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Sign-up for the paper presentation at the end of next week (after proj assignment) Ashish and Stephan volunteer for the next

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Overview

Integrity Key distribution and certification Mobile malcode

Overview Viruses Worms Automatic updates

History of Viruses (if time allowed)

Digital Signatures

Cryptographic technique analogous to hand-written signatures.

sender (Bob) digitally signs document, establishing he is document owner/creator.

verifiable, nonforgeable: recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document

Digital Signatures

Simple digital signature for message m: Bob signs m by encrypting with his private

key KB, creating “signed” message, KB(m)--

Dear Alice

Oh, how I have missed you. I think of you all the time! …(blah blah blah)

Bob

Bob’s message, m

Public keyencryptionalgorithm

Bob’s privatekey

K B-

Bob’s message, m, signed

(encrypted) with his private key

K B-(m)

Digital Signatures (more) Suppose Alice receives msg m, digital signature KB(m)

Alice verifies m signed by Bob by applying Bob’s public key KB to KB(m) then checks KB(KB(m) ) = m.

If KB(KB(m) ) = m, whoever signed m must have used

Bob’s private key.

+ +

-

-

- -

+

Alice thus verifies that: Bob signed m. No one else signed m. Bob signed m and not m’.

Non-repudiation: Alice can take m, and signature KB(m) to court and

prove that Bob signed m. -

Message Digests

Computationally expensive to public-key-encrypt long messages

Goal: fixed-length, easy- to-compute digital “fingerprint”

apply hash function H to m, get fixed size message digest, H(m).

Hash function properties: many-to-1 produces fixed-size msg

digest (fingerprint) given message digest x,

computationally infeasible to find m such that x = H(m)

large message

m

H: HashFunction

H(m)

Internet checksum: poor crypto hash function

Internet checksum has some properties of hash function:

produces fixed length digest (16-bit sum) of message

is many-to-oneBut given message with given hash value, it is easy to find another message with same hash value:

I O U 10 0 . 99 B O B

49 4F 55 3130 30 2E 3939 42 D2 42

message ASCII format

B2 C1 D2 AC

I O U 90 0 . 19 B O B

49 4F 55 3930 30 2E 3139 42 D2 42

message ASCII format

B2 C1 D2 ACdifferent messagesbut identical checksums!

large message

mH: Hashfunction H(m)

digitalsignature(encrypt)

Bob’s private

key K B-

+

Bob sends digitally signed message:

Alice verifies signature and integrity of digitally signed message:

KB(H(m))-

encrypted msg digest

KB(H(m))-

encrypted msg digest

large message

m

H: Hashfunction

H(m)

digitalsignature(decrypt)

H(m)

Bob’s public

key K B+

equal ?

Digital signature = signed message digest

Hash Function Algorithms

MD5 hash function widely used (RFC 1321) computes 128-bit message digest in 4-step

process. arbitrary 128-bit string x, appears difficult to

construct msg m whose MD5 hash is equal to x.

SHA-1 is also used US standard [NIST, FIPS PUB 180-1]

160-bit message digest SHA-2 is the new variant

Is the collective name of one-way hash functions developed by the NIST: SHA-256, SHA-384, and SHA-512

Overview

Integrity Key distribution and certification Mobile malcode

Overview Viruses Worms Automatic updates

Trusted Intermediaries

Symmetric key problem:

How do two entities establish shared secret key over network?

Solution: trusted key distribution

center (KDC) acting as intermediary between entities

Public key problem: When Alice obtains

Bob’s public key (from web site, e-mail, diskette), how does she know it is Bob’s public key, not Trudy’s?

Solution: trusted certification

authority (CA)

Key Distribution Center (KDC)

Alice, Bob need shared symmetric key. KDC: server shares different secret key with each

registered user (many users) Alice, Bob know own symmetric keys, KA-KDC KB-KDC ,

for communicating with KDC.

KB-KDC

KX-KDC

KY-KDC

KZ-KDC

KP-KDC

KB-KDC

KA-KDC

KA-KDC

KP-KDC

KDC

Key Distribution Center (KDC)

Aliceknows

R1

Bob knows to use R1 to communicate with Alice

Alice and Bob communicate: using R1 as session key for shared symmetric

encryption

Q: How does KDC allow Bob, Alice to determine shared symmetric secret key to communicate with each other?

