Internet security

Chapter 8
Network Security

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Network Security 7-1

Chapter 7: Network Security
Chapter goals:
Ì understand principles of network security:
r cryptography and its many uses beyond
“confidentiality”
r authentication
r message integrity
r key distribution

Ì security in practice:
r firewalls
r security in application, transport, network, link
layers

Chapter 7 roadmap

7.1 What is network security?
7.2 Principles of cryptography
7.3 Authentication
7.4 Integrity
7.5 Key Distribution and certification
7.6 Access control: firewalls
7.7 Attacks and counter measures
7.8 Security in many layers


What is network security?
Confidentiality: only sender, intended receiver
should “understand” message contents
r sender encrypts message
r receiver decrypts message

Authentication: sender, receiver want to confirm
identity of each other
Message Integrity: sender, receiver want to ensure
message not altered (in transit, or afterwards)
without detection
Access and Availability: services must be accessible
and available to users


Friends and enemies: Alice, Bob, Trudy
Ì well-known in network security world
Ì Bob, Alice (lovers!) want to communicate “securely”
Ì Trudy (intruder) may intercept, delete, add messages

Alice Bob
data, control
channel
messages

data secure secure data
sender receiver

Trudy

Who might Bob, Alice be?
Ì … well, real-life Bobs and Alices!
Ì Web browser/server for electronic
transactions (e.g., on-line purchases)
Ì on-line banking client/server
Ì DNS servers
Ì routers exchanging routing table updates
Ì other examples?


There are bad guys (and girls) out there!
Q: What can a “bad guy” do?
A: a lot!
r eavesdrop: intercept messages
r actively insert messages into connection
r impersonation: can fake (spoof) source address
in packet (or any field in packet)
r hijacking: “take over” ongoing connection by
removing sender or receiver, inserting himself
in place
r denial of service: prevent service from being
used by others (e.g., by overloading resources)
more on this later ……

Chapter 7 roadmap

7.3 Authentication
7.4 Integrity


The language of cryptography
Alice’s Bob’s
K encryption K decryption
A
key B key

plaintext encryption ciphertext decryption plaintext
algorithm algorithm

symmetric key crypto: sender, receiver keys identical
public-key crypto: encryption key public, decryption key
secret (private)

Symmetric key cryptography
substitution cipher: substituting one thing for another
r monoalphabetic cipher: substitute one letter for another

plaintext: abcdefghijklmnopqrstuvwxyz

ciphertext: mnbvcxzasdfghjklpoiuytrewq

E.g.: Plaintext: bob. i love you. alice
ciphertext: nkn. s gktc wky. mgsbc

Q: How hard to break this simple cipher?:
 brute force (how hard?)
 other?


Symmetric key cryptography

KA-B KA-B

message, m algorithm algorithm
K (m)
A-B
m=K ( K (m) )
A-B A-B

symmetric key crypto: Bob and Alice share know same
(symmetric) key: K
A-B
Ì e.g., key is knowing substitution pattern in mono
alphabetic substitution cipher
Ì Q: how do Bob and Alice agree on key value?


Symmetric key crypto: DES
DES: Data Encryption Standard
Ì US encryption standard [NIST 1993]
Ì 56-bit symmetric key, 64-bit plaintext input
Ì How secure is DES?
rDES Challenge: 56-bit-key-encrypted phrase
(“Strong cryptography makes the world a safer
place”) decrypted (brute force) in 4 months
r no known “backdoor” decryption approach
Ì making DES more secure:
r use three keys sequentially (3-DES) on each datum
r use cipher-block chaining


Symmetric key
crypto: DES
DES operation
initial permutation
16 identical “rounds” of
function application,
each using different
48 bits of key
final permutation


AES: Advanced Encryption Standard

Ì new (Nov. 2001) symmetric-key NIST
standard, replacing DES
Ì processes data in 128 bit blocks
Ì 128, 192, or 256 bit keys
Ì brute force decryption (try each key)
taking 1 sec on DES, takes 149 trillion
years for AES


Public Key Cryptography

symmetric key crypto public key cryptography
Ì requires sender, Ì radically different
receiver know shared approach [Diffie-
secret key Hellman76, RSA78]
Ì Q: how to agree on key Ì sender, receiver do
in first place not share secret key
(particularly if never Ì public encryption key
“met”)? known to all
Ì private decryption
key known only to
receiver


