Network Security 7-1
Chapter 7
Network Security
Computer Networking,
A Top Down Approach
Featuring the Internet,
2nd edition,
Jim Kurose,Keith Ross
Addison-Wesley,July
2002,
The PowerPoint Slides are based on the
material provided by
J.F Kurose and K.W,Ross.
Network Security 7-2
Chapter 7,Network Security
Chapter goals:
?understand principles of network security:
? cryptography and its many uses beyond
“confidentiality”
? authentication
? message integrity
? key distribution
?security in practice:
? firewalls
? security in application,transport,network,link
layers
Network Security 7-3
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
Network Security 7-4
What is network security?
Confidentiality,only sender,intended receiver
should,understand” message contents
? sender encrypts message
? 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
Network Security 7-5
Friends and enemies,Alice,Bob,Trudy
? well-known in network security world
? Bob,Alice (lovers!) want to communicate,securely”
? Trudy (intruder) may intercept,delete,add messages
secure
sender
secure
receiver
channel data,control messages
data data
Alice Bob
Trudy
Network Security 7-6
Who might Bob,Alice be?
?… well,real-life Bobs and Alices!
?Web browser/server for electronic
transactions (e.g.,on-line purchases)
?on-line banking client/server
?DNS servers
?routers exchanging routing table updates
?other examples?
Network Security 7-7
There are bad guys (and girls) out there!
Q,What can a,bad guy” do?
A,a lot!
? eavesdrop,intercept messages
? actively insert messages into connection
? impersonation,can fake (spoof) source address
in packet (or any field in packet)
? hijacking:,take over” ongoing connection by
removing sender or receiver,inserting himself
in place
? denial of service,prevent service from being
used by others (e.g.,by overloading resources)
more on this later ……
Network Security 7-8
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
Network Security 7-9
The language of cryptography
symmetric key crypto,sender,receiver keys identical
public-key crypto,encryption key public,decryption key
secret (private)
plaintext plaintextciphertext
KA
encryption
algorithm
decryption
algorithm
Alice’s
encryption
key
Bob’s
decryption
key
KB
Network Security 7-10
Symmetric key cryptography
substitution cipher,substituting one thing for another
? monoalphabetic cipher,substitute one letter for another
plaintext,abcdefghijklmnopqrstuvwxyz
ciphertext,mnbvcxzasdfghjklpoiuytrewq
Plaintext,bob,i love you,alice
ciphertext,nkn,s gktc wky,mgsbc
E.g.:
Q,How hard to break this simple cipher?:
? brute force (how hard?)
? other?
Network Security 7-11
Symmetric key cryptography
symmetric key crypto,Bob and Alice share know same
(symmetric) key,K
? e.g.,key is knowing substitution pattern in mono
alphabetic substitution cipher
? Q,how do Bob and Alice agree on key value?
plaintextciphertext
KA-B
encryption
algorithm
decryption
algorithm
A-B
KA-B
plaintext
message,m K (m)
A-B K (m)A-Bm = K ( )A-B
Network Security 7-12
Symmetric key crypto,DES
DES,Data Encryption Standard
? US encryption standard [NIST 1993]
? 56-bit symmetric key,64-bit plaintext input
? How secure is DES?
? DES Challenge,56-bit-key-encrypted phrase
(“Strong cryptography makes the world a safer
place”) decrypted (brute force) in 4 months
?no known,backdoor” decryption approach
? making DES more secure:
? use three keys sequentially (3-DES) on each datum
? use cipher-block chaining
Network Security 7-13
Symmetric key
crypto,DES
initial permutation
16 identical,rounds” of
function application,
each using different
48 bits of key
final permutation
DES operation
Network Security 7-14
AES,Advanced Encryption Standard
?new (Nov,2001) symmetric-key NIST
standard,replacing DES
?processes data in 128 bit blocks
?128,192,or 256 bit keys
?brute force decryption (try each key)
taking 1 sec on DES,takes 149 trillion
years for AES
Network Security 7-15
Public Key Cryptography
symmetric key crypto
? requires sender,
receiver know shared
secret key
? Q,how to agree on key
in first place
(particularly if never
“met”)?
public key cryptography
? radically different
approach [Diffie-
Hellman76,RSA78]
? sender,receiver do
not share secret key
? public encryption key
known to all
? private decryption
key known only to
receiver
Network Security 7-16
Public key cryptography
plaintext
message,m
ciphertextencryption
algorithm
decryption
algorithm
Bob’s public
key
plaintext
messageK (m)
B
+
K B+
Bob’s private
key K B
-
m = K (K (m))B+B-
Network Security 7-17
Public key encryption algorithms
need K ( ) and K ( ) such thatB B.,
given public key K,it should be
impossible to compute
private key K B
B
Requirements:
1
2
RSA,Rivest,Shamir,Adelson algorithm
+ -
K (K (m)) = mBB- +
+
-
Network Security 7-18
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-
Network Security 7-19
RSA,Encryption,decryption
0,Given (n,e) and (n,d) as computed above
1,To encrypt bit pattern,m,compute
c = m mod ne (i.e.,remainder when m is divided by n)e
2,To decrypt received bit pattern,c,compute
m = c mod nd (i.e.,remainder when c is divided by n)d
m = (m mod n)e mod ndMagichappens!
c
Network Security 7-20
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 = m mod ne
l 12 1524832 17
c m = c mod nd
17 481968572106750915091411825223071697 12
cd letter
l
encrypt:
decrypt:
Network Security 7-21
RSA,Why is that m = (m mod n)e mod nd
(m mod n)e mod n = m mod nd ed
Useful number theory result,If p,q prime and
n = pq,then:
x mod n = x mod ny y mod (p-1)(q-1)
= m mod ned mod (p-1)(q-1)
= m mod n1
= m
(using number theory result above)
(since we chose ed to be divisible by
(p-1)(q-1) with remainder 1 )
Network Security 7-22
RSA,another important property
The following property will be very useful later:
K (K (m)) = mBB- + K (K (m))BB+ -=
use public key
first,followed
by private key
use private key
first,followed
by public key
Result is the same!
