8-1Network Security
Chapter 8Network Security
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Thanks and enjoy! JFK/KWR
All material copyright 1996-2010J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking: A Top Down Approach ,5th edition. Jim Kurose, Keith RossAddison-Wesley, April 2009.
8-2Network Security
Chapter 8: Network Security
Chapter goals: understand principles of network security:
cryptography and its many uses beyond “confidentiality”
authentication message integrity
security in practice: firewalls and intrusion detection systems security in application, transport, network, link
layers
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Chapter 8 roadmap
8.1 What is network security?8.2 Principles of cryptography8.3 Message integrity8.4 Securing e-mail8.5 Securing TCP connections: SSL8.6 Network layer security: IPsec8.7 Securing wireless LANs8.8 Operational security: firewalls and IDS
8-4Network Security
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
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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
securesender
securereceiver
channel data, control messages
data data
Alice Bob
Trudy
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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?
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There are bad guys (and girls) out there!Q: What can a “bad guy” do?A: A lot! See section 1.6
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)
8-8Network Security
Chapter 8 roadmap
8.1 What is network security?8.2 Principles of cryptography8.3 Message integrity8.4 Securing e-mail8.5 Securing TCP connections: SSL8.6 Network layer security: IPsec8.7 Securing wireless LANs8.8 Operational security: firewalls and IDS
8-9Network Security
The language of cryptography
m plaintext messageKA(m) ciphertext, encrypted with key KA
m = KB(KA(m))
plaintext plaintextciphertext
KA
encryptionalgorithm
decryption algorithm
Alice’s encryptionkey
Bob’s decryptionkey
KB
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Simple encryption schemesubstitution cipher: substituting one thing for another
monoalphabetic cipher: substitute one letter for another
plaintext: abcdefghijklmnopqrstuvwxyz
ciphertext: mnbvcxzasdfghjklpoiuytrewq
Plaintext: bob. i love you. aliceciphertext: nkn. s gktc wky. mgsbc
E.g.:
Key: the mapping from the set of 26 letters to the set of 26 letters
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Polyalphabetic encryption n monoalphabetic ciphers, M1,M2,…,Mn
Cycling pattern: e.g., n=4, M1,M3,M4,M3,M2; M1,M3,M4,M3,M2;
For each new plaintext symbol, use subsequent monoalphabetic pattern in cyclic pattern dog: d from M1, o from M3, g from M4
Key: the n ciphers and the cyclic pattern
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Breaking an encryption scheme Cipher-text only
attack: Trudy has ciphertext that she can analyze
Two approaches: Search through all
keys: must be able to differentiate resulting plaintext from gibberish
Statistical analysis
Known-plaintext attack: Trudy has some plaintext corresponding to some ciphertext e.g., in monoalphabetic
cipher, Trudy determines pairings for a,l,i,c,e,b,o,
Chosen-plaintext attack: Trudy can get the ciphertext for some chosen plaintext
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Types of Cryptography
Crypto often uses keys: Algorithm is known to everyone Only “keys” are secret
Public key cryptography Involves the use of two keys
Symmetric key cryptography Involves the use one key
Hash functions Involves the use of no keys Nothing secret: How can this be useful?
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Symmetric key cryptography
symmetric key crypto: Bob and Alice share 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
K S
encryptionalgorithm
decryption algorithm
S
K S
plaintextmessage, m
K (m)S
m = KS(KS(m))
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Two types of symmetric ciphers
Stream ciphers encrypt one bit at time
Block ciphers Break plaintext message in equal-size
blocks Encrypt each block as a unit
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Stream Ciphers
Combine each bit of keystream with bit of plaintext to get bit of ciphertext
m(i) = ith bit of message ks(i) = ith bit of keystream c(i) = ith bit of ciphertext c(i) = ks(i) m(i) ( = exclusive or) m(i) = ks(i) c(i)
keystreamgeneratorkey keystream
pseudo random
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RC4 Stream Cipher
RC4 is a popular stream cipher Extensively analyzed and considered good Key can be from 1 to 256 bytes Used in WEP for 802.11 Can be used in SSL
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Block ciphers
Message to be encrypted is processed in blocks of k bits (e.g., 64-bit blocks).
1-to-1 mapping is used to map k-bit block of plaintext to k-bit block of ciphertext
Example with k=3:
input output000 110001 111010 101011 100
input output100 011101 010110 000111 001
What is the ciphertext for 010110001111 ?
