network security
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Network Security. CPE 401 / 601 Computer Network Systems. slides are modified from Dave Hollinger. slides are modified from Jim Kurose, Keith Ross. Chapter goals: understand principles of network security: cryptography and its many uses beyond “confidentiality” authentication - PowerPoint PPT PresentationTRANSCRIPT
Network Security
CPE 401 / 601
Computer Network Systems
slides are modified from Dave Hollingerslides are modified from Jim Kurose, Keith Ross
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
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
CPE 401/601
Lecture 17: Network Security
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by Peter Steiner, New York, July 5, 1993
Early Hacking – Phreaking In1957, a blind seven-year old, Joe Engressia
Joybubbles, discovered a whistling tone that resets trunk lines Blow into receiver – free phone calls
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Cap’n Crunch cereal prizeGiveaway whistle produces 2600 MHz tone
The Seventies John Draper
a.k.a. Captain Crunch “If I do what I do, it is onlyto explore a system”
In 1971, built Bluebox
Pranksters, free calls Mark Bernay and Al Bernay Steve Jobs and Steve Wozniak
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The Eighties Robert Morris worm - 1988
Developed to measure the size of the Internet• However, a computer could be infected multiple times
Brought down a large fraction of the Internet • ~ 6K computers
Academic interest in network security
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The Nineties Kevin Mitnick
First hacker on FBI’s Most Wanted list Hacked into many networks
• including FBI Stole intellectual property
• including 20K credit card numbers In 1995, caught 2nd time
• served five years in prison
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Security Trends
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9www.cert.org (Computer Emergency Readiness Team)
Top Security Threats
10Computing Technology Industry Association, 2009 survey
Changes on the technology landscape affecting security
11
Concern for Security Explosive growth of desktops started in ‘80s
No emphasis on security• Who wants military security, I just want to run my spreadsheet!
Internet was originally designed for a group of mutually trusting users By definition, no need for security Users can send a packet to any other user Identity (source IP address) taken by default to be true
Explosive growth of Internet in mid ’90s Security was not a priority until recently
• Only a research network, who will attack it?
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Concern for Security Explosive growth of desktops started in ‘80s
No emphasis on security• Who wants military security, I just want to run my spreadsheet!
Internet was originally designed for a group of mutually trusting users By definition, no need for security Users can send a packet to any other user Identity (source IP address) taken by default to be true
Explosive growth of Internet in mid ’90s Security was not a priority until recently
• Only a research network, who will attack it?
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Lecture 17: Network Security
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Friends and enemies: Alice, Bob, Trudy well-known in network security world Bob, Alice want to communicate “securely” Trudy (intruder) may intercept, delete, add
messages
securesender
securereceiver
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 DNS servers 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) 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)
Alice’s Online Bank Alice opens Alice’s Online Bank (AOB) What are Alice’s security concerns? If Bob is a customer of AOB, what are his
security concerns? How are Alice and Bob concerns similar? How
are they different? How does Trudy view the situation?
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Alice’s Online Bank
AOB must prevent Trudy from learning Bob’s balance Confidentiality (prevent unauthorized reading of information)
Trudy must not be able to change Bob’s balance Bob must not be able to improperly change his
own account balance Integrity (prevent unauthorized writing of information)
AOB’s info must be available when needed Availability (data is available in a timely manner when needed
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Alice’s Online Bank How does Bob’s computer know that “Bob” is
really Bob and not Trudy? When Bob logs into AOB, how does AOB
know that “Bob” is really Bob? Authentication (assurance that other party is the claimed one)
Bob can’t view someone else’s account info Bob can’t install new software, etc.
Authorization (allowing access only to permitted resources)
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Think Like Trudy Good guys must think like bad guys! A police detective
Must study and understand criminals In network security
We must try to think like Trudy We must study Trudy’s methods We can admire Trudy’s cleverness Often, we can’t help but laugh at Alice and Bob’s
carelessness But, we cannot act like Trudy
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Aspects of Security Security Services
Enhance the security of data processing systems and information transfers of an organization.
Counter security attacks. Security Attack
Action that compromises the security of information owned by an organization.
Security Mechanisms Designed to prevent, detect or recover from a
security attack.