Network Security 7-23
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
Network Security 7-24
Authentication
Goal,Bob wants Alice to,prove” her identity
to him
Protocol ap1.0,Alice says,I am Alice”
Failure scenario
“I am Alice”
Network Security 7-25
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
declares
herself to be Alice
“I am Alice”
Network Security 7-26
Authentication,another try
Protocol ap2.0,Alice says,I am Alice” in an IP packet
containing her source IP address
Failure scenario
“I am Alice”Alice’s IP address
Network Security 7-27
Authentication,another try
Protocol ap2.0,Alice says,I am Alice” in an IP packet
containing her source IP address
Trudy can create
a packet
“spoofing”
Alice’s address“I am Alice”Alice’s IP address
Network Security 7-28
Authentication,another try
Protocol ap3.0,Alice says,I am Alice” and sends her
secret password to,prove” it.
Failure scenario
“I’m Alice”Alice’s IP addr Alice’s password
OKAlice’s IP addr
Network Security 7-29
Authentication,another try
Protocol ap3.0,Alice says,I am Alice” and sends her
secret password to,prove” it.
playback attack,Trudy
records Alice’s packet
and later
plays it back to Bob
“I’m Alice”Alice’s IP addr Alice’s password
OKAlice’s IP addr
“I’m Alice”Alice’s IP addr Alice’s password
Network Security 7-30
Authentication,yet another try
Protocol ap3.1,Alice says,I am Alice” and sends her
encrypted secret password to,prove” it.
Failure scenario
“I’m Alice”Alice’s IP addr encrypted password
OKAlice’s IP addr
Network Security 7-31
Authentication,another try
Protocol ap3.1,Alice says,I am Alice” and sends her
encrypted secret password to,prove” it.
record
and
playback
still works!
“I’m Alice”Alice’s IP addr encrypptedpassword
OKAlice’s IP addr
“I’m Alice”Alice’s IP addr encryptedpassword
Network Security 7-32
Authentication,yet another try
Goal,avoid playback attack
Failures,drawbacks?
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
K (R)A-B Alice is live,and only Alice knows
key to encrypt
nonce,so it must
be Alice!
Network Security 7-33
Authentication,ap5.0
ap4.0 requires shared symmetric key
? can we authenticate using public key techniques?
ap5.0,use nonce,public key cryptography
“I am Alice”
R
Bob computes
K (R)A-
“send me your public key”
K A+
(K (R)) = RA-K A+
and knows only Alice
could have the private
key,that encrypted R
such that
(K (R)) = RA-K A+
Network Security 7-34
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
TK (R)
-
Send me your public key
TK
+AK (R)-
Send me your public key
AK
+
TK (m)
+
Tm = K (K (m))
+
T
-
Trudy gets
sends m to Alice
ennrypted with
Alice’s public key
AK (m)
+
Am = K (K (m))
+
A
-
R
Network Security 7-35
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!
Network Security 7-36
Chapter 7 roadmap
7.1 What is network security?
7.2 Principles of cryptography
7.3 Authentication
7.4 Message integrity
7.5 Key Distribution and certification
7.6 Access control,firewalls
7.7 Attacks and counter measures
7.8 Security in many layers
Network Security 7-37
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
Network Security 7-38
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 key
encryption
algorithm
Bob’s private
key K B
-
Bob’s message,
m,signed
(encrypted) with
his private key
K B-(m)
Network Security 7-39
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,
-
Network Security 7-40
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,Hash
Function
H(m)
Network Security 7-41
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-one
But given message with given hash value,it is easy to find
another message with same hash value,
I O U 1
0 0, 9
9 B O B
49 4F 55 31
30 30 2E 39
39 42 D2 42
message ASCII format
B2 C1 D2 AC
I O U 9
0 0, 1
9 B O B
49 4F 55 39
30 30 2E 31
39 42 D2 42
message ASCII format
B2 C1 D2 ACdifferent messages
but identical checksums!
Network Security 7-42
large
message
m
H,Hash
function H(m)
digital
signature
(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,Hash
function
H(m)
digital
signature
(decrypt)
H(m)
Bob’s
public
key K
B
+
equal
Digital signature = signed message digest
Network Security 7-43
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
Network Security 7-44
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
Network Security 7-45
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)
Network Security 7-46
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
Network Security 7-47
Key Distribution Center (KDC)
Alice
knows
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
generates
R1
KB-KDC(A,R1)
KA-KDC(A,B)
KA-KDC(R1,KB-KDC(A,R1) )
Network Security 7-48
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
information
digital
signature
(encrypt)
CA
private
key K CA
-
K B+
certificate for
Bob’s public key,
signed by CA
Network Security 7-49
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+
digital
signature
(decrypt)
CA
public
key K CA
+
K B+
Network Security 7-50
A certificate contains:
? Serial number (unique to issuer)
? info about certificate owner,including algorithm
and key value itself (not shown)
? info about
certificate
issuer
? valid dates
? digital
signature by
issuer
Network Security 7-51
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
Network Security 7-52
Firewalls
isolates organization’s internal net from larger
Internet,allowing some packets to pass,
blocking others.
firewall
administered
network
public
Internet
firewall
Network Security 7-53
Firewalls,Why
prevent denial of service attacks:
? SYN flooding,attacker establishes many bogus
TCP connections,no resources left for,real”
connections,
prevent illegal modification/access of internal data.
