N)(c) = ckd mod N
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.
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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Symmetric Encryption
Same key used to encrypt and decrypt
Asymmetric Encryption Example
For example. make p = 7and q = 13 We then calculate N = 713 = 91 and (p1)(q1) = 72 We next select ke relatively prime to 72 and< 72, yielding 5 Finally,we calculate kd such that kekd mod 72 = 1, yielding 29 We how have our keys
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
Triple-DES considered more secure Advanced Encryption Standard (AES), twofish up and coming RC4 is most common symmetric stream cipher, but known to have
Public key, ke, N = 5, 91 Private key, kd , N = 29, 91
vulnerabilities
Encrypting the message 69 with the public key results in the cyphertext 62
Encrypts/decrypts a stream of bytes (i.e wireless transmission) Key is a input to psuedo-random-bit generator
Generates
Cyphertext can be decoded with the private key
Public key can be distributed in cleartext to anyone who wants to communicate with holder of public key
an infinite keystream
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Encryption and Decryption using RSA Asymmetric Cryptography
Authentication (Cont.)
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)
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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Cryptography (Cont.)
Note symmetric cryptography based on transformations, asymmetric based
Authentication Hash Functions
Basis of authentication Creates small, fixed-size block of data (message digest, hash value) from
on mathematical functions
Asymmetric much more compute intensive Typically not used for bulk data encryption
m
Hash Function H must be collision resistant on m
Must be infeasible to find an m m such that H(m) = H(m) The message has not been modified
If H(m) = H(m), then m = m
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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Authentication
Authentication - MAC
Symmetric encryption used in message-authentication code (MAC)
Constraining set of potential senders of a message
Complementary and sometimes redundant to encryption Also can prove message unmodified A set K of keys A set M of messages A set A of authenticators A function S : K (M A)
authentication algorithm
Simple example:
Algorithm components
MAC defines S(k)(m) = f (k, H(m))
Where
f is a function that is one-way on its first argument
k cannot be derived from f (k, H(m))
Because
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
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) that k is needed to compute both S(k) and V(k), so anyone able to compute one can compute the other
Note
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 Digital Signature
Man-in-the-middle Attack on Asymmetric Cryptography
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 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))
Consider the RSA digital-signature algorithm
Where kv satisfies kvks mod (p 1)(q 1) = 1
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Authentication (Cont.)
Why authentication if a subset of encryption?
Digital Certificates
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
Fewer computations (except for RSA digital signatures) Authenticator usually shorter than message Sometimes want authentication but not confidentiality
Signed
public key belongs to entity
Certificate authority are trusted party their public keys included with web
patches et al
Can be basis for non-repudiation
browser distributions
They vouch for other authorities via digitally signing their keys, and so on
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Key Distribution
Delivery of symmetric key is huge challenge
Encryption Example - SSL
Insertion of cryptography at one layer of the ISO network model (the
Sometimes done out-of-band Even asymmetric key distribution needs care man-in-the-middle attack
transport layer)
Asymmetric keys can proliferate stored on key ring
SSL Secure Socket Layer (also called TLS) Cryptographic protocol that limits two computers to only exchange
messages with each other
Very complicated, with many variations
Used between web servers and browsers for secure communication (credit
card numbers) server
The server is verified with a certificate assuring client is talking to correct Asymmetric cryptography used to establish a secure session key
(symmetric encryption) for bulk of communication during session cryptography
Communication between each computer theb uses symmetric key
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User Authentication
Crucial to identify user correctly, as protection systems depend on user ID User identity most often established through passwords, can be considered
Network Security Through Domain Separation Via Firewall
a special case of either keys or capabilities
Also can include something user has and /or a user attribute Frequent change of passwords Use of non-guessable passwords Log all invalid access attempts
Passwords must be kept secret
Passwords may also either be encrypted or allowed to be used only once
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Implementing Security Defenses
Defense in depth is most common security theory multiple layers of
Computer Security Classifications
U.S. Department of Defense outlines four divisions of computer security: A,
security Security policy describes what is being secured to security policy
B, C, and D.
D Minimal security. C Provides discretionary protection through auditing. Divided into C1 and
Vulnerability assessment compares real state of system / network compared Intrusion detection endeavors to detect attempted or successful intrusions
C2. C1 identifies cooperating users with the same level of protection. C2 allows user-level access control.
B All the properties of C, however each object may have unique sensitivity
Signature-based detection spots known bad patterns Anomaly detection spots differences from normal behavior
Can
labels. Divided into B1, B2, and B3.
A Uses formal design and verification techniques to ensure security.
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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Firewalling to Protect Systems and Networks
A network firewall is placed between trusted and untrusted hosts
Example: Windows XP
Security is based on user accounts
The firewall limits network access between these two security domains Can be tunneled or spoofed
Each user has unique security ID Login to ID creates security access token
Includes Every
Tunneling allows disallowed protocol to travel within allowed protocol (i.e. telnet inside of HTTP) Firewall rules typically based on host name or IP address which can be spoofed Can monitor / limit traffic to and from the host
security ID for user, for users groups, and special privileges process gets copy of token checks token to determine if access allowed or denied
Personal firewall is software layer on given host
System
Uses a subject model to ensure access security. A subject tracks and
Application proxy firewall understands application protocol and can
manages permissions for each program that a user runs
Each object in Windows XP has a security attribute defined by a
control them (i.e. SMTP)
System-call firewall monitors all important system calls and apply rules to
them (i.e. this program can execute that system call)
security descriptor
For example, a file has a security descriptor that indicates the access permissions for all users
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