12. Protection/Security Interface - PowerPoint Presentation

Slide1

12. Protection/Security Interface

12.1 Security Threats Types of Damage Vulnerable Resources Types of Attacks 12.2 Functions of a Protection System12.3 User AuthenticationApproaches to AuthenticationPasswords12.4 Secure CommunicationPrinciples of CryptographySecret-Key CryptosystemsPublic-Key Cryptosystems

Operating Systems

1Slide2

Security Threats

Types of damageInformation Disclosuretheft Information Destructionpossible without disclosureUnauthorized Use of servicesi

nstall SW without license, pirated copies (theft) use fake ID/password to use online service

Denial

of Servicedifficult to quantifyVulnerable resourcesHardware (CPU, memory, communications, devices)Software (files, processes, VM)

Operating Systems

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Types of Attacks

Browsing of InformationUnauthorized search for residual informationUnused memory and disk space is generally not deletedTypically done by a user who is already insideInformation leakingA trusted service leaks confidential/secret information (Confinement Problem)

Operating Systems

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Types of Attacks

Trojan Horse Greek mythology—the siege of Troywooden horse = “present” by Greekssoldiers hidden insideTrojans 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 email attachments)

Trap door

a

n

undocumented

feature

i

nserted on

purpose

or as a

flaw

to enter later

Operating Systems

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Types of Attacks

VirusesDesigned to replicate themselvesremovable storage media, email, file transferIntended to cause damageNeed a host programattach to and modify hostexecute as part of hostVirus detectioncheck program length(but virus can hide or compress program)c

heck for virus “signature”—bit pattern used by virus to mark already infected program (viruses use encryption)

Operating Systems

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Types of Attacks

WormsIntended to cause damageExploit some system weakness to replicateNo host neededExample: 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 startfinger: Buffer overflow

not checked – major weakness to take over the system

Operating Systems

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Types of Attacks

Buffer Overflow: Example:foo calls fingerAttack hijacks return address by supplying aparameter that islonger than the buffer (overflows)When

finger terminates,control goes to a placeset by the attack and

is not returned to

foo

.

Operating Systems

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Types of Attacks

Remote executionService to upload and start code on remote machineMobile agent: may migrate among machinesLike worm but legitimate migrationMust be designed carefully to prevent abuse of privileges of remote host environmentWire tappingInsert a device into line or listen to wirelessPassive (listen) or Active (modify)Waste searchingLook for passwords or sensitive data

Operating Systems

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Types of Attacks

MasqueradingImpersonate process, user, serviceUsed from outside: Use stolen password (impersonate a legitimate user)Break communication line, assume sessionUsed from within (spoofing):Impersonate login shell, steal passwordTrial and errore.g., try to guess passwordOperating Systems

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Types of Attacks -- Classification

From withindirect access as a valid processindirect Access via agent (attacker not present during attack)From outsidechannels provided for legitimate purposesillegitimate channels

Operating Systems

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Functions of a Protection System

External safeguardsguard physical access (locks, badges, cameras)Verification of user identity (User Authentication) Communication safeguardsprotect public/vulnerable lines: cryptographyAccess control (Ch 13)c

an Subject perform function on R

esource

Information flow

control (Ch 13)can S get information contained in R (indirectly)

Operating Systems

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User Authentication

Approaches:Knowledge of some informationPassword, dialogPossession of some artifactMachine-readable cards (ATM)Combine with knowledge (PIN)Biometrics: Physical characteristics of personFingerprintHand geometryFace geometryRetina or iris scanVoice printSignature dynamics

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Fingerprint Recognition

Extremely useful biometrics technologyFingerprints are a primary and accurate identification method

Operating Systems

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Fingerprint Recognition

Uses the ridge endings and bifurcation's to plot points known as minutiaeThe number and locations of the minutiae vary from finger to finger and from person to person

Minutiae

Finger Image

+ Minutiae

Finger Image

Operating Systems

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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

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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

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Hand geometry

one of the most deployed biometrics world wide

Ben

Gurion

Airport

(Israel)

Operating Systems

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Signature Verification

Static/off-line: match pattern (image)can easily be reproducedDynamic/On-line:match movement

of the pen during signing process (pressure, speed)

Many commercial products

Operating Systems

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Iris recognition

Based on visible features, i.e. rings, furrows, freckles and the corona Safest, most accurate biometrics technology

Heathrow Airport (London

)

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Retina recognition

Capture the pattern of blood vessels throughout the retinaNo two retinas are the same, even in identical twins

More difficult/less convenient than iris scan

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Thermograms

infrared camera to detect the heat patterns

Other techniques

DNA

Unique

(except for identical twins) but many

imitations

:

n

ot fully automated, slow, expensive

p

rivacy

issue – DNA contains information about

race

, paternity, medical

conditions

r

equires

a physical sample of tissue

Operating Systems

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User Authentication

Problem with biometrics:uncertainty in recognitionSystem generates anumber 0  n  1Bimodal distributionThreshold must be chosen to minimizefalse alarmsimposter acceptance