?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:
? application-level
? packet-filtering
Network Security 7-54
Packet Filtering
? internal network connected to Internet via
router firewall
? router filters packet-by-packet,decision to
forward/drop packet based on:
? source IP address,destination IP address
? TCP/UDP source and destination port numbers
? ICMP message type
? TCP SYN and ACK bits
Should arriving
packet be allowed
in? Departing packet
let out?
Network Security 7-55
Packet Filtering
? Example 1,block incoming and outgoing
datagrams with IP protocol field = 17 and with
either source or dest port = 23.
? All incoming and outgoing UDP flows and telnet
connections are blocked.
? Example 2,Block inbound TCP segments with
ACK=0.
? Prevents external clients from making TCP
connections with internal clients,but allows
internal clients to connect to outside.
Network Security 7-56
Application gateways
? Filters packets on
application data as well
as on IP/TCP/UDP fields.
? Example,allow select
internal users to telnet
outside.
host-to-gateway
telnet session
gateway-to-remote
host telnet session
application
gateway
router and filter
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.
Network Security 7-57
Limitations of firewalls and gateways
? IP spoofing,router
can’t know if data
“really” comes from
claimed source
? if multiple app’s,need
special treatment,each
has own app,gateway.
? client software must
know how to contact
gateway.
? e.g.,must set IP address
of proxy in Web
browser
? filters often use all or
nothing policy for UDP.
? tradeoff,degree of
communication with
outside world,level of
security
? many highly protected
sites still suffer from
attacks.
Network Security 7-58
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
Network Security 7-59
Internet security threats
Mapping:
?before attacking:,case the joint” – find out
what services are implemented on network
? Use ping to determine what hosts have
addresses on network
? Port-scanning,try to establish TCP connection
to each port in sequence (see what happens)
? nmap (http://www.insecure.org/nmap/) mapper,
“network exploration and security auditing”
Countermeasures?
Network Security 7-60
Internet security threats
Mapping,countermeasures
? record traffic entering network
? look for suspicious activity (IP addresses,pots
being scanned sequentially)
Network Security 7-61
Internet security threats
Packet sniffing:
? broadcast media
? promiscuous NIC reads all packets passing by
? can read all unencrypted data (e.g,passwords)
?e.g.,C sniffs B’s packets
A
B
C
src:B dest:A payload
Countermeasures?
Network Security 7-62
Internet security threats
Packet sniffing,countermeasures
? all hosts in orgnization run software that
checks periodically if host interface in
promiscuous mode.
? one host per segment of broadcast media
(switched Ethernet at hub)
A
B
C
src:B dest:A payload
Network Security 7-63
Internet security threats
IP Spoofing:
?can generate,raw” IP packets directly from
application,putting any value into IP source
address field
?receiver can’t tell if source is spoofed
? e.g.,C pretends to be B
A
B
C
src:B dest:A payload
Countermeasures?
Network Security 7-64
Internet security threats
IP Spoofing,ingress filtering
? routers should not forward outgoing packets
with invalid source addresses (e.g.,datagram
source address not in router’s network)
? great,but ingress filtering can not be mandated
for all networks
A
B
C
src:B dest:A payload
Network Security 7-65
Internet security threats
Denial of service (DOS):
?flood of maliciously generated packets,swamp”
receiver
? Distributed DOS (DDOS),multiple coordinated
sources swamp receiver
? e.g.,C and remote host SYN-attack A
A
B
C
SYN
SYNSYNSYN
SYN
SYN
SYN
Countermeasures?
Network Security 7-66
Internet security threats
Denial of service (DOS),countermeasures
? filter out flooded packets (e.g.,SYN) before
reaaching host,throw out good with bad
? traceback to source of floods (most likely an
innocent,compromised machine)
A
B
C
SYN
SYNSYNSYN
SYN
SYN
SYN
Network Security 7-67
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
7.8.1,Secure email
7.8.2,Secure sockets
7.8.3,IPsec
8.8.4,802.11 WEP
Network Security 7-68
Secure e-mail
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.
?Alice wants to send confidential e-mail,m,to Bob.
KS( ).
KB( ).+
+ -
KS(m )
KB(KS )+
m
KS
KS
KB+
Internet
KS( ).
KB( ).-
KB-
KS
mKS(m )
KB(KS )+
Network Security 7-69
Secure e-mail
Bob:
? uses his private key to decrypt and recover KS
? uses KS to decrypt KS(m) to recover m
?Alice wants to send confidential e-mail,m,to Bob.
KS( ).
KB( ).+
+ -
KS(m )
KB(KS )+
m
KS
KS
KB+
Internet
KS( ).
KB( ).-
KB-
KS
mKS(m )
KB(KS )+
Network Security 7-70
Secure e-mail (continued)
? Alice wants to provide sender authentication
message integrity.
? Alice digitally signs message.
? sends both message (in the clear) and digital signature.
H( ),KA( ).-
+ -
H(m )KA(H(m))
-
m
KA-
Internet
m
KA( ).+
KA+
KA(H(m))-
m H( )
,H(m )
compare
Network Security 7-71
Secure e-mail (continued)
? Alice wants to provide secrecy,sender authentication,
message integrity.
Alice uses three keys,her private key,Bob’s public
key,newly created symmetric key
H( ),KA( ).-
+
KA(H(m))-m
KA-
m
KS( ).