Public key cryptography
+ Bob’s public
K
B key

- Bob’s private
K
B key

message, m algorithm + algorithm message
K (m) - +
B m = K B(K (m))
B


Public key encryption algorithms

Requirements:
+ . .
1 need K B( ) and K - ( ) such that
B
- +
K (K (m)) = m
B B
+
2 given public key KB , it should be
impossible to compute private
-
key K B

RSA: Rivest, Shamir, Adelson algorithm

RSA: Choosing keys
1. Choose two large prime numbers p, q.
(e.g., 1024 bits each)

2. Compute n = pq, z = (p-1)(q-1)

3. Choose e (with e<n) that has no common factors
with z. (e, z are “relatively prime”).

4. Choose d such that ed-1 is exactly divisible by z.
(in other words: ed mod z = 1 ).

5. Public key is (n,e). Private key is (n,d).
+ -
KB KB

RSA: Encryption, decryption
0. Given (n,e) and (n,d) as computed above

1. To encrypt bit pattern, m, compute
e
c = m e mod n (i.e., remainder when m is divided by n)

2. To decrypt received bit pattern, c, compute
d
m = c d mod n (i.e., remainder when c is divided by n)

Magic d
m = (m e mod n) mod n
happens!
c


RSA example:
Bob chooses p=5, q=7. Then n=35, z=24.
e=5 (so e, z relatively prime).
d=29 (so ed-1 exactly divisible by z.

letter m me c = me mod n
encrypt:
l 12 1524832 17

d
decrypt:
c c m = cd mod n letter
17 481968572106750915091411825223071697 12 l


RSA: Why is that d
m = (m e mod n) mod n

Useful number theory result: If p,q prime and
n = pq, then: y y mod (p-1)(q-1)
x mod n = x mod n

e
(m mod n) d mod n = med mod n
ed mod (p-1)(q-1)
= m mod n
(using number theory result above)
1
= m mod n
(since we chose ed to be divisible by
(p-1)(q-1) with remainder 1 )

= m

RSA: another important property
The following property will be very useful later:

- + + -
K (K (m)) = m = K (K (m))
B B B B

use public key use private key
first, followed first, followed
by private key by public key

Result is the same!


Chapter 7 roadmap

7.3 Authentication
7.4 Integrity


Authentication
Goal: Bob wants Alice to “prove” her identity
to him
Protocol ap1.0: Alice says “I am Alice”

“I am Alice”
Failure scenario??


Authentication
Goal: Bob wants Alice to “prove” her identity
to him
Protocol ap1.0: Alice says “I am Alice”

in a network,
Bob can not “see”
Alice, so Trudy simply
“I am Alice” declares
herself to be Alice


Authentication: another try
Protocol ap2.0: Alice says “I am Alice” in an IP packet
containing her source IP address

Alice’s
IP address
“I am Alice”

Failure scenario??


Protocol ap2.0: Alice says “I am Alice” in an IP packet
containing her source IP address

Trudy can create
a packet
Alice’s
“spoofing”
IP address
“I am Alice” Alice’s address


Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.

Alice’s Alice’s
“I’m Alice”
IP addr password

Alice’s Failure scenario??
OK
IP addr


secret password to “prove” it.

Alice’s Alice’s
“I’m Alice”
IP addr password
playback attack: Trudy
Alice’s records Alice’s packet
OK
IP addr and later
plays it back to Bob

Alice’s Alice’s
“I’m Alice”
IP addr password


Authentication: yet another try
encrypted secret password to “prove” it.

Alice’s encrypted
“I’m Alice”
IP addr password

Alice’s Failure scenario??
OK
IP addr


encrypted secret password to “prove” it.

Alice’s encryppted
IP addr password
“I’m Alice” record
and
Alice’s
OK playback
IP addr
still works!

Alice’s encrypted
“I’m Alice”
IP addr password


Authentication: yet another try
Goal: avoid playback attack
Nonce: number (R) used only once –in-a-lifetime
ap4.0: to prove Alice “live”, Bob sends Alice nonce, R.
Alice
must return R, encrypted with shared secret key
“I am Alice”

R
KA-B(R) Alice is live, and
only Alice knows
key to encrypt
nonce, so it must
Failures, drawbacks? be Alice!

