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Fundamentals of Network Security

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This presentation was delivered as an invited lecture in the National Conference ETCC 2008, which was organized by the National Institute of Technology (NIT), Hamirpur, India, during December 30 - 31, 2008. It discusses various concepts in security related issues in computer networks. More Info: Keynote lecture delivered in the National Conference ETCC’08 Event Date: Dec 31, 2008 Organization: Department of Computer Science and Engineering, National Institute of Technology (NIT), Hamirpur, Himachal Pradesh, India
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Fundamentals
of
Network Security
8-1
Jaydip Sen
Innovation Lab, TCS, Kolkata, India
ETCC’08, NIT Hamirpur, December 30 - 31, 2008
Network Security
Goals:
Understanding the Principles of Network
Security:
Cryptography and its many uses beyond
“confidentiality”
8-2
“confidentiality”
Authentication
Message Integrity
Security in Practice:
Firewalls
Security in Application, Transport, Network, Link
Layers
Roadmap
1. What is Network Security?
2. Principles of Cryptography
3. Authentication
4.
Integrity
8-3
4.
Integrity
5. Key Distribution and Certification
6. Access Control: Firewalls
7. Attacks and Counter Measures
8. Security in Many Layers
What is Network Security?
Confidentiality: only sender, intended receiver should
“understand” message contents.
sender encrypts the message before sending.
receiver decrypts the message after receiving.
Authentication:
sender, receiver want to confirm identity
8-4
Authentication:
sender, receiver want to confirm identity
of each other.
Message Integrity: sender, receiver want to ensure that
the message is not altered (in transit, or afterwards)
without detection.
Access and Availability: services must be accessible
and available to authorized users.
Friends and Enemies: Alice, Bob, Trudy
Well-known in network security world
Bob, Alice (lovers!) want to communicate “securely”
Olga (intruder) may intercept, delete, add messages
data, control
Alice
Bob
8-5
secure
sender
secure
receiver
channel
data, control
messages
data data
Alice
Bob
Trudy
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
8-6
routers exchanging routing table updates
other examples?
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)
8-7
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 ……
Roadmap
1. What is Network Security?
2. Principles of Cryptography
3. Authentication
4.
Integrity
8-8
4.
Integrity
5. Key Distribution and Certification
6. Access Control: Firewalls
7. Attacks and Counter Measures
8. Security in Many Layers
The Language of Cryptography
plaintext plaintext
ciphertext
KA
encryption
algorithm decryption
algorithm
Alice’s
encryption
key
Bob’s
decryption
key
KB
8-9
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
monoalphabetic cipher: substitute one letter for another
plaintext: abcdefghijklmnopqrstuvwxyz
ciphertext: mnbvcxzasdfghjklpoiuytrewq
8-10
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?
Symmetric Key Cryptography
plaintext
ciphertext
K
A-B
encryption
algorithm
decryption
algorithm
K
A-B
plaintext
message, m
K (m)
A
-
B
K (m)
A
-
B
m = K
(
)
A
-
B
8-11
Symmetric-key Crypto: Bob and Alice share know same
(symmetric) key: K
e.g., in mono alphabetic substitution cipher, the key is
the substitution cipher
Q: how do Bob and Alice agree on key value?
A-B
K (m)
A
-
B
K (m)
A
-
B
m = K
(
)
A
-
B
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
8-12
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
Symmetric Key
Crypto: DES
initial permutation
16 identical “rounds” of
function application,
DES operation
8-13
function application,
each using different
48 bits of key
final permutation
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
8-14
128, 192, or 256 bit keys
brute force decryption (try each key) taking 1
sec on DES, takes 149 trillion years for AES
Public Key Cryptography
Symmetric Key Crypto
requires sender,
receiver know shared
secret key
Q: how to agree on key
Public Key Cryptography
radically different
approach [Diffie-
Hellman76, RSA78]
sender, receiver do
not
8-15
Q: how to agree on key
in first place
(particularly if never
“met”)?
sender, receiver do
not
share secret key
public encryption key
known to all
private decryption key
known only to receiver
Public Key Cryptography
Bob’s public
key
K
B
+
Bob’s private
key
K B
-
8-16
plaintext
message, m
ciphertext
encryption
algorithm decryption
algorithm
plaintext
message
K (m)
B
+
m = K (K (m))
B
+
B
-
Public Key Encryption Algorithms
need K ( ) and K ( ) such that
BB
..