KB( ).+
+
KB(KS )+
KS
KB+
Internet
KS
Network Security 7-72
Pretty good privacy (PGP)
? Internet e-mail encryption
scheme,de-facto standard.
? uses symmetric key
cryptography,public key
cryptography,hash
function,and digital
signature as described.
? provides secrecy,sender
authentication,integrity.
? inventor,Phil Zimmerman,
was target of 3-year
federal investigation.
---BEGIN PGP SIGNED MESSAGE---
Hash,SHA1
Bob:My husband is out of town
tonight.Passionately yours,
Alice
---BEGIN PGP SIGNATURE---
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Network Security 7-73
Secure sockets layer (SSL)
? transport layer
security to any TCP-
based app using SSL
services,
? used between Web
browsers,servers for
e-commerce (shttp).
? security services:
? server authentication
? data encryption
? client authentication
(optional)
? server authentication:
? SSL-enabled browser
includes public keys for
trusted CAs.
? Browser requests
server certificate,
issued by trusted CA.
? Browser uses CA’s
public key to extract
server’s public key from
certificate,
? check your browser’s
security menu to see
its trusted CAs.
Network Security 7-74
SSL (continued)
Encrypted SSL session:
? Browser generates
symmetric session key,
encrypts it with server’s
public key,sends
encrypted key to server.
? Using private key,server
decrypts session key.
? Browser,server know
session key
? All data sent into TCP
socket (by client or server)
encrypted with session key.
? SSL,basis of IETF
Transport Layer
Security (TLS).
? SSL can be used for
non-Web applications,
e.g.,IMAP.
? Client authentication
can be done with client
certificates.
Network Security 7-75
IPsec,Network Layer Security
? Network-layer secrecy:
? sending host encrypts the
data in IP datagram
? TCP and UDP segments;
ICMP and SNMP
messages.
? Network-layer authentication
? destination host can
authenticate source IP
address
? Two principle protocols:
? authentication header
(AH) protocol
? encapsulation security
payload (ESP) protocol
? For both AH and ESP,source,
destination handshake:
? create network-layer
logical channel called a
security association (SA)
? Each SA unidirectional.
? Uniquely determined by:
? security protocol (AH or
ESP)
? source IP address
? 32-bit connection ID
Network Security 7-76
Authentication Header (AH) Protocol
? provides source
authentication,data
integrity,no
confidentiality
? AH header inserted
between IP header,
data field.
? protocol field,51
? intermediate routers
process datagrams as
usual
AH header includes:
? connection identifier
? authentication data,
source- signed message
digest calculated over
original IP datagram.
? next header field,
specifies type of data
(e.g.,TCP,UDP,ICMP)
IP header data (e.g.,TCP,UDP segment)AH header
Network Security 7-77
ESP Protocol
? provides secrecy,host
authentication,data
integrity.
? data,ESP trailer
encrypted.
? next header field is in ESP
trailer.
? ESP authentication
field is similar to AH
authentication field.
? Protocol = 50,
IP header TCP/UDP segmentESPheader ESPtrailer ESPauthent.
encrypted
authenticated
Network Security 7-78
IEEE 802.11 security
? War-driving,drive around Bay area,see what 802.11
networks available?
? More than 9000 accessible from public roadways
? 85% use no encryption/authentication
? packet-sniffing and various attacks easy!
? Wired Equivalent Privacy (WEP),authentication as in
protocol ap4.0
? host requests authentication from access point
? access point sends 128 bit nonce
? host encrypts nonce using shared symmetric key
? access point decrypts nonce,authenticates host
Network Security 7-79
IEEE 802.11 security
? Wired Equivalent Privacy (WEP),data encryption
? Host/AP share 40 bit symmetric key (semi-
permanent)
? Host appends 24-bit initialization vector (IV) to
create 64-bit key
? 64 bit key used to generate stream of keys,kiIV
? kiIV used to encrypt ith byte,di,in frame:
ci = di XOR kiIV
? IV and encrypted bytes,ci sent in frame
Network Security 7-80
802.11 WEP encryption
IV
( p e r f r a m e )
K S, 40 - b i t
s e c r e t
s y m m e t r i c
k e y
k 1
IV
k 2
IV
k 3
IV
… k N
IV
k N + 1
IV
… k N + 1
IV
d 1
d 2 d 3 … d N
C R C 1 … C R C 4
c 1
c 2 c 3 … c N
c N + 1 … c N + 4
p l a i n t e x t
f r a m e d a t a
p l u s C R C
k e y s e q u e n c e g e n e r a to r
( f o r g i v e n K S,I V )
8 0 2, 1 1
h e a d e r
IV
W E P - e n c r y p t e d d a ta
p l u s CR C
F ig ur e 7, 8 - ne w 1, 8 0 2, 1 1 W E P p r o to c o l
Sender-side WEP encryption
Network Security 7-81
Breaking 802.11 WEP encryption
Security hole,
? 24-bit IV,one IV per frame,-> IV’s eventually reused
? IV transmitted in plaintext -> IV reuse detected
?Attack:
? Trudy causes Alice to encrypt known plaintext d1 d2
d3 d4 …
? Trudy sees,ci = di XOR kiIV
? Trudy knows ci di,so can compute kiIV
? Trudy knows encrypting key sequence k1IV k2IV k3IV …
? Next time IV is used,Trudy can decrypt!
Network Security 7-82
Network Security (summary)
Basic techniques…...
? cryptography (symmetric and public)
? authentication
? message integrity
? key distribution
…,used in many different security scenarios
? secure email
? secure transport (SSL)
? IP sec
? 802.11 WEP
Chapter 7
Network Security
Computer Networking,
A Top Down Approach
Featuring the Internet,
2nd edition,
Jim Kurose,Keith Ross
Addison-Wesley,July
2002,
The PowerPoint Slides are based on the
material provided by
J.F Kurose and K.W,Ross.