KDC generate

s R1

KB-KDC(A,R1)

KA-KDC(A,B)

KA-KDC(R1, KB-KDC(A,R1) )

Certification Authorities

Certification authority (CA): binds public key to particular entity, E.

E (person, router) registers its public key with CA. E provides “proof of identity” to CA. CA creates certificate binding E to its public key. certificate containing E’s public key digitally signed by

CA – CA says “this is E’s public key”Bob’s public

key K B+

Bob’s identifying informatio

n

digitalsignature(encrypt)

CA private

key K CA-

K B+

certificate for Bob’s public

key, signed by CA

Certification Authorities When Alice wants Bob’s public key:

gets Bob’s certificate (Bob or elsewhere). apply CA’s public key to Bob’s certificate,

get Bob’s public key

Bob’s public

key K B+

digitalsignature(decrypt)

CA public

key K CA+

K B+

Overview

Integrity Key distribution and certification Mobile malcode

Overview Viruses Worms Automatic updates

Mobile Malcode Overview Malicious programs which spread from

machine to machine without the consent of the owners/operators/users Windows Automatic Update is (effectively)

consensual Many strains possible

Viruses Worms Compromised Auto-updates

• No user action required, very dangerous

Viruses Definition from RFC 1135: A virus is a piece of

code that inserts itself into a host, including operating systems, to propagate. It cannot run independently. It requires that its host program be run to activate it.

On execution Search for valid target files

• Usually executable files• Often only infect uninfected files

Insert a copy into targeted files• When the target is executed, the virus starts running

Only spread when contaminated files are moved from machine to machine

Mature defenses available

1988: Less than 10 known viruses 1990: New virus found every day 1993: 10-30 new viruses per week 1999: 45,000 viruses and variants

Source: McAfee

Virus Growth

0

10000

20000

30000

40000

50000

60000

1988 1990 1993 1999

Virus Infectables Executable files: .com, .exe, .bat Macros

With macro languages the line between pure data files and executable files is blurring

An infected file might be attached to an E-mail E-mail programs may use other programs (e.g.,

word) with macros to display incoming mail System sector viruses

Infect control sectors on a disk• DOS boot sectors• Partition (MBR) sectors

System sector viruses spread easily via floppy disk infections

Virus Infectables (cont’d) Companion viruses

Create a .com files for each .exe files DOS runs COM files before EXE files Relatively easy to find and eliminate

Cluster viruses Change the DOS directory info so that directory

entries point to the virus code instead of the real program

Even though every program on the disk may be "infected“, there is only one copy of the virus on the disk

Anatomy of a Virus

Two primary components Propagation mechanism Payload

Propagation Method by which the virus spreads itself. Old days: single PC, transferred to other

hosts by ways of floppy diskettes. Nowadays: Internet.

Structure of A Virus

Virus() { infectExecutable(); if (triggered()) { doDamage(); } jump to main of infected program;}

void infectExecutable() { file = choose an uninfected executable file; prepend V to file; }

void doDamage() { ... }int triggered() { return (some test? 1 : 0); }

Variable Viruses Polymorphic viruses

Change with each infection• Executables virus code changing (macros: var name,

line spacing, etc.)• Control flow permutations (rearrange code with goto’s)

Attempt to defeat scanners Virus writing tool kits have been created to

"simplify" creation of new viruses Current tool kits create viruses that can be

detected easily with existing scanner technology

But just a matter of time …

Virus Detection/Evasion Look for changes in

size Check time stamp on

file Look for bad behavior

False alarm prone

Look for patterns (byte streams) in virus code that are unique

Look for changes in file checksum

Compression of virus and target code

Modify time stamp to original

Do bad thing insidiously

Change patterns – polymorphism

Rearrange data in the file

Disable anti-virus programs

More on Virus Detection Scanning

Depend on prior knowledge of a virus Check programs before execution Need to be regularly updated

Integrity Checking Read entire disk and record integrity data that

acts as a signature for the files and system sectors

Analyze the nature of the changes to differentiate file corruption caused by a bug from corruption caused by a virus

Use cryptographic computation technique instead of simple checksum

More on Virus Detection Interception

Monitoring for system-level routines that perform destructive acts

Good for detecting logic bomb and Trojoran horse Cannot depend entirely upon behavior monitors

as they are easily bypassed. Combination of all three techniques can

detect most viruses

Logic Bomb:

Logic Bomb: A logic bomb executes when specific conditions occur.