Authentication: ap5.0
ap4.0 requires shared symmetric key
Ì can we authenticate using public key techniques?
ap5.0: use nonce, public key cryptography

“I am Alice”
Bob computes
R + -
- KA(KA (R)) = R
K A (R) and knows only Alice
“send me your public key”
could have the private
+ key, that encrypted R
KA such that
+ -
K (K (R)) = R
A A


ap5.0: security hole
Man (woman) in the middle attack: Trudy poses as
Alice (to Bob) and as Bob (to Alice)

I am Alice I am Alice
R -
K (R)
T
R - Send me your public key
K (R) +
A K
T
Send me your public key
+
K
A +
K (m)
Trudy gets T
- +
+ m = K (K (m))
K (m)
A sends T to Alice
m T
- + ennrypted with
m = K (K (m))
A A Alice’s public key

ap5.0: security hole
Man (woman) in the middle attack: Trudy poses as
Alice (to Bob) and as Bob (to Alice)

Difficult to detect:
 Bob receives everything that Alice sends, and vice
versa. (e.g., so Bob, Alice can meet one week later and
recall conversation)
 problem is that Trudy receives all messages as well!


Chapter 7 roadmap

7.3 Authentication
7.4 Message integrity


Digital Signatures

Cryptographic technique analogous to hand-
written signatures.
Ì sender (Bob) digitally signs document,
establishing he is document owner/creator.
Ì 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:
Ì Bob signs m by encrypting with his private key
- -
KB, creating “signed” message, KB(m)
-
Bob’s message, m K B Bob’s private -
K B(m)
key
Dear Alice
Bob’s message,
Oh, how I have missed Public key m, signed
you. I think of you all the
time! …(blah blah blah) encryption (encrypted) with
algorithm his private key
Bob


Digital Signatures (more)
-
Ì Suppose Alice receives msg m, digital signature KB(m)
Ì Alice verifies m signed by Bob by applying Bob’s
+ - + -
public key KB to KB(m) then checks KB(KB(m) ) = m.
+ -
Ì If KB(KB(m) ) = m, whoever signed m must have used
Bob’s private key.
Alice thus verifies that:
½ Bob signed m.
½ No one else signed m.
½ 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 large
H: Hash
message
Function
m
Computationally expensive
to public-key-encrypt
H(m)
long messages
Goal: fixed-length, easy- Hash function properties:
to-compute digital Ì many-to-1
“fingerprint”
Ì produces fixed-size msg
Ì apply hash function H
digest (fingerprint)
to m, get fixed size
Ì given message digest x,
message digest, H(m).
computationally
infeasible to find m such
that x = H(m)

Internet checksum: poor crypto hash
function
Internet checksum has some properties of hash function:
½ produces fixed length digest (16-bit sum) of message
½ is many-to-one

But given message with given hash value, it is easy to find
another message with same hash value:

message ASCII format message ASCII format
I O U 1 49 4F 55 31 I O U 9 49 4F 55 39
0 0 . 9 30 30 2E 39 0 0 . 1 30 30 2E 31
9 B O B 39 42 D2 42 9 B O B 39 42 D2 42
B2 C1 D2 AC different messages B2 C1 D2 AC
but identical checksums!

Digital signature = signed message digest
Alice verifies signature and
Bob sends digitally signed integrity of digitally signed
message: message:
large
message H: Hash encrypted
m function H(m)
msg digest
-
KB(H(m))
Bob’s digital large
private signature message
- Bob’s
key KB (encrypt) m digital
public
+ signature
key KB
encrypted H: Hash (decrypt)
msg digest function
-
+ KB(H(m))
H(m) H(m)

equal
?

Hash Function Algorithms
Ì MD5 hash function widely used (RFC 1321)
r computes 128-bit message digest in 4-step
process.
r arbitrary 128-bit string x, appears difficult to
construct msg m whose MD5 hash is equal to x.
Ì SHA-1 is also used.
r US standard [NIST, FIPS PUB 180-1]
r 160-bit message digest


Chapter 7 roadmap

7.3 Authentication
7.4 Integrity
7.5 Key distribution and certification


Trusted Intermediaries
Symmetric key problem: Public key problem:
Ì How do two entities Ì When Alice obtains
establish shared secret Bob’s public key (from
key over network? web site, e-mail,
Solution: diskette), how does she
know it is Bob’s public
Ì trusted key distribution
key, not Trudy’s?
center (KDC) acting as
intermediary between Solution:
entities Ì trusted certification
authority (CA)