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User Authentication

PasswordsMust protect stored password files from accessMust prevent trial and error (guessing)Protecting password filesMaintain unencrypted; rely on access controlEncrypt using “one-way” function H:H-1 is unknownknowing H(x) does not yield x

keep only H(pw) with user namea

t login, compute

H(pw’)

and compare with H(pw)Operating Systems23Slide24

User Authentication

Preventing password guessingSystem-generated pwRandom string: difficult to memorize“Pronounceable” wordsSystem-validatedAccept only passwords that obey specifications (length, mix of letters/digits, upper/lower case)Employ

password-cracking programs toreject easy-to-guess passwords

Time-limited

Expiration

date or number of usesOperating Systems

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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/2Use one-way function to generate series ofone-time passwords from one password

pw… H(H(H(pw))) H(H(pw)) H(pw) pwIntruder can derive H(H(pw)) from

H(pw)

but not

H(pw) from H(H(pw))because H-1 is unknownOperating Systems

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User Authentication

guess any valid password:repeatedly generate strings s (dictionary, random, …), check if H(s) is in tableSystem-extended pwfor each pw, generate randomnumber slt (called “salt”)store: UserName,

slt,H(slt,pw

)

g

uessing: must check H(slt,s) for every slt in tablesalting does not make it harder to guess the password of a

specific user

Operating Systems

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Functions of a Protection System

External safeguardsGuard physical access (locks, badges, cameras)Verification of user identity (User Authentication)Communication safeguards Protect public/vulnerable lines: cryptographyAccess controlCan Subject perform function on ResourceInformation flow controlCan S get information contained in R

(indirectly)Operating Systems

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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 revealedIntegrity = message not modifiedAuthenticity = establish identity of sender

Nonrepudiability

= establish identity of creator (regardless of who sent it)

an actor cannot deny creation of message (signature)

Operating Systems

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Secure Communication

Secret-key Cryptosystems

Symmetric

:

S

and

R

s

hare

a common

secret

key

K

which is used for both encrypting and decryptingOperating Systems29Slide30

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

e

xample: Cesar cypheralgorithm: replace every letter by the letter k positions down in the alphabetkey: ke.g., k=1: 'fly at once' → 'gmz bu podf‘Easy to break using statistical analysisSecure CommunicationOperating Systems30Slide31

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

s

ubstitutions

XOR with a 56-bit key

Triple-DESC = DEA(DEA(DEA(P, K3), K2, K1)Secure CommunicationOperating Systems

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Secure Communication

With Secret-key cryptosystems:Secrecy: only R can decrypt CIntegrity: intruder cannot produce valid messageNonrepudiation: not possible, S can denyAuthenticity of sender: must prevent

replay

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Secure Communication

Use nonce N to prevent replay of message: S R(1)  N(2)

C=

E

({

P,N},

K) 

Capturing either message does not help;

both are different every time

Use

timestamp

T

to prevent replay

S R C=E({P,T},K)  Limits possible replay to a chosen time intervalOperating Systems

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Secure Communication

Key distribution and authenticationBoth S and R must have the same key KTrusted 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},K

A

) 

(3)

ticket



ticket

=

E

({KAB,A},KB)Operating Systems34Slide35

Secure Communication

Public-key cryptosystems (Diffie-Hellman, 1976)Asymmetric: different keys for encryption and decryption One cannot be derived from the otherOne is Public key, other is Private

Operating Systems

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Secure Communication

With Public-key cryptosystemsSecrecy: only R can decrypt message using KRprivIntegrity: intruder cannot produce valid message without KSprivAuthenticity of creator: same as integrity: only

S knows KSpriv

Authenticity of

sender

: use nonce or timestamp to prevent replay

Operating Systems

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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 eOperating Systems

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Secure Communication

RSA Key GenerationChoose large primes p and q; compute n=p*qExample: p=5, q=7, n=35Choose 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 35Operating Systems38Slide39

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 derivedsimilarly, d cannot be derived from

e without p and q

Operating Systems

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Secure Communication

Public key distribution and authenticationMaking 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},K

KDCpriv) 

KDC provides B’s public key

K

B

publ

K

KDC

priv

guarantees authenticity (KDC sent it)

Operating Systems40Slide41

Secure Communication

Digital Signatures: How can a document be “signed” and transmitted electronically?Here is my signatureAnyone can copy and attach it to any documentSign on paper, scanAny document is

digitized and can be modified

Public-key cryptosystems permit

unforgeable

electronic “signatures”?

Operating Systems

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Secure Communication

Digital Signature: document M is to be “signed”Sender generates unique digest: d = H(M)Sender encrypts E(d,KS

priv),

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 MOperating Systems42