Network Security 7-2
Chapter 7,Network Security
Chapter goals:
?understand principles of network security:
? cryptography and its many uses beyond
“confidentiality”
? authentication
? message integrity
? key distribution
?security in practice:
? firewalls
? security in application,transport,network,link
layers
Network Security 7-3
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
Network Security 7-4
What is network security?
Confidentiality,only sender,intended receiver
should,understand” message contents
? sender encrypts message
? 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
Network Security 7-5
Friends and enemies,Alice,Bob,Trudy
? well-known in network security world
? Bob,Alice (lovers!) want to communicate,securely”
? Trudy (intruder) may intercept,delete,add messages
secure
sender
secure
receiver
channel data,control messages
data data
Alice Bob
Trudy
Network Security 7-6
Who might Bob,Alice be?
?… well,real-life Bobs and Alices!
?Web browser/server for electronic
transactions (e.g.,on-line purchases)
?on-line banking client/server
?DNS servers
?routers exchanging routing table updates
?other examples?
Network Security 7-7
There are bad guys (and girls) out there!
Q,What can a,bad guy” do?
A,a lot!
? eavesdrop,intercept messages
? actively insert messages into connection
? impersonation,can fake (spoof) source address
in packet (or any field in packet)
? hijacking:,take over” ongoing connection by
removing sender or receiver,inserting himself
in place
? denial of service,prevent service from being
used by others (e.g.,by overloading resources)
more on this later ……
Network Security 7-8
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
Network Security 7-9
The language of cryptography
symmetric key crypto,sender,receiver keys identical
public-key crypto,encryption key public,decryption key
secret (private)
plaintext plaintextciphertext
KA
encryption
algorithm
decryption
algorithm
Alice’s
encryption
key
Bob’s
decryption
key
KB
Network Security 7-10
Symmetric key cryptography
substitution cipher,substituting one thing for another
? monoalphabetic cipher,substitute one letter for another
plaintext,abcdefghijklmnopqrstuvwxyz
ciphertext,mnbvcxzasdfghjklpoiuytrewq
Plaintext,bob,i love you,alice
ciphertext,nkn,s gktc wky,mgsbc
E.g.:
Q,How hard to break this simple cipher?:
? brute force (how hard?)
? other?
Network Security 7-11
Symmetric key cryptography
symmetric key crypto,Bob and Alice share know same
(symmetric) key,K
? e.g.,key is knowing substitution pattern in mono
alphabetic substitution cipher
? Q,how do Bob and Alice agree on key value?
plaintextciphertext
KA-B
encryption
algorithm
decryption
algorithm
A-B
KA-B
plaintext
message,m K (m)
A-B K (m)A-Bm = K ( )A-B
Network Security 7-12
Symmetric key crypto,DES
DES,Data Encryption Standard
? US encryption standard [NIST 1993]
? 56-bit symmetric key,64-bit plaintext input
? How secure is DES?
? DES Challenge,56-bit-key-encrypted phrase
(“Strong cryptography makes the world a safer
place”) decrypted (brute force) in 4 months
?no known,backdoor” decryption approach
? making DES more secure:
? use three keys sequentially (3-DES) on each datum
? use cipher-block chaining
Network Security 7-13
Symmetric key
crypto,DES
initial permutation
16 identical,rounds” of
function application,
each using different
48 bits of key
final permutation
DES operation
Network Security 7-14
AES,Advanced Encryption Standard
?new (Nov,2001) symmetric-key NIST
standard,replacing DES
?processes data in 128 bit blocks
?128,192,or 256 bit keys
?brute force decryption (try each key)
taking 1 sec on DES,takes 149 trillion
years for AES
Network Security 7-15
Public Key Cryptography
symmetric key crypto
? requires sender,
receiver know shared
secret key
? Q,how to agree on key
in first place
(particularly if never
“met”)?
public key cryptography
? radically different
approach [Diffie-
Hellman76,RSA78]
? sender,receiver do
not share secret key
? public encryption key
known to all
? private decryption
key known only to
receiver
Network Security 7-16
Public key cryptography
plaintext
message,m
ciphertextencryption
algorithm
decryption
algorithm
Bob’s public
key
plaintext
messageK (m)
B
+
K B+
Bob’s private
key K B
-
m = K (K (m))B+B-
Network Security 7-17
Public key encryption algorithms
need K ( ) and K ( ) such thatB B.,
given public key K,it should be
impossible to compute
private key K B
B
Requirements:
1
2
RSA,Rivest,Shamir,Adelson algorithm
+ -
K (K (m)) = mBB- +
+
-
Network Security 7-18
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-
Network Security 7-19
RSA,Encryption,decryption
0,Given (n,e) and (n,d) as computed above
1,To encrypt bit pattern,m,compute
c = m mod ne (i.e.,remainder when m is divided by n)e
2,To decrypt received bit pattern,c,compute
m = c mod nd (i.e.,remainder when c is divided by n)d
m = (m mod n)e mod ndMagichappens!
c
Network Security 7-20
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 = m mod ne
l 12 1524832 17
c m = c mod nd
17 481968572106750915091411825223071697 12
cd letter
l
encrypt:
decrypt:
Network Security 7-21
RSA,Why is that m = (m mod n)e mod nd
(m mod n)e mod n = m mod nd ed
Useful number theory result,If p,q prime and
n = pq,then:
x mod n = x mod ny y mod (p-1)(q-1)
= m mod ned mod (p-1)(q-1)
= m mod n1
= m
(using number theory result above)
(since we chose ed to be divisible by
(p-1)(q-1) with remainder 1 )
Network Security 7-22
RSA,another important property
The following property will be very useful later:
K (K (m)) = mBB- + K (K (m))BB+ -=
use public key
first,followed
by private key
use private key
first,followed
by public key
Result is the same!