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Prototype function64-bit input
S1
8bits
8 bits
S2
8bits
8 bits
S3
8bits
8 bits
S4
8bits
8 bits
S7
8bits
8 bits
S6
8bits
8 bits
S5
8bits
8 bits
S8
8bits
8 bits
64-bit intermediate
64-bit output
Loop for n rounds
8-bit to8-bitmapping
From Kaufmanet al
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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
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Public key cryptography
plaintextmessage, m
ciphertextencryptionalgorithm
decryption algorithm
Bob’s public key
plaintextmessageK (m)
B+
K B+
Bob’s privatekey
K B-
m = K (K (m))B+
B-
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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)) = m BB
- +
+
-
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Prerequisite: modular arithmetic
x mod n = remainder of x when divide by n
Facts:[(a mod n) + (b mod n)] mod n = (a+b) mod n[(a mod n) - (b mod n)] mod n = (a-b) mod n[(a mod n) * (b mod n)] mod n = (a*b) mod n
Thus (a mod n)d mod n = ad mod n Example: x=14, n=10, d=2:
(x mod n)d mod n = 42 mod 10 = 6xd = 142 = 196 xd mod 10 = 6
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RSA: getting ready
A message is a bit pattern. A bit pattern can be uniquely represented by
an integer number. Thus encrypting a message is equivalent to
encrypting a number.Example m= 10010001 . This message is uniquely
represented by the decimal number 145. To encrypt m, we encrypt the corresponding
number, which gives a new number (the ciphertext).
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RSA: Creating public/private key pair
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).
K B+ K B
-
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RSA: Encryption, decryption
0. Given (n,e) and (n,d) as computed above
1. To encrypt message m (<n), compute
c = m mod n
e
2. To decrypt received bit pattern, c, compute
m = c mod n
d
m = (m mod n)
e mod n
dMagichappens!
c
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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).
bit pattern m me c = m mod ne
00001100 12 248832 17
c m = c mod nd
17 481968572106750915091411825223071697 12
cd
encrypt:
decrypt:
Encrypting 8-bit messages.
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Why does RSA work?
Must show that cd mod n = m where c = me mod n
Fact: for any x and y: xy mod n = x(y mod z) mod n where n= pq and z = (p-1)(q-1)
Thus, cd mod n = (me mod n)d mod n
= med mod n = m(ed mod z) mod n = m1 mod n = m
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RSA: another important property
The following property will be very useful later:
K (K (m)) = m BB
- +K (K (m))
BB+ -
=
use public key first, followed
by private key
use private key first,
followed by public key
Result is the same!
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Follows directly from modular arithmetic:
(me mod n)d mod n = med mod n = mde mod n = (md mod n)e mod n
K (K (m)) = m BB
- +K (K (m))
BB+ -
=Why ?
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Why is RSA Secure? suppose you know Bob’s public key
(n,e). How hard is it to determine d? essentially need to find factors of n
without knowing the two factors p and q. fact: factoring a big number is hard.
Generating RSA keys have to find big primes p and q approach: make good guess then apply
testing rules (see Kaufman)
8-32Network Security
Chapter 8 roadmap
8.1 What is network security?8.2 Principles of cryptography8.3 Message integrity8.4 Securing e-mail8.5 Securing TCP connections: SSL8.6 Network layer security: IPsec8.7 Securing wireless LANs8.8 Operational security: firewalls and IDS
8-33Network Security
Message Integrity allows communicating parties to verify
that received messages are authentic. Content of message has not been altered Source of message is who/what you think it
is Message has not been replayed Sequence of messages is maintained
let’s first talk about message digests
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Message Digests
function H( ) that takes as input an arbitrary length message and outputs a fixed-length string: “message signature”
note that H( ) is a many-to-1 function
H( ) is often called a “hash function”
desirable properties: easy to calculate irreversibility: Can’t
determine m from H(m) collision resistance:
computationally difficult to produce m and m’ such that H(m) = H(m’)
seemingly random output
large message
m
H: HashFunction
H(m)
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Internet checksum: poor message digest
Internet checksum has some properties of hash function: produces fixed length digest (16-bit sum) of input is many-to-one
but given message with given hash value, it is easy to find another message with same hash value. e.g.,: simplified checksum: add 4-byte chunks at a time:
I O U 10 0 . 99 B O B
49 4F 55 3130 30 2E 3939 42 D2 42
message ASCII format
B2 C1 D2 AC
I O U 90 0 . 19 B O B
49 4F 55 3930 30 2E 3139 42 D2 42
message ASCII format
B2 C1 D2 ACdifferent messagesbut identical checksums!