Key Distribution Center (KDC)
Ì Alice, Bob need shared symmetric key.
Ì KDC: server shares different secret key with each
registered user (many users)
Ì Alice, Bob know own symmetric keys, KA-KDC KB-KDC , for
communicating with KDC.
KDC

KA-KDC KP-KDC
KX-KDC
KP-KDC KB-KDC
KY-KDC

KZ-KDC
KA-KDC KB-KDC


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

KDC
generates
KA-KDC(A,B)
R1

Alice KA-KDC(R1, KB-KDC(A,R1) )
Bob knows to
knows use R1 to
R1 KB-KDC(A,R1) communicate
with Alice

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

Certification Authorities
Ì Certification authority (CA): binds public key to
particular entity, E.
Ì E (person, router) registers its public key with CA.
r E provides “proof of identity” to CA.
r CA creates certificate binding E to its public key.
r certificate containing E’s public key digitally signed by CA
– CA says “this is E’s public key”

Bob’s digital
+
public +
signature KB
key KB (encrypt)
CA
certificate for
K-
Bob’s private
identifying key CA Bob’s public key,
information signed by CA

Certification Authorities
Ì When Alice wants Bob’s public key:
r gets Bob’s certificate (Bob or elsewhere).
r apply CA’s public key to Bob’s certificate, get
Bob’s public key

+ digital Bob’s
KB signature public
+
(decrypt) K B key

CA
public +
K CA
key


A certificate contains:
Ì Serial number (unique to issuer)
Ì info about certificate owner, including algorithm
and key value itself (not shown)
Ì info about
certificate
issuer
Ì valid dates
Ì digital
signature by
issuer


Chapter 7 roadmap

7.3 Authentication
7.4 Integrity


Firewalls
firewall
isolates organization’s internal net from larger
Internet, allowing some packets to pass,
blocking others.

administered public
network Internet

firewall


Firewalls: Why
prevent denial of service attacks:
r SYN flooding: attacker establishes many bogus
TCP connections, no resources left for “real”
connections.
prevent illegal modification/access of internal data.
r e.g., attacker replaces CIA’s homepage with
something else
allow only authorized access to inside network (set of
authenticated users/hosts)
two types of firewalls:
r application-level
r packet-filtering

Should arriving
Packet Filtering packet be allowed
in? Departing packet
let out?

Ì internal network connected to Internet via
router firewall
Ì router filters packet-by-packet, decision to
forward/drop packet based on:
r source IP address, destination IP address
r TCP/UDP source and destination port numbers
r ICMP message type
r TCP SYN and ACK bits

Packet Filtering
Ì Example 1: block incoming and outgoing
datagrams with IP protocol field = 17 and with
either source or dest port = 23.
r All incoming and outgoing UDP flows and telnet
connections are blocked.
Ì Example 2: Block inbound TCP segments with
ACK=0.
r Prevents external clients from making TCP
connections with internal clients, but allows
internal clients to connect to outside.


Application gateways gateway-to-remote
host telnet session
host-to-gateway
telnet session
Ì Filters packets on
application data as well application
gateway
router and filter

as on IP/TCP/UDP fields.
Ì Example: allow select
internal users to telnet
outside.
1. Require all telnet users to telnet through gateway.
2. For authorized users, gateway sets up telnet connection to
dest host. Gateway relays data between 2 connections
3. Router filter blocks all telnet connections not originating
from gateway.


Limitations of firewalls and gateways

Ì IP spoofing: router Ì filters often use all or
can’t know if data nothing policy for UDP.
“really” comes from Ì tradeoff: degree of
claimed source communication with
Ì if multiple app’s. need outside world, level of
special treatment, each security
has own app. gateway. Ì many highly protected
Ì client software must sites still suffer from
know how to contact attacks.
gateway.
r e.g., must set IP address
of proxy in Web
browser

Chapter 7 roadmap

7.3 Authentication
7.4 Integrity


Internet security threats
Mapping:
r before attacking: “case the joint” – find out
what services are implemented on network
r Use ping to determine what hosts have
addresses on network
r Port-scanning: try to establish TCP connection
to each port in sequence (see what happens)
r nmap (http://www.insecure.org/nmap/) mapper:
“network exploration and security auditing”

Countermeasures?


