1. 12. Protection/Security Interface
12.1 Security Threats
Types of Damage
Vulnerable Resources
Types of Attacks
12.2 Functions of a Protection System
12.3 User Authentication
Approaches to Authentication
Passwords
12.4 Secure Communication
Principles of Cryptography
Secret-Key Cryptosystems
Public-Key Cryptosystems
Operating Systems

2. Security Threats Types of damage Information Disclosure theft
Information Destruction
possible without disclosure
Unauthorized Use of services
install SW without license, pirated copies (theft)
use fake ID/password to use online service
Denial of Service
difficult to quantify
Vulnerable resources
Hardware (CPU, memory, communications, devices)
Software (files, processes, VM)
Operating Systems

3. Types of Attacks Browsing of Information
Unauthorized search for residual information
Unused memory and disk space is generally not deleted
Typically done by a user who is already inside
Information leaking
A trusted service leaks confidential/secret information (Confinement Problem)
Operating Systems

4. Types of Attacks Trojan Horse Greek mythology—the siege of Troy
wooden horse = “present” by Greeks
soldiers hidden inside
Trojans pulled the horse into the city
soldiers opened the gates for the Greeks, causing the destruction of Troy
Attack: trusting user accepts a “present”, e.g. a free program, that causes damage (don’t open attachments)
Trap door
an undocumented feature
inserted on purpose or as a flaw to enter later
Operating Systems

5. Types of Attacks Viruses Designed to replicate themselves
removable storage media, , file transfer
Intended to cause damage
Need a host program
attach to and modify host
execute as part of host
Virus detection
check program length (but virus can hide or compress program)
check for virus “signature”—bit pattern used by virus to mark already infected program (viruses use encryption)
Operating Systems

6. Types of Attacks Worms Intended to cause damage
Exploit some system weakness to replicate
No host needed
Example: Robert Morris Internet Worm (Nov 2, 1988)
Excessive replication caused major havoc on the internet (denial of service)
3 separate attacks:
rsh: Spawn process on remote machine without pw (using a list of “trusted” systems)
sendmail: Exploited an error that allows a message to send itself and start
finger: Buffer overflow not checked – major weakness to take over the system
Operating Systems

7. Types of Attacks Buffer Overflow: Example: foo calls finger
Attack hijacks return address by supplying a parameter that is longer than the buffer (overflows)
When finger terminates, control goes to a place set by the attack and is not returned to foo.
Operating Systems

8. Types of Attacks Remote execution
Service to upload and start code on remote machine
Mobile agent: may migrate among machines
Like worm but legitimate migration
Must be designed carefully to prevent abuse of privileges of remote host environment
Wire tapping
Insert a device into line or listen to wireless
Passive (listen) or Active (modify)
Waste searching
Look for passwords or sensitive data
Operating Systems

9. Types of Attacks Masquerading Impersonate process, user, service
Used from outside:
Use stolen password (impersonate a legitimate user)
Break communication line, assume session
Used from within (spoofing):
Impersonate login shell, steal password
Trial and error
e.g., try to guess password
Operating Systems

10. Types of Attacks -- Classification
From within
direct access as a valid process
indirect Access via agent (attacker not present during attack)
From outside
channels provided for legitimate purposes
illegitimate channels
Operating Systems

11. Functions of a Protection System
External safeguards
guard physical access (locks, badges, cameras)
Verification of user identity (User Authentication) 
Communication safeguards
protect public/vulnerable lines: cryptography
Access control (Ch 13)
can Subject perform function on Resource
Information flow control (Ch 13)
can S get information contained in R (indirectly)
Operating Systems

12. User Authentication Approaches: Knowledge of some information
Password, dialog
Possession of some artifact
Machine-readable cards (ATM)
Combine with knowledge (PIN)
Biometrics: Physical characteristics of person
Fingerprint
Hand geometry
Face geometry
Retina or iris scan
Voice print
Signature dynamics
Operating Systems

13. Fingerprint Recognition
Extremely useful biometrics technology
Fingerprints are a primary and accurate identification method
Operating Systems

14. Fingerprint Recognition
Uses the ridge endings and bifurcation's to plot points known as minutiae
The number and locations of the minutiae vary from finger to finger and from person to person
Finger Image
Finger Image
+ Minutiae
Minutiae
Operating Systems