Triggers for logic bombs can include change in a file, by a particular series of keystrokes, or at a specific time or date.

Trojan Horse

Programs that appear to have one function but actually perform another function.

Modern Trojan Horse: resemble a program that the user wishes to run - a game, a spreadsheet, or an editor.

While the program appears to be doing what the user wants, it is also doing something else unrelated to its advertised purpose, and without the user's knowledge.

Virus Recovery Extricate the virus from the infected file to

leave the original behind Remove the redirection to the virus code Recover the file from backup Delete the files and move on with life

Email-Borne Viruses/Worms The logic/code is contained in an email,

usually as an executable attachment Some mail agents execute content automatically Upon execution

• Search for email addresses in address book, emails• Send out copies of itself to all the addresses it can find

Taxonomy: are they viruses or worms? Email viruses: when human or human-agents

asynchronously trigger the worm Email worms: when they are fully automatic

Worms

Autonomous, active code that can replicate to remote hosts without any triggering Typically, a worm is a process not a file

Because they propagate autonomously, they can spread much more quickly than viruses!

Much of the class focuses on worms Speed and general lack of user interaction

make them the most significant threats

Compromised Auto-updates Auto-update program: At a given time, call to

central server If new programs are available, download and install

• Probably a good thing when correctly implemented

Hijack the auto-update May be trivial

• DNS cache poisoning or router redirection May be very difficult

• Capture the code-signing and the server’s private keys

Present a fake update to the world Unlike other mobile malcode, it only travels a

single step: from server to the clients

Malcode Attributes

Virus Email-borne Virus

Worm Email-borne Worm

Auto-Update

Autonomous

X X X

Triggered

X X X

Domain/Universe

File/ File system

Email User /Email Users

Host/Network

Email User /Email Users

Host/Network

Overview

Integrity Key distribution and certification Mobile malcode

Overview Viruses Worms Automatic updates

A History of Viruses

First Wild Viruses Apple I/II/III: 1981

Three viruses for the Apple machines emerged in 1981 Boot sector viruses

Floppies of that time had the disk operating system (DOS) on them by default Wrote it without malice

Fred Cohen’s Work: 1983 First documented work with viruses

Cohen’s PhD advisor, Leo Adelman, coined the term “virus”

Virus: “a program that can infect other programs by modifying them to include a … version of itself”

Viruses can quickly (~30 min) spread through a networked file system

Dissertation (1986) conclusion: "universal" detection of a virus is undecidable Gave insights to some heuristic detection

mechanisms that are widely used now No 100% guaranteed detection for virus/worm

First PC Virus: Pakistani Brain Virus (1986)

Written by Pakistani brothers to protect their copyright Claim: infect only machines that had an

unlicensed copy of their software Boot sector, memory resident Printed“Welcome to the Dungeon (c) 1986 Basit * Amjad

(pvt) Ltd. BRAIN COMPUTER SERVICES 730 NIZAB BLOCK ALLAMA IQBAL TOWN LAHORE-PAKISTAN

PHONE :430791,443248,280530. Beware of this VIRUS.... Contact us for

vaccination ............. !!"

Destructive Virus: Chernobyl (1998)

Designed to inflict harm Flash BIOS: would cause permanent hardware

damage to vulnerable motherboards Also overwrote first 2K sectors of each disk

• Typically resulted in a loss of data and made it unbootable

Previously believed that being benign was necessary for virus longevity Chernobyl provided evidence to the contrary

Early Mail Virus: Happy99 (1999)

One of the earliest viruses that propagated automatically when an infected attachment is executed

Did not infect files, only email user accounts

Email sent from infected person to others in address book (novelty at the time)

Early Macro Virus: Melissa (1999)

Microsoft Word 97 Macro virus Target first 50 entries in Outlook’s address

book Adjusted subject “Important messages

from ______” Points to attachment as a document

requested Contains a list of porn sites

Macro security was greatly increased with Melissa