Requirements:
1+-
K (K (m)) = m
B
B
-+
8-17
given public key K , it should be
impossible to compute private
key K
B
B
2
RSA: Rivest, Shamir, Adlleman algorithm
K (K (m)) = m
B
B
+
-
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
8-18
3.
Choose
e
(
with
e<n)
that has no common factors
with z. (e, z are “relatively prime”).
4. Choose dsuch 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
c
= m mod n
e
(i.e., remainder when mis divided by n)
e
8-19
2. To decrypt received bit pattern, c, compute
m = c mod n
d
(i.e., remainder when cis divided by n)
d
m = (m
mod
n)
e
mod
n
d
Magic
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
m
e
c = m mod n
e
8-20
letter
m
m
e
c = m mod n
e
l12 1524832 17
cm = c mod n
d
17 481968572106750915091411825223071697 12
cdletter
l
encrypt:
decrypt:
RSA: Why is that
m = (m
mod
n)
e
mod
n
d
(m
mod
n)
e
mod
n = m
mod
n
ded
Useful number theory result: If
p,q
prime and
n = pq,
then:
x
mod
n = x
mod
n
y y
mod
(p-1)(q-1)
8-21
(m
mod
n)
mod
n = m
mod
n
= m
mod
n
ed
mod
(p-1)(q-1)
= m
mod
n
1
= m
(using number theory result above)
(since we chose
ed
to be divisible by
(p-1)(q-1)
with remainder 1 )
RSA: Another Important Property
The following property will be
very
useful later:
K (K (m))= m
B
B
-+K (K (m))
B
B
+-
=
8-22
use public key
first, followed
by private key
use private key
first, followed
by public key
Result is the same!
Roadmap
1. What is Network Security?
2. Principles of Cryptography
3. Authentication
4.
Integrity
8-23
4.
Integrity
5. Key Distribution and Certification
6. Access Control: Firewalls
7. Attacks and Counter Measures
8. Security in Many Layers
Authentication
Goal: Bob wants Alice to “prove” her identity to
him
Protocol ap1.0: Alice says “I am Alice”
8-24
Failure scenario??
“I am Alice”
Authentication
Goal: Bob wants Alice to “prove” her identity to
him
Protocol ap1.0: Alice says “I am Alice”
8-25
in a network,
Bob can not “see” Alice,
so Trudy simply
declares
herself to be Alice
“I am Alice”
Authentication: Another Try
Protocol ap2.0: Alice says “I am Alice” in an IP packet
containing her source IP address
8-26
Failure scenario??
“I am Alice”
Alice’s
IP address
Authentication: Another Try
Protocol ap2.0: Alice says “I am Alice” in an IP packet
containing her source IP address
8-27
Trudy can create
a packet “spoofing”
Alice’s address
“I am Alice”
Alice’s
IP address
Authentication: Another Try
Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.
Alice’s
Alice’s
8-28
Failure scenario??
“I’m Alice”
Alice’s
IP addr
Alice’s
password
OK
Alice’s
IP addr
Authentication: Another Try
Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.
Alice’s
Alice’s
8-29
playback attack: Trudy
records Alice’s packet
and later
plays it back to Bob
“I’m Alice”
Alice’s
IP addr
Alice’s
password
OK
Alice’s
IP addr
“I’m Alice”
Alice’s
IP addr
Alice’s
password
Authentication: Yet Another Try
Protocol ap3.1: Alice says “I am Alice” and sends her
encrypted secret password to “prove” it.
Alice’s
encrypted
8-30
Failure scenario??