15. Face Recognition Uses an image or series of images
Principle: analysis of the unique shape, pattern and positioning of facial features
Passive: does not require a person’s cooperation
Highly complex technology
Common approach: Face geometry
Operating Systems

16. Voice Recognition Not speech recognition, it is speaker recognition
Low-cost (cheap hardware)
Not very accurate (voice varies, noise)
Can be stolen (recorded)
Operating Systems

17. Hand geometry one of the most deployed biometrics world wide
Ben Gurion Airport
(Israel)
Operating Systems

18. Signature Verification
Static/off-line:
match pattern (image)
can easily be reproduced
Dynamic/On-line:
match movement of the pen during signing process (pressure, speed)
Many commercial products
Operating Systems

19. Iris recognition
Based on visible features, i.e. rings, furrows, freckles and the corona
Safest, most accurate biometrics technology
Heathrow Airport (London)
Operating Systems

20. Retina recognition
Capture the pattern of blood vessels throughout the retina
No two retinas are the same, even in identical twins
More difficult/less convenient than iris scan
Operating Systems

21. Other techniques
DNA
Unique (except for identical twins) but many imitations:
not fully automated, slow, expensive
privacy issue – DNA contains information about race, paternity, medical conditions
requires a physical sample of tissue
Thermograms
infrared camera to detect the heat patterns
Operating Systems

22. User Authentication
Problem with biometrics: uncertainty in recognition
System generates a number 0  n  1
Bimodal distribution
Threshold must be chosen to minimize
false alarms
imposter acceptance
Operating Systems

23. User Authentication Passwords
Must protect stored password files from access
Must prevent trial and error (guessing)
Protecting password files
Maintain unencrypted; rely on access control
Encrypt using “one-way” function H:
H-1 is unknown
knowing H(x) does not yield x
keep only H(pw) with user name
at login, compute H(pw’) and compare with H(pw)
Operating Systems

24. User Authentication Preventing password guessing System-generated pw
Random string: difficult to memorize
“Pronounceable” words
System-validated
Accept only passwords that obey specifications (length, mix of letters/digits, upper/lower case)
Employ password-cracking programs to reject easy-to-guess passwords
Time-limited
Expiration date or number of uses
Operating Systems

25. User Authentication One-time passwords
Smart card (can be lost or stolen)
Use secret function;
System generates a challenge n, user replies with f(n) as password; e.g. f(n)=3*n/2
Use one-way function to generate series of one-time passwords from one password pw
… H(H(H(pw))) H(H(pw)) H(pw) pw
Intruder can derive H(H(pw)) from H(pw) but not H(pw) from H(H(pw)) because H-1 is unknown
Operating Systems

26. User Authentication guess any valid password:
repeatedly generate strings s (dictionary, random, …), check if H(s) is in table
System-extended pw
for each pw, generate random number slt (called “salt”)
store: UserName,slt,H(slt,pw)
guessing: must check H(slt,s) for every slt in table
salting does not make it harder to guess the password of a specific user
Operating Systems

27. Functions of a Protection System
External safeguards
Guard physical access (locks, badges, cameras)
Verification of user identity (User Authentication)
Communication safeguards 
Protect public/vulnerable lines: cryptography
Access control
Can Subject perform function on Resource
Information flow control
Can S get information contained in R (indirectly)
Operating Systems

28. Secure Communication Principles of cryptography:
Cipher text, Plain text, Key(s)
Encrypt: C = E(P,Ke)
Decrypt: D(C,Kd) = P
Goals:
Secrecy = message content not revealed
Integrity = message not modified
Authenticity = establish identity of sender
Nonrepudiability = establish identity of creator (regardless of who sent it)
an actor cannot deny creation of message (signature)
Operating Systems

29. Secure Communication Secret-key Cryptosystems Symmetric:
S and R share a common secret key K which is used for both encrypting and decrypting
Operating Systems

30. Secure Communication Examples transposition cipher:
rearranges the order of letters
example algorithm: swap 2 letters, skip n
key: n
e.g., n=1: 'hello world' → 'ehlol owrdlnd’
substitution cipher
replace letters or groups of letters
example: Cesar cypher
algorithm: replace every letter by the letter k positions down in the alphabet
key: k
e.g., k=1: 'fly at once' → 'gmz bu podf‘
Easy to break using statistical analysis
Operating Systems