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Security Services Enhance security of data processing systems and
information transfers
Authentication Assurance that the communicating entity is the
one claimed
Authorization Prevention of the unauthorized use of a resource
Availability Data is available in a timely manner when needed
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Security Services Confidentiality
Protection of data from unauthorized disclosure
Integrity Assurance that data received is as sent by an
authorized entity
Non-Repudiation Protection against denial by one of the parties in
a communication
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Security Attacks
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Informationsource
Informationdestination
Normal Flow
Security Attacks
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Informationsource
Informationdestination
Interruption
Attack on availability(ability to use desired information or
resources)
Denial of Service
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Internet
PerpetratorVictim
ICMP echo (spoofed source address of victim) Sent to IP broadcast address
ICMP echo reply
ICMP = Internet Control Message Protocol
Innocentreflector sites
Smurf Attack
1 SYN
10,000 SYN/ACKs – Victim is dead
Security Attacks
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Informationsource
Informationdestination
Interception
Attack on confidentiality(concealment of information)
Packet Sniffing
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Packet Sniffer
Client
Server
Network Interface Card allows only packets for this MAC address
Every network interface card has a unique 48-bit Media Access Control (MAC) address, e.g. 00:0D:84:F6:3A:10 24 bits assigned by IEEE; 24 by card vendor
Packet sniffer sets his card to promiscuous mode to allow all packets
Security Attacks
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Informationsource
Informationdestination
Fabrication
Attack on authenticity(identification and assurance of origin of information)
IP Address Spoofing IP addresses are filled in by the originating
host Using source address for authentication
r-utilities (rlogin, rsh, rhosts etc..)
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• Can A claim it is B to the server S?
• ARP Spoofing
• Can C claim it is B to the server S?
• Source Routing
InternetInternet
2.1.1.1 C
1.1.1.1 1.1.1.2A B
1.1.1.3 S
Security Attacks
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Informationsource
Informationdestination
Modification
Attack on integrity(prevention of unauthorized changes)
TCP Session Hijack When is a TCP packet valid?
Address / Port / Sequence Number in window
How to get sequence number? Sniff traffic Guess it
• Many earlier systems had predictable Initial Sequence Number
Inject arbitrary data to the connection
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Security Attacks
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Message interception
Trafficanalysis
eavesdropping, monitoring transmissions
Passive attacks
Masquerade Denial ofservice
some modification of the data stream
Active attacks
Replay Modification of message contents
Model for Network Security
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Security Mechanism Feature designed to
Prevent attackers from violating security policy Detect attackers’ violation of security policy Recover, continue to function correctly even if
attack succeeds.
No single mechanism that will support all services Authentication, authorization, availability,
confidentiality, integrity, non-repudiation
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What is network security about ? It is about secure communication
Everything is connected by the Internet
There are eavesdroppers that can listen on the communication channels
Information is forwarded through packet switches which can be reprogrammed to listen to or modify data in transit
Tradeoff between security and performance
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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
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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
39
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
40
Polyalphabetic encryption n monoalphabetic cyphers, 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
41
Breaking an encryption scheme Cipher-text only
attack: Trudy has ciphertext that she can analyze 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 eg, in monoalphabetic
cipher, trudy determines pairings for a,l,i,c,e,b,o,
Chosen-plaintext attack: trudy can get the cyphertext for some chosen plaintext
Cryptanalytic Attacks rely on:
nature of the algorithm plus some knowledge of the
general characteristics of the plaintext
even some sample plaintext-ciphertext pairs
exploits the characteristics of the algorithm to attempt to deduce a specific plaintext or the key being used if successful all future and past
messages encrypted with that key are compromised
Brute-Force Attack try all possible keys
on some ciphertext until an intelligible translation into plaintext is obtained on average half of all
possible keys must be tried to achieve success
Attacking Symmetric Encryption
43
Types of Cryptography
Crypto often uses keys: Algorithm is known to everyone Only “keys” are secret
Symmetric key cryptography Involves the use one key
Public key cryptography Involves the use of two keys
Hash functions Involves the use of no keys Nothing secret: How can this be useful?
44
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))
Symmetric Encryption
universal technique for providing confidentiality
also referred to as single-key encryption
two requirements for secure use: need a strong encryption algorithm sender and receiver must have obtained
copies of the secret key in a secure fashion• and must keep the key secure
46
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
47
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) = k.s(i) m(i) ( = exclusive or) m(i) = k.s(i) c(i)
keystreamgeneratorkey keystream
pseudo random
48
RC4 Stream Cipher
RC4 is a popular stream cipher Extensively analyzed and considered good Key can be from 1 to 256 bytes
Block & Stream Ciphers
•processes the input one block of elements at a time
•produces an output block for each input block•can reuse keys•more common
Block Block CipherCipherBlock Block CipherCipher
•processes the input elements continuously•produces output one element at a time•primary advantage is that they are almost
always faster and use far less code•encrypts plaintext one byte at a time•pseudorandom stream is one that is
unpredictable without knowledge of the input key
Stream Stream CipherCipherStream Stream CipherCipher
50
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 ?
51
Block ciphers
How many possible mappings are there for k=3? How many 3-bit inputs? How many permutations of the 3-bit inputs? Answer: 40,320 ; not very many!
In general, 2k! mappings; huge for k=64 Problem:
Table approach requires table with 264 entries, each entry with 64 