“I’m Alice”
Alice’s
IP addr
encrypted
password
OK
Alice’s
IP addr
Authentication: Another Try
Protocol ap3.1: Alice says “I am Alice” and sends her
encrypted secret password to “prove” it.
Alice’s
encrypted
8-31
record
and
playback
still works!
“I’m Alice”
Alice’s
IP addr
encrypted
password
OK
Alice’s
IP addr
“I’m Alice”
Alice’s
IP addr
encrypted
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
8-32
Failures, drawbacks?
“I am Alice”
R
K (R)
A-B Alice is live, and
only Alice knows
key to encrypt
nonce, so it must
be Alice!
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”
Bob computes
8-33
“I am Alice”
R
Bob computes
K (R)
A
-
“send me your public key”
K
A
+
(K (R)) = R
A
-
K
A
+
and knows only Alice
could have the private
key, that encrypted R
such that
(K (R)) = R
A
-
K
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
T
K (R)
-
Send me your public key
-
R
8-34
Send me your public key
T
K
+
A
K (R)
-
Send me your public key
A
K
+
T
K (m)
+
T
m = K (K (m))
+
T
-
Trudy gets
sends m to Alice
encrypted with
Alice’s public key
A
K (m)
+
A
m = K (K (m))
+
A
-
R
ap5.0: Security Hole
Man (woman) in the middle attack: Trudy poses as
Alice (to Bob) and as Bob (to Alice)
8-35
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!
Roadmap
1. What is Network Security?
2. Principles of Cryptography
3. Authentication
4. Message Integrity
8-36
4. Message Integrity
5. Key Distribution and Certification
6. Access Control: Firewalls
7. Attacks and Counter Measures
8. Security in Many Layers
Digital Signatures
Cryptographic Technique Analogous to Hand-
Written Signatures.
sender (Bob) digitally signs document, establishing
that he is the owner/creator of the document.
8-37
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)-
-
Bob’s message, m Bob’s private
key
K
B
-
K
B
-
(m)
8-38
Dear Alice
Oh, how I have missed
you. I think of you all the
time! …(blah blah blah)
Bob
Public key
encryption
algorithm
key
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 KBto KB(m) then checks KB(KB(m) ) = m.
If KB(KB(m) ) = m, whoever signed m must have used
Bob’s private key.
++
-
-
- -
+
8-39
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
Hash function properties:
large
message
m
H: Hash
Function
H(m)
8-40
to
-
compute digital
“fingerprint”
apply hash function H to
m, get fixed size
message digest, H(m).
many-to-1
produces fixed-size msg
digest (fingerprint)
given message digest x,
computationally infeasible
to find m such that x =
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-one
But given message with given hash value, it is easy to find
8-41
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 AC
different messages
but identical checksums!
large
message
m
H: Hash
function H(m)
digital
Bob’s
Bob sends digitally signed
message:
Alice verifies signature and
integrity of digitally signed
message:
KB(H(m))
-
encrypted
msg digest
large
Digital Signature = Signed Message Digest
8-42
digital
signature
(encrypt)
Bob’s
private
key K
B
-
+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
?
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.
8-43
SHA
-
1 is also used.
US standard [NIST, FIPS PUB 180-1]
160-bit message digest
Roadmap
1. What is Network Security?
2. Principles of Cryptography
3. Authentication
4.
Integrity
8-44
4.
Integrity
5. Key Distribution and Certification
6. Access Control: Firewalls
7. Attacks and Counter Measures
8. Security in Many Layers
Trusted Intermediaries
Symmetric Key Problem:
How do two entities
establish shared secret key
over network?
Solution:
Public Key Problem:
When Alice obtains
Bob’s public key (from
web site, e-mail,
diskette), how does she
8-45
Solution:
trusted key distribution
center (KDC) acting as
intermediary between
entities
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.
8-46
KB-KDC
KX-KDC
KY-KDC
KZ-KDC
KP-KDC
KB-KDC
KA-KDC
KA-KDC
KP-KDC
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
R1
KA-KDC(A,B)
8-47
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
KB-KDC(A,R1)
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
8-48
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
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
8-49
Bob’s
public
key
K
B
+
digital
signature
(decrypt)
CA
public
key K
CA
+
K
B
+
Roadmap
1. What is Network Security?
2. Principles of Cryptography
3. Authentication
4.