31. Secure Communication Example: DES US standard (1977) Blocks of 64 bits
Block is divided into L and R half
F applies Key to R
result is XOR’d with L, becomes new R
old R becomes new L
repeat 16 times
F uses:
permutations
substitutions
XOR with a 56-bit key
Triple-DES
C = DEA(DEA(DEA(P, K3), K2, K1)
Operating Systems

32. Secure Communication With Secret-key cryptosystems:
Secrecy: only R can decrypt C
Integrity: intruder cannot produce valid message
Nonrepudiation: not possible, S can deny
Authenticity of sender: must prevent replay
Operating Systems

33. Secure Communication Use nonce N to prevent replay of message: S R
(2) C=E({P,N},K) 
Capturing either message does not help; both are different every time
Use timestamp T to prevent replay
C=E({P,T},K) 
Limits possible replay to a chosen time interval
Operating Systems

34. Secure Communication Key distribution and authentication
Both S and R must have the same key K
Trusted server approach:
Each process has its own secret key for communication with trusted Key Distribution Center (KDC)
At runtime, process A asks KDC for a Session Key KAB for communication with process B
KDC A B (1)  A,B (2) E({KAB,B,ticket},KA)  (3) ticket  ticket = E({KAB,A},KB)
Operating Systems

35. Secure Communication Public-key cryptosystems (Diffie-Hellman, 1976)
Asymmetric: different keys for encryption and decryption
One cannot be derived from the other
One is Public key, other is Private
Operating Systems

36. Secure Communication With Public-key cryptosystems
Secrecy: only R can decrypt message using KRpriv
Integrity: intruder cannot produce valid message without KSpriv
Authenticity of creator: same as integrity: only S knows KSpriv
Authenticity of sender: use nonce or timestamp to prevent replay
Operating Systems

37. Secure Communication
How to make a key/function so that the other cannot be derived from it?
RSA (Rivest, Shamir, Adelman) Public Key C = E(P) = Pe mod n P = D(C) = Cd mod n
(e,n): Public encryption key
(d,n): Private/secret decryption key; d cannot be derived from e
Operating Systems

38. Secure Communication RSA Key Generation
Choose large primes p and q; compute n=p*q
Example: p=5, q=7, n=35
Choose d to be a (large) prime number having no factors in common with (p1)*(q1)
Example: (51)*(71)=24; d=5 or 7 or 11 (choose 11)
Choose e such that e*d mod (p1)*(q1) = 1
Example: e*11 mod 24 = 1; e = 11 or 35 or 59 or 83 … C = E(P) = P59 mod 35 P = D(C) = C11 mod 35
Operating Systems

39. Secure Communication Why is RSA encryption secure?
n is derived from p and q; (n=p*q)
d is also derived from p and q; (no common factors)
e is derived from d but also needs p and q;
only d is known/public, p and q have been discarded → e cannot be derived
similarly, d cannot be derived from e without p and q
Operating Systems

40. Secure Communication Public key distribution and authentication
Making key public is easy, but need to authenticate it:
How does A safely get B’s public key KBpubl ?
Trusted server approach:
KDC A (1)  A,B (2) E({B,KBpubl},KKDCpriv) 
KDC provides B’s public key KBpubl
KKDCpriv guarantees authenticity (KDC sent it)
Operating Systems

41. Secure Communication Digital Signatures:
How can a document be “signed” and transmitted electronically?
Here is my signature
Anyone can copy and attach it to any document
Sign on paper, scan
Any document is digitized and can be modified
Public-key cryptosystems permit unforgeable electronic “signatures”?
Operating Systems

42. Secure Communication Digital Signature: document M is to be “signed”
Sender generates unique digest: d = H(M)
Sender encrypts E(d,KSpriv), receiver decrypts with KSpubl
Receiver computes d’ = H(M);
d’ is a unique signature of document M
d=d’ means that d is a also a unique signature of M;
Decryption authenticates sender, proving sender sent d
i.e., sender signed M
Operating Systems