Network Security 7-23
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
Network Security 7-24
Authentication
Goal,Bob wants Alice to,prove” her identity
to him
Protocol ap1.0,Alice says,I am Alice”
Failure scenario
“I am Alice”
Network Security 7-25
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
declares
herself to be Alice
“I am Alice”
Network Security 7-26
Authentication,another try
Protocol ap2.0,Alice says,I am Alice” in an IP packet
containing her source IP address
Failure scenario
“I am Alice”Alice’s IP address
Network Security 7-27
Authentication,another try
Protocol ap2.0,Alice says,I am Alice” in an IP packet
containing her source IP address
Trudy can create
a packet
“spoofing”
Alice’s address“I am Alice”Alice’s IP address
Network Security 7-28
Authentication,another try
Protocol ap3.0,Alice says,I am Alice” and sends her
secret password to,prove” it.
Failure scenario
“I’m Alice”Alice’s IP addr Alice’s password
OKAlice’s IP addr
Network Security 7-29
Authentication,another try
Protocol ap3.0,Alice says,I am Alice” and sends her
secret password to,prove” it.
playback attack,Trudy
records Alice’s packet
and later
plays it back to Bob
“I’m Alice”Alice’s IP addr Alice’s password
OKAlice’s IP addr
“I’m Alice”Alice’s IP addr Alice’s password
Network Security 7-30
Authentication,yet another try
Protocol ap3.1,Alice says,I am Alice” and sends her
encrypted secret password to,prove” it.
Failure scenario
“I’m Alice”Alice’s IP addr encrypted password
OKAlice’s IP addr
Network Security 7-31
Authentication,another try
Protocol ap3.1,Alice says,I am Alice” and sends her
encrypted secret password to,prove” it.
record
and
playback
still works!
“I’m Alice”Alice’s IP addr encrypptedpassword
OKAlice’s IP addr
“I’m Alice”Alice’s IP addr encryptedpassword
Network Security 7-32
Authentication,yet another try
Goal,avoid playback attack
Failures,drawbacks?
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
K (R)A-B Alice is live,and only Alice knows
key to encrypt
nonce,so it must
be Alice!
Network Security 7-33
Authentication,ap5.0
ap4.0 requires shared symmetric key
? can we authenticate using public key techniques?
ap5.0,use nonce,public key cryptography
“I am Alice”
R
Bob computes
K (R)A-
“send me your public key”
K A+
(K (R)) = RA-K A+
and knows only Alice
could have the private
key,that encrypted R
such that
(K (R)) = RA-K A+
Network Security 7-34
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
TK (R)
-
Send me your public key
TK
+AK (R)-
Send me your public key
AK
+
TK (m)
+
Tm = K (K (m))
+
T
-
Trudy gets
sends m to Alice
ennrypted with
Alice’s public key
AK (m)
+
Am = K (K (m))
+
A
-
R
Network Security 7-35
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!
Network Security 7-36
Chapter 7 roadmap
7.1 What is network security?
7.2 Principles of cryptography
7.3 Authentication
7.4 Message integrity
7.5 Key Distribution and certification
7.6 Access control,firewalls
7.7 Attacks and counter measures
7.8 Security in many layers
Network Security 7-37
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
Network Security 7-38
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 key
encryption
algorithm
Bob’s private
key K B
-
Bob’s message,
m,signed
(encrypted) with
his private key
K B-(m)
Network Security 7-39
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,
-
Network Security 7-40
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,Hash
Function
H(m)
Network Security 7-41
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-one
But given message with given hash value,it is easy to find
another message with same hash value,
I O U 1
0 0, 9
9 B O B
49 4F 55 31
30 30 2E 39
39 42 D2 42
message ASCII format
B2 C1 D2 AC
I O U 9
0 0, 1
9 B O B
49 4F 55 39
30 30 2E 31
39 42 D2 42
message ASCII format
B2 C1 D2 ACdifferent messages
but identical checksums!
Network Security 7-42
large
message
m
H,Hash
function H(m)
digital
signature
(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,Hash
function
H(m)
digital
signature
(decrypt)
H(m)
Bob’s
public
key K
B
+
equal
Digital signature = signed message digest
Network Security 7-43
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
Network Security 7-44
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
Network Security 7-45
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)
Network Security 7-46
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
Network Security 7-47
Key Distribution Center (KDC)
Alice
knows
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
generates
R1
KB-KDC(A,R1)
KA-KDC(A,B)
KA-KDC(R1,KB-KDC(A,R1) )
Network Security 7-48
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
information
digital
signature
(encrypt)
CA
private
key K CA
-
K B+
certificate for
Bob’s public key,
signed by CA
Network Security 7-49
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+
digital
signature
(decrypt)
CA
public
key K CA
+
K B+
Network Security 7-50
A certificate contains:
? Serial number (unique to issuer)
? info about certificate owner,including algorithm
and key value itself (not shown)
? info about
certificate
issuer
? valid dates
? digital
signature by
issuer
Network Security 7-51
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
Network Security 7-52
Firewalls
isolates organization’s internal net from larger
Internet,allowing some packets to pass,
blocking others.
firewall
administered
network
public
Internet
firewall
Network Security 7-53
Firewalls,Why
prevent denial of service attacks:
? SYN flooding,attacker establishes many bogus
TCP connections,no resources left for,real”
connections,
prevent illegal modification/access of internal data.