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Hash Function Algorithms
MD5 hash function widely used (RFC 1321) computes 128-bit message digest in 4-step
process. SHA-1 is also used.
US standard [NIST, FIPS PUB 180-1]
160-bit message digest
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Message Authentication Code (MAC)
mess
ag
e
H( )
s
mess
ag
e
mess
ag
e
s
H( )
compare
s = shared secret
Authenticates sender Verifies message integrity No encryption ! Also called “keyed hash” Notation: MDm = H(s||m) ; send m||MDm
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HMAC
popular MAC standard addresses some subtle security flaws operation:
concatenates secret to front of message. hashes concatenated message concatenates secret to front of digest hashes combination again
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Example: OSPF
Recall that OSPF is an intra-AS routing protocol
Each router creates map of entire AS (or area) and runs shortest path algorithm over map.
Router receives link-state advertisements (LSAs) from all other routers in AS.
Attacks: Message insertion Message deletion Message
modification
How do we know if an OSPF message is authentic?
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OSPF Authentication
within an Autonomous System, routers send OSPF messages to each other.
OSPF provides authentication choices no authentication shared password:
inserted in clear in 64-bit authentication field in OSPF packet
cryptographic hash
cryptographic hash with MD5 64-bit authentication
field includes 32-bit sequence number
MD5 is run over a concatenation of the OSPF packet and shared secret key
MD5 hash then appended to OSPF packet; encapsulated in IP datagram
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End-point authentication
want to be sure of the originator of the message – end-point authentication
assuming Alice and Bob have a shared secret, will MAC provide end-point authentication? we do know that Alice created message. … but did she send it?
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MACTransfer $1Mfrom Bill to Trudy
MACTransfer $1M fromBill to Trudy
Playback attack
MAC =f(msg,s)
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“I am Alice”
R
MACTransfer $1M from Bill to Susan
MAC =f(msg,s,R)
Defending against playback attack: nonce
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Digital Signatures
cryptographic technique analogous to hand-written signatures.
sender (Bob) digitally signs document, establishing he is document owner/creator.
goal is similar to that of MAC, except now use public-key cryptography
verifiable, nonforgeable: recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document
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Digital Signatures
simple digital signature for message m: Bob signs m by encrypting with his private
key KB, creating “signed” message, KB(m)--
Dear Alice
Oh, how I have missed you. I think of you all the time! …(blah blah blah)
Bob
Bob’s message, m
Public keyencryptionalgorithm
Bob’s privatekey
K B-
Bob’s message, m, signed
(encrypted) with his private key
K B-(m)
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Public-key certification
motivation: Trudy plays pizza prank on Bob Trudy creates e-mail order:
Dear Pizza Store, Please deliver to me four pepperoni pizzas. Thank you, Bob
Trudy signs order with her private key Trudy sends order to Pizza Store Trudy sends to Pizza Store her public key, but
says it’s Bob’s public key. Pizza Store verifies signature; then delivers
four pizzas to Bob. Bob doesn’t even like Pepperoni
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Certification Authorities
Certification authority (CA): binds public key to particular entity, E.
