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CMSC 412
Fall 2004
Security
Announcements
• Reading
– Chapter 19 (today)
– Chapter 20 (next time)
• Final exam review
– Second half of class Wednesday
• Finals week office hours
– Tuesday (12/14) 2-4pm
– Wednesday (12/15) 2-4pm
Prepare for your exams
Study with the several resources on Docsity
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Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Lecture Notes on Security - Operating Systems | CMSC 412, Study notes of Operating Systems
Material Type: Notes; Professor: Hicks; Class: Operating Systems; Subject: Computer Science; University: University of Maryland; Term: Fall 2004;
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CMSC 412
Fall 2004
Security
Announcements
- Reading
- Chapter 19 (today)
- Chapter 20 (next time)
- Final exam review
- Second half of class Wednesday
- Finals week office hours
- Tuesday (12/14) 2-4pm
- Wednesday (12/15) 2-4pm
Who do you trust?
- Do I trust a login prompt?
- Do I trust the OS that I got from the vendor?
- Do I trust the system staff?
- should I encrypt all my files?
- Networking
- do you trust the network provider?
- do you trust the phone company?
- How do you bootstrap security?
- need one “out of band” transfer to get going
The Security Problem
- Security must consider external
environment of the system, and
protect it from:
- unauthorized access
- modification or destruction of data
- denial of service
- Easier to protect against accidental
than malicious misuse.
Example (UNIX passwords)
- use a function that is hard to invert
- “easy” to compute f(x) given x
- hard to compute x given f(x)
- the function used is a variation on the DES algorithm
- changes selected items in the transformation matrix to prevent hardware attacks
- store only f(x) in the filesystem
- to login
- user supplies a password x’
- compute f(x’) and compare to f(x)
- salt
- add an extra two characters to x so that the same x will produce different values on different machines
- dictionary attack
- if it’s to easy to compute f(x)
- can “guess” many passwords and try them out
Other authenticators
- Biometric data
- One-time passwords
- Password function; checked with challenge/response
- On-time pad; password, once used, is discarded. Pad obtained from trusted source
Computer Authentication
- How does a user know what computer they are using?
- Need to have mutual authentication
- computer presents some information that only it could contain
- example: Windows
- - to login- user software can’t trap that information
- assumes that the kernel itself is secure
- telephone example
- never give banking/credit card info over the phone unless you placed the phone call - i.e. you use the telco namespace for authentication
Program Threats
- Trojan Horse
- Code segment that misuses its environment.
- Exploits mechanisms for allowing programs written by users to be executed by other users.
- Trap Door
- Specific user identifier or password that circumvents normal security procedures.
- Could be included in a compiler.
- Stack and Buffer Overflow
System Attacks
- Viruses – fragment of code embedded in a legitimate program. (Popularly speaking, worm and virus tend to be used synonymously.) - Located in acquired program; set in motion when the program is run. - Possible to write system independent viruses - MS Word virus uses macros to call into the OS
- Denial of Service
- Overload the targeted computer preventing it from doing any useful work.
Threat Monitoring
- Check for suspicious patterns of activity – i.e., several incorrect password attempts may signal password guessing.
- Audit log – records the time, user, and type of all accesses to an object; useful for recovery from a violation and developing better security measures.
- Scan the system periodically for security holes; done when the computer is relatively unused.
Threat Monitoring (Cont.)
- Check for:
- Short or easy-to-guess passwords
- Unauthorized set-uid programs
- Unauthorized programs in system directories
- Unexpected long-running processes
- Improper directory protections
- Improper protections on system data files
- Dangerous entries in the program search path (Trojan horse)
- Changes to system programs: monitor checksum values
Tripwire
- Compute a set of expectations about system
- Hash of file contents
- Dates on files
- Store database of values
- On read-only media
- Offline
- Periodically
- Compare database to current system
- Report any differences
Intrusion Detection
- Detect attempts to intrude into computer systems. - Anomaly detection: know what “normal” behavior is and look for anomalies - Signature detection: know what “bad” behavior is and look for that
- Behavior is determined by auditing and logging - Process log messages - Network traffic - System call monitoring
Encryption
- Encrypt clear text into cipher text.
- Properties of good encryption technique:
- Relatively simple for authorized users to incrypt and decrypt data.
- Encryption scheme depends not on the secrecy of the algorithm but on a parameter of the algorithm called the encryption key.
- Extremely difficult for an intruder to determine the encryption key.
Encryption: protecting info
from being read
- Given a message m
- use a key k, and function Ek to compute Ek(m)
- store or send only Ek(m)
- use a second key k’ and function Dk’ such that
- Dk’(Ek(m)) = m
- E and D need not be kept a secret
Encryption Techniques
- For Dk’(Ek(m)) = m:
- If k=k’ it’s called symmetric key encryption
- need to keep k secret
- example Data Encryption Standard (DES)
- if k != k’, it’s called public key encryption
- By keeping k’ secret, anyone can send a private message using k
- still need a way to authenticate k or k’ for a user
- example RSA
Public Key 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 decrypt the messages. - Efficient algorithm for testing whether or not a number is prime. - No efficient algorithm is know for finding the prime factors of a number.
RSA
- RSA (Rivest-Shamir-Adelman) first public key system. - algorithm for computing public/private key pairs - based on problems involved in factoring large primes - for an n bit message P, C = (Pe^ mod n), and P = (Cd^ mod n)
- Other PE schemes: El-Gamal, Knapsack, DSA, …
Message Authentication Schemes
- Use a digital signature to ensure authenticity
- Does not require signed document to be encrypted
- Given a message m
- use a key k, and function Sk to compute Sk(m)
- send (m,Sk(m))
- use a second key k’ and function Vk’ such that
- Vk’(m,Sk(m)) = true or false
- S and V need not be kept a secret
- Most encryption schemes support MAC
Secure Communications - SSL
- SSL – Secure Socket Layer
- Cryptographic protocol that limits two computers to only exchange messages with each other.
- Used between web servers and browsers for secure communication (credit card numbers)
- The server is verified with a certificate.
- Communication between each computers uses symmetric key cryptography.
Computer Security Classifications
- U.S. Department of Defense four divisions of computer security: A , B , C , and D.
- D – Minimal security.
- C – Provides discretionary protection through auditing. Divided into C1 and C. 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 labels. Divided into B1 , B2 , and B.
- A – Uses formal design and verification techniques to ensure security.