Integrity
8-50
4.
Integrity
5. Key Distribution and Certification
6. Access Control: Firewalls
7. Attacks and Counter Measures
8. Security in Many Layers
Firewalls
isolates organization’s secure internal network
(LAN) from insecure public Internet, allowing
some packets to pass, blocking others.
firewall
8-51
administered
network
public
Internet
firewall
Firewalls: Why?
Prevent Denial Of Service (DOS) Attacks:
SYN flooding: attacker establishes many bogus
TCP connections, no resources left for “real”
connections.
Prevent Illegal Modification/Access of Internal Data.
8-52
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
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
8-53
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.
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
filters often use all or
nothing policy for UDP.
tradeoff: degree of
communication with
outside world, level of
security
8-54
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
security
many highly protected
sites still suffer from
attacks.
Roadmap
1. What is Network Security?
2Principles of Cryptography
3. Authentication
4.
Integrity
8-55
4.
Integrity
5. Key Distribution and Certification
6. Access Control: Firewalls
7. Attacks and Counter Measures
8. Security in Many Layers
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
8-56
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?
Internet Security Threats
Mapping: Countermeasures
record traffic entering network
look for suspicious activity (IP addresses, pots
being scanned sequentially)
8-57
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
8-58
A
B
C
src:B dest:A payload
Countermeasures?
Internet Security Threats
Packet Sniffing: Countermeasures
all hosts in organization run software that checks
periodically if host interface in promiscuous mode.
one host per segment of broadcast media (switched
Ethernet at hub)
8-59
A
B
C
src:B dest:A payload
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
8-60
e.g.: C pretends to be B
A
B
C
src:B dest:A payload
Countermeasures?
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
8-61
A
B
C
src:B dest:A payload
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
8-62
A
B
C
SYN
SYNSYNSYN
SYN
SYN
SYN
Countermeasures?
Internet Tecurity Threats
Denial of Service (DOS): Countermeasures
filter out flooded packets (e.g., SYN) before reaching host:
throw out good with bad
traceback to source of floods (most likely an innocent,
compromised machine)
8-63
A
B
C
SYN
SYNSYNSYN
SYN
SYN
SYN
Roadmap
1. What is Network Security?
2. Principles of Cryptography
3. Authentication
4. Integrity
5. Key Distribution and Certification
6.
Access Control: Firewalls
8-64
6.
Access Control: Firewalls
7. Attacks and Counter Measures
8. Security in Many Layers
8.1. Secure email
8.2. Secure Sockets
8.3. IPsec
8.4. Security in 802.11
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
---BEGIN PGP SIGNED MESSAGE---
Hash: SHA1
Bob:My husband is out of town
tonight.Passionately yours,
Alice
A PGP signed message:
8-65
and digital signature as
described.
provides secrecy, sender
authentication, integrity.
inventor, Phil Zimmerman,
was target of 3-year federal
investigation.
Alice
---BEGIN PGP SIGNATURE---
Version: PGP 5.0
Charset: noconv
yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJ
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Secure Sockets Layer (SSL)
transport layer security
to any TCP-based app
using SSL services.
used between Web
browsers, servers for e
-
server authentication:
SSL-enabled browser
includes public keys for
trusted CAs.
Browser requests server
certificate, issued by
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browsers, servers for e
-
commerce (shttp).
security services:
server authentication
data encryption
client authentication
(optional)
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.
SSL (continued)
Encrypted SSL session:
Browser generates
symmetric session key,
encrypts it with server’s
public key, sends
encrypted key to server.
SSL: basis of IETF
Transport Layer
Security (TLS).
SSL can be used for
non-Web applications,
e.g., IMAP.
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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.
e.g., IMAP.
Client authentication
can be done with client
certificates.