E (person, router) registers its public key with CA. E provides “proof of identity” to CA. CA creates certificate binding E to its public key. certificate containing E’s public key digitally signed by
CA – CA says “this is E’s public key”Bob’s public
key K B+
Bob’s identifying informatio
n
digitalsignature(encrypt)
CA private
key K CA-
K B+
certificate for Bob’s public
key, signed by CA
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Certification Authorities when Alice wants Bob’s public key:
gets Bob’s certificate (Bob or elsewhere). apply CA’s public key to Bob’s certificate,
get Bob’s public key
Bob’s public
key K B+
digitalsignature(decrypt)
CA public
key K CA+
K B+
8-49Network Security
Chapter 8 roadmap
8.1 What is network security?8.2 Principles of cryptography8.3 Message integrity8.4 Securing e-mail8.5 Securing TCP connections: SSL8.6 Network layer security: IPsec8.7 Securing wireless LANs8.8 Operational security: firewalls and IDS
8-50Network Security
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 )+
8-51Network Security
Chapter 8 roadmap
8.1 What is network security?8.2 Principles of cryptography8.3 Message integrity8.4 Securing e-mail8.5 Securing TCP connections: SSL8.6 Network layer security: IPsec8.7 Securing wireless LANs8.8 Operational security: firewalls and IDS
8-52Network Security
SSL: Secure Sockets Layerwidely deployed security
protocol supported by almost all
browsers, web servers https billions $/year over SSL
original design: Netscape, 1993
variation -TLS: transport layer security, RFC 2246
provides confidentiality integrity authentication
original goals: Web e-commerce
transactions encryption (especially
credit-card numbers) Web-server
authentication optional client
authentication minimum hassle in doing
business with new merchant
available to all TCP applications secure socket interface
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SSL and TCP/IP
Application
TCP
IP
Normal Application
Application
SSL
TCP
IP
Application with SSL
• SSL provides application programming interface (API)to applications• C and Java SSL libraries/classes readily available
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Could do something like PGP:
but want to send byte streams & interactive datawant set of secret keys for entire connectionwant certificate exchange as part of protocol:
handshake phase
H( ). KA( ).-
+
KA(H(m))-
m
KA-
m
KS( ).
KB( ).+
+
KB(KS )+
KS
KB+
Internet
KS
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Toy SSL: a simple secure channel
handshake: Alice and Bob use their certificates, private keys to authenticate each other and exchange shared secret
key derivation: Alice and Bob use shared secret to derive set of keys
data transfer: data to be transferred is broken up into series of records
connection closure: special messages to securely close connection
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Toy: A simple handshake
MS = master secret EMS = encrypted master secret
hello
certificate
KB+(MS) = EMS
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Real SSL: Handshake (1)
Purpose1. server authentication2. negotiation: agree on crypto
algorithms3. establish keys4. client authentication (optional)
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Virtual Private Networks (VPNs)
institutions often want private networks for security. costly: separate routers, links, DNS
infrastructure. VPN: institution’s inter-office traffic is sent
over public Internet instead encrypted before entering public Internet logically separate from other traffic
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IPheader
IPsecheader
Securepayload
IPhe
ader
IPse
che
ader
Sec
ure
payl
oad
IP
header
IPsec
header
Secure
payload
IPhe
ader
payl
oad
IPheader
payload
headquartersbranch office
salespersonin hotel
PublicInternet
laptop w/ IPsec
Router w/IPv4 and IPsec
Router w/IPv4 and IPsec
Virtual Private Network (VPN)
8-60Network Security
Chapter 8 roadmap
8.1 What is network security?8.2 Principles of cryptography8.3 Message integrity8.4 Securing e-mail8.5 Securing TCP connections: SSL8.6 Network layer security: IPsec8.7 Securing wireless LANs8.8 Operational security: firewalls and IDS
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WEP Design Goals
symmetric key crypto confidentiality end host authorization data integrity
self-synchronizing: each packet separately encrypted given encrypted packet and key, can decrypt; can
continue to decrypt packets when preceding packet was lost (unlike Cipher Block Chaining (CBC) in block ciphers)
efficient can be implemented in hardware or software
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Review: Symmetric Stream Ciphers
combine each byte of keystream with byte of plaintext to get ciphertext m(i) = ith unit of message ks(i) = ith unit of keystream c(i) = ith unit of ciphertext c(i) = ks(i) m(i) ( = exclusive or) m(i) = ks(i) c(i)
WEP uses RC4
keystreamgeneratorkey keystream
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WEP Authentication
APauthentication request
nonce (128 bytes)
nonce encrypted shared key
success if decrypted value equals nonce
Not all APs do it, even if WEPis being used. AP indicates if authentication is necessary in beacon frame. Done before association.
8-64Network Security
Chapter 8 roadmap
8.1 What is network security?8.2 Principles of cryptography8.3 Message integrity8.4 Securing e-mail8.5 Securing TCP connections: SSL8.6 Network layer security: IPsec8.7 Securing wireless LANs8.8 Operational security: firewalls and IDS
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Firewalls
isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others
firewall
administerednetwork
publicInternet
firewall
8-66Network Security
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 elseallow only authorized access to inside network (set of
authenticated users/hosts)three types of firewalls:
stateless packet filters stateful packet filters application gateways