Mapping: countermeasures
r record traffic entering network
r look for suspicious activity (IP addresses, pots
being scanned sequentially)


Packet sniffing:
r broadcast media
r promiscuous NIC reads all packets passing by
r can read all unencrypted data (e.g. passwords)
r e.g.: C sniffs B’s packets

A C

src:B dest:A payload
B
Countermeasures?

Packet sniffing: countermeasures
r all hosts in orgnization run software that
checks periodically if host interface in
promiscuous mode.
r one host per segment of broadcast media
(switched Ethernet at hub)

A C

B


IP Spoofing:
r can generate “raw” IP packets directly from
application, putting any value into IP source
address field
r receiver can’t tell if source is spoofed
r e.g.: C pretends to be B

A C


B
Countermeasures?

IP Spoofing: ingress filtering
r routers should not forward outgoing packets
with invalid source addresses (e.g., datagram
source address not in router’s network)
r great, but ingress filtering can not be mandated
for all networks

A C


B


Denial of service (DOS):
r flood of maliciously generated packets “swamp”
receiver
r Distributed DOS (DDOS): multiple coordinated
sources swamp receiver
r e.g., C and remote host SYN-attack A

A C
SYN
SYN
SYN SYN SYN

B
SYN
Countermeasures? SYN

Denial of service (DOS): countermeasures
r filter out flooded packets (e.g., SYN) before
reaaching host: throw out good with bad
r traceback to source of floods (most likely an
innocent, compromised machine)

A C
SYN
SYN
SYN SYN SYN

B
SYN
SYN

Chapter 7 roadmap
7.3 Authentication
7.4 Integrity
7.8.1. Secure email
7.8.2. Secure sockets
7.8.3. IPsec
8.8.4. 802.11 WEP


Secure e-mail
 Alice wants to send confidential e-mail, m, to Bob.
KS

m K (.
S )
KS(m ) KS(m )
KS( ) . m

+ Internet - KS

KS
+.
K ()
B + +
- .
KB ( )
KB(KS ) KB(KS )
+ -
KB
KB

Alice:
 generates random symmetric private key, KS.
 encrypts message with KS (for efficiency)
 also encrypts KS with Bob’s public key.
 sends both KS(m) and KB(KS) to Bob.

Secure e-mail
 Alice wants to send confidential e-mail, m, to Bob.
KS

m K (.
S )
KS(m ) KS(m )
KS( ) . m

+ Internet - KS

KS
+.
K ()
B + +
- .
KB ( )
KB(KS ) KB(KS )
+ -
KB
KB

Bob:
 uses his private key to decrypt and recover KS
 uses KS to decrypt KS(m) to recover m


Secure e-mail (continued)
• Alice wants to provide sender authentication
message integrity.

- KA
+
KA
- -
m .
H( )
-.
K ()
A
KA(H(m)) KA(H(m)) +
KA ( )
. H(m )

+ Internet - compare

m H( ) . H(m )
m

• Alice digitally signs message.
• sends both message (in the clear) and digital signature.


Secure e-mail (continued)
• Alice wants to provide secrecy, sender authentication,
message integrity.
-
KA
-
m .
H( )
- .
K A( )
KA(H(m))
KS

+ KS( ) .
m + Internet

KS
+
KB( )
. +
KB(KS )
+
KB

Alice uses three keys: her private key, Bob’s public
key, newly created symmetric key

Pretty good privacy (PGP)
Ì Internet e-mail encryption A PGP signed message:
scheme, de-facto standard.
---BEGIN PGP SIGNED MESSAGE---
Ì uses symmetric key Hash: SHA1
cryptography, public key
cryptography, hash Bob:My husband is out of town
tonight.Passionately yours,
function, and digital Alice
signature as described.
Ì provides secrecy, sender ---BEGIN PGP SIGNATURE---
Version: PGP 5.0
authentication, integrity. Charset: noconv
Ì inventor, Phil Zimmerman, yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJh
was target of 3-year FEvZP9t6n7G6m5Gw2
---END PGP SIGNATURE---
federal investigation.