IPsec: Network Layer Security
Network-layer secrecy:
sending host encrypts the
data in IP datagram
TCP and UDP segments;
ICMP and SNMP
messages.
For both AH and ESP, source,
destination handshake:
create network-layer logical
channel called a security
association (SA)
Each SA unidirectional.
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Network-layer authentication
destination host can
authenticate source IP
address
Two principle protocols:
authentication header (AH)
protocol
encapsulation security
payload (ESP) protocol
Each SA unidirectional.
Uniquely determined by:
security protocol (AH or
ESP)
source IP address
32-bit connection ID
Authentication Header (AH) Protocol
provides source
authentication, data
integrity, no
confidentiality
AH header inserted
between IP header, data
AH header includes:
connection identifier
authentication data:
source- signed message
digest calculated over
original IP datagram.
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between IP header, data
field.
protocol field: 51
intermediate routers
process datagrams as
usual
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
ESP Protocol
provides secrecy, host
authentication, data
integrity.
data, ESP trailer encrypted.
next header field is in ESP
ESP authentication field
is similar to AH
authentication field.
Protocol = 50.
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next header field is in ESP
trailer.
IP header TCP/UDP segment
ESP
header
ESP
trailer
ESP
authent.
encrypted
authenticated
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!
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packet
-
sniffing and various attacks easy!
Securing 802.11
encryption, authentication
first attempt at 802.11 security: Wired Equivalent
Privacy (WEP): a failure
current attempt: 802.11i
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
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access point decrypts nonce, authenticates host
no key distribution mechanism
authentication: knowing the shared key is enough
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, ki
IV
k
IV
used to encrypt ith byte, d
, in frame:
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k
i
IV
used to encrypt ith byte, d
i
, in frame:
ci= diXOR ki
IV
IV and encrypted bytes, ci sent in frame
802.11 WEP Encryption
IV
(per frame)
K
S
: 40-
bit
secret
symmetric
key
k
1
IV
k
2
IV
k
3
IV
… k
N
IV
k
N+1
IV
… k
N+1
IV
p
laintext
key sequence generator
( for given K
S
, IV)
802.11
header
IV
WEP-encrypted data
plus CRC
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d
1
d
2
d
3
… d
N
CRC
1
CRC
4
c
1
c
2
c
3
… c
N
c
N+1
c
N+4
p
laintext
frame
data
plus CRC
Figure 7.8-new1: 802.11 WEP protocol
Sender-side WEP encryption
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 d
d
d
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Trudy causes Alice to encrypt known plaintext d
1
d
2
d
3
d4
Trudy sees: ci= diXOR ki
IV
Trudy knows cidi, so can compute ki
IV
Trudy knows encrypting key sequence k1
IV k2
IV k3
IV
Next time IV is used, Trudy can decrypt!
802.11i: Improved Security
numerous (stronger) forms of encryption
possible
provides key distribution
uses authentication server separate from
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uses authentication server separate from
access point
AP: access point AS:
Authentication
server
wired
network
STA:
client station
1 Discovery of
security capabilities
802.11i: Four Phases of Operation
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3
STA and AS mutually authenticate, together
generate Master Key (MK). AP servers as “pass through”
2
3STA derives
Pairwise Master
Key (PMK)
AS derives
same PMK,
sends to AP
4STA, AP use PMK to derive
Temporal Key (TK) used for message
encryption, integrity
EAP: Extensible Authentication Protocol
EAP: end-end client (mobile) to authentication
server protocol
EAP sent over separate “links”
mobile-to-AP (EAP over LAN)
AP to authentication server (RADIUS over UDP)
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wired
network
EAP TLS
EAP
EAP over LAN (EAPoL)
IEEE 802.11
RADIUS
UDP/IP
AP to authentication server (RADIUS over UDP)
Network Security (summary)
Basic techniques…...
cryptography (symmetric and public)
authentication
message integrity
key distribution
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key distribution
…. used in many different security scenarios
secure email
secure transport (SSL)
IP sec
802.11
Thank You!
8-80
Questions?
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