?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:
? application-level
? packet-filtering
Network Security 7-54
Packet Filtering
? internal network connected to Internet via
router firewall
? router filters packet-by-packet,decision to
forward/drop packet based on:
? source IP address,destination IP address
? TCP/UDP source and destination port numbers
? ICMP message type
? TCP SYN and ACK bits
Should arriving
packet be allowed
in? Departing packet
let out?
Network Security 7-55
Packet Filtering
? Example 1,block incoming and outgoing
datagrams with IP protocol field = 17 and with
either source or dest port = 23.
? All incoming and outgoing UDP flows and telnet
connections are blocked.
? Example 2,Block inbound TCP segments with
ACK=0.
? Prevents external clients from making TCP
connections with internal clients,but allows
internal clients to connect to outside.
Network Security 7-56
Application gateways
? Filters packets on
application data as well
as on IP/TCP/UDP fields.
? Example,allow select
internal users to telnet
outside.
host-to-gateway
telnet session
gateway-to-remote
host telnet session
application
gateway
router and filter
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.
Network Security 7-57
Limitations of firewalls and gateways
? IP spoofing,router
can’t know if data
“really” comes from
claimed source
? if multiple app’s,need
special treatment,each
has own app,gateway.
? client software must
know how to contact
gateway.
? e.g.,must set IP address
of proxy in Web
browser
? filters often use all or
nothing policy for UDP.
? tradeoff,degree of
communication with
outside world,level of
security
? many highly protected
sites still suffer from
attacks.
Network Security 7-58
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
Network Security 7-59
Internet security threats
Mapping:
?before attacking:,case the joint” – find out
what services are implemented on network
? Use ping to determine what hosts have
addresses on network
? Port-scanning,try to establish TCP connection
to each port in sequence (see what happens)
? nmap (http://www.insecure.org/nmap/) mapper,
“network exploration and security auditing”
Countermeasures?
Network Security 7-60
Internet security threats
Mapping,countermeasures
? record traffic entering network
? look for suspicious activity (IP addresses,pots
being scanned sequentially)
Network Security 7-61
Internet security threats
Packet sniffing:
? broadcast media
? promiscuous NIC reads all packets passing by
? can read all unencrypted data (e.g,passwords)
?e.g.,C sniffs B’s packets
A
B
C
src:B dest:A payload
Countermeasures?
Network Security 7-62
Internet security threats
Packet sniffing,countermeasures
? all hosts in orgnization run software that
checks periodically if host interface in
promiscuous mode.
? one host per segment of broadcast media
(switched Ethernet at hub)
A
B
C
src:B dest:A payload
Network Security 7-63
Internet security threats
IP Spoofing:
?can generate,raw” IP packets directly from
application,putting any value into IP source
address field
?receiver can’t tell if source is spoofed
? e.g.,C pretends to be B
A
B
C
src:B dest:A payload
Countermeasures?
Network Security 7-64
Internet security threats
IP Spoofing,ingress filtering
? routers should not forward outgoing packets
with invalid source addresses (e.g.,datagram
source address not in router’s network)
? great,but ingress filtering can not be mandated
for all networks
A
B
C
src:B dest:A payload
Network Security 7-65
Internet security threats
Denial of service (DOS):
?flood of maliciously generated packets,swamp”
receiver
? Distributed DOS (DDOS),multiple coordinated
sources swamp receiver
? e.g.,C and remote host SYN-attack A
A
B
C
SYN
SYNSYNSYN
SYN
SYN
SYN
Countermeasures?
Network Security 7-66
Internet security threats
Denial of service (DOS),countermeasures
? filter out flooded packets (e.g.,SYN) before
reaaching host,throw out good with bad
? traceback to source of floods (most likely an
innocent,compromised machine)
A
B
C
SYN
SYNSYNSYN
SYN
SYN
SYN
Network Security 7-67
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
7.8.1,Secure email
7.8.2,Secure sockets
7.8.3,IPsec
8.8.4,802.11 WEP
Network Security 7-68
Secure e-mail
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.
?Alice wants to send confidential e-mail,m,to Bob.
KS( ).
KB( ).+
+ -
KS(m )
KB(KS )+
m
KS
KS
KB+
Internet
KS( ).
KB( ).-
KB-
KS
mKS(m )
KB(KS )+
Network Security 7-69
Secure e-mail
Bob:
? uses his private key to decrypt and recover KS
? uses KS to decrypt KS(m) to recover m
?Alice wants to send confidential e-mail,m,to Bob.
KS( ).
KB( ).+
+ -
KS(m )
KB(KS )+
m
KS
KS
KB+
Internet
KS( ).
KB( ).-
KB-
KS
mKS(m )
KB(KS )+
Network Security 7-70
Secure e-mail (continued)
? Alice wants to provide sender authentication
message integrity.
? Alice digitally signs message.
? sends both message (in the clear) and digital signature.
H( ),KA( ).-
+ -
H(m )KA(H(m))
-
m
KA-
Internet
m
KA( ).+
KA+
KA(H(m))-
m H( )
,H(m )
compare
Network Security 7-71
Secure e-mail (continued)
? Alice wants to provide secrecy,sender authentication,
message integrity.
Alice uses three keys,her private key,Bob’s public
key,newly created symmetric key
H( ),KA( ).-
+
KA(H(m))-m
KA-
m
KS( ).
KB( ).+
+
KB(KS )+
KS
KB+
Internet
KS
Network Security 7-72
Pretty good privacy (PGP)
? Internet e-mail encryption
scheme,de-facto standard.