Secure sockets layer (SSL)

Ì transport layer Ì server authentication:
r SSL-enabled browser
security to any TCP- includes public keys for
based app using SSL trusted CAs.
services. r Browser requests

Ì used between Web server certificate,
issued by trusted CA.
browsers, servers for r Browser uses CA’s
e-commerce (shttp). public key to extract
Ì security services: server’s public key from
certificate.
r server authentication Ì check your browser’s
r data encryption security menu to see
r client authentication its trusted CAs.
(optional)

SSL (continued)
Encrypted SSL session: Ì SSL: basis of IETF
Ì Browser generates Transport Layer
symmetric session key, Security (TLS).
encrypts it with server’s Ì SSL can be used for
public key, sends non-Web applications,
encrypted key to server. e.g., IMAP.
Ì Using private key, server Ì Client authentication
decrypts session key. can be done with client
Ì Browser, server know certificates.
session key
r All data sent into TCP
socket (by client or server)
encrypted with session key.


IPsec: Network Layer Security
Ì Network-layer secrecy:
Ì For both AH and ESP, source,
r sending host encrypts the
destination handshake:
data in IP datagram
r create network-layer
r TCP and UDP segments;
logical channel called a
ICMP and SNMP
security association (SA)
messages.
Ì Each SA unidirectional.
Ì Network-layer authentication
Ì Uniquely determined by:
r destination host can
r security protocol (AH or
authenticate source IP
address ESP)
Ì Two principle protocols: r source IP address

r authentication header r 32-bit connection ID

(AH) protocol
r encapsulation security
payload (ESP) protocol

Authentication Header (AH) Protocol
Ì provides source AH header includes:
authentication, data Ì connection identifier
integrity, no Ì authentication data:
confidentiality source- signed message
Ì AH header inserted digest calculated over
between IP header, original IP datagram.
data field. Ì next header field:
Ì protocol field: 51 specifies type of data
Ì intermediate routers (e.g., TCP, UDP, ICMP)
process datagrams as
usual
IP header AH header data (e.g., TCP, UDP segment)


ESP Protocol
Ì provides secrecy, host Ì ESP authentication
authentication, data field is similar to AH
integrity. authentication field.
Ì data, ESP trailer Ì Protocol = 50.
encrypted.
Ì next header field is in ESP
trailer.
authenticated
encrypted
ESP ESP ESP
IP header TCP/UDP segment
header trailer authent.


IEEE 802.11 security
Ì War-driving: drive around Bay area, see what 802.11
networks available?
r More than 9000 accessible from public roadways
r 85% use no encryption/authentication
r packet-sniffing and various attacks easy!
Ì Wired Equivalent Privacy (WEP): authentication as in
protocol ap4.0
r host requests authentication from access point
r access point sends 128 bit nonce
r host encrypts nonce using shared symmetric key
r access point decrypts nonce, authenticates host


IEEE 802.11 security

Ì Wired Equivalent Privacy (WEP): data encryption
r Host/AP share 40 bit symmetric key (semi-
permanent)
r Host appends 24-bit initialization vector (IV) to
create 64-bit key
r 64 bit key used to generate stream of keys, kiIV
r kiIV used to encrypt ith byte, di, in frame:
ci = di XOR kiIV
r IV and encrypted bytes, ci sent in frame


802.11 WEP encryption
IV
(per frame)
KS: 40-bit key sequence generator
secret ( for given KS, IV)
symmetric
k1IV k2IV k3IV … kNIV kN+1IV… kN+1IV 802.11 WEP-encrypted data
key IV
header plus CRC
plaintext
frame data d1 d2 d3 … dN CRC1 … CRC4
plus CRC
c1 c2 c3 … cN cN+1 … cN+4

Sender-side WEP encryption
Figure 7.8-new1: 802.11 WEP protocol


Breaking 802.11 WEP encryption
Security hole:
Ì 24-bit IV, one IV per frame, -> IV’s eventually reused
Ì IV transmitted in plaintext -> IV reuse detected
Ì Attack:
r Trudy causes Alice to encrypt known plaintext d1 d2
d3 d4 …
r Trudy sees: ci = di XOR kiIV
r Trudy knows ci di, so can compute kiIV
r Trudy knows encrypting key sequence k1IV k2IV k3IV …
r Next time IV is used, Trudy can decrypt!

Network Security (summary)
Basic techniques…...
r cryptography (symmetric and public)
r authentication
r message integrity
r key distribution

…. used in many different security scenarios
r secure email
r secure transport (SSL)
r IP sec
r 802.11 WEP


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