? uses symmetric key
cryptography,public key
cryptography,hash
function,and digital
signature as described.
? provides secrecy,sender
authentication,integrity.
? inventor,Phil Zimmerman,
was target of 3-year
federal investigation.
---BEGIN PGP SIGNED MESSAGE---
Hash,SHA1
Bob:My husband is out of town
tonight.Passionately yours,
Alice
---BEGIN PGP SIGNATURE---
Version,PGP 5.0
Charset,noconv
yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJ
hFEvZP9t6n7G6m5Gw2
---END PGP SIGNATURE---
A PGP signed message:
Network Security 7-73
Secure sockets layer (SSL)
? transport layer
security to any TCP-
based app using SSL
services,
? used between Web
browsers,servers for
e-commerce (shttp).
? security services:
? server authentication
? data encryption
? client authentication
(optional)
? server authentication:
? SSL-enabled browser
includes public keys for
trusted CAs.
? Browser requests
server certificate,
issued by trusted CA.
? Browser uses CA’s
public key to extract
server’s public key from
certificate,
? check your browser’s
security menu to see
its trusted CAs.
Network Security 7-74
SSL (continued)
Encrypted SSL session:
? Browser generates
symmetric session key,
encrypts it with server’s
public key,sends
encrypted key to server.
? Using private key,server
decrypts session key.
? Browser,server know
session key
? All data sent into TCP
socket (by client or server)
encrypted with session key.
? SSL,basis of IETF
Transport Layer
Security (TLS).
? SSL can be used for
non-Web applications,
e.g.,IMAP.
? Client authentication
can be done with client
certificates.
Network Security 7-75
IPsec,Network Layer Security
? Network-layer secrecy:
? sending host encrypts the
data in IP datagram
? TCP and UDP segments;
ICMP and SNMP
messages.
? Network-layer authentication
? destination host can
authenticate source IP
address
? Two principle protocols:
? authentication header
(AH) protocol
? encapsulation security
payload (ESP) protocol
? For both AH and ESP,source,
destination handshake:
? create network-layer
logical channel called a
security association (SA)
? Each SA unidirectional.
? Uniquely determined by:
? security protocol (AH or
ESP)
? source IP address
? 32-bit connection ID
Network Security 7-76
Authentication Header (AH) Protocol
? provides source
authentication,data
integrity,no
confidentiality
? AH header inserted
between IP header,
data field.
? protocol field,51
? intermediate routers
process datagrams as
usual
AH header includes:
? connection identifier
? authentication data,
source- signed message
digest calculated over
original IP datagram.
? next header field,
specifies type of data
(e.g.,TCP,UDP,ICMP)
IP header data (e.g.,TCP,UDP segment)AH header
Network Security 7-77
ESP Protocol
? provides secrecy,host
authentication,data
integrity.
? data,ESP trailer
encrypted.
? next header field is in ESP
trailer.
? ESP authentication
field is similar to AH
authentication field.
? Protocol = 50,
IP header TCP/UDP segmentESPheader ESPtrailer ESPauthent.
encrypted
authenticated
Network Security 7-78
IEEE 802.11 security
? War-driving,drive around Bay area,see what 802.11
networks available?
? More than 9000 accessible from public roadways
? 85% use no encryption/authentication
? packet-sniffing and various attacks easy!
? Wired Equivalent Privacy (WEP),authentication as in
protocol ap4.0
? host requests authentication from access point
? access point sends 128 bit nonce
? host encrypts nonce using shared symmetric key
? access point decrypts nonce,authenticates host
Network Security 7-79
IEEE 802.11 security
? Wired Equivalent Privacy (WEP),data encryption
? Host/AP share 40 bit symmetric key (semi-
permanent)
? Host appends 24-bit initialization vector (IV) to
create 64-bit key
? 64 bit key used to generate stream of keys,kiIV
? kiIV used to encrypt ith byte,di,in frame:
ci = di XOR kiIV
? IV and encrypted bytes,ci sent in frame
Network Security 7-80
802.11 WEP encryption
IV
( p e r f r a m e )
K S, 40 - b i t
s e c r e t
s y m m e t r i c
k e y
k 1
IV
k 2
IV
k 3
IV
… k N
IV
k N + 1
IV
… k N + 1
IV
d 1
d 2 d 3 … d N
C R C 1 … C R C 4
c 1
c 2 c 3 … c N
c N + 1 … c N + 4
p l a i n t e x t
f r a m e d a t a
p l u s C R C
k e y s e q u e n c e g e n e r a to r
( f o r g i v e n K S,I V )
8 0 2, 1 1
h e a d e r
IV
W E P - e n c r y p t e d d a ta
p l u s CR C
F ig ur e 7, 8 - ne w 1, 8 0 2, 1 1 W E P p r o to c o l
Sender-side WEP encryption
Network Security 7-81
Breaking 802.11 WEP encryption
Security hole,
? 24-bit IV,one IV per frame,-> IV’s eventually reused
? IV transmitted in plaintext -> IV reuse detected
?Attack:
? Trudy causes Alice to encrypt known plaintext d1 d2
d3 d4 …
? Trudy sees,ci = di XOR kiIV
? Trudy knows ci di,so can compute kiIV
? Trudy knows encrypting key sequence k1IV k2IV k3IV …
? Next time IV is used,Trudy can decrypt!
Network Security 7-82
Network Security (summary)
Basic techniques…...
? cryptography (symmetric and public)
? authentication
? message integrity
? key distribution
…,used in many different security scenarios
? secure email
? secure transport (SSL)
? IP sec
? 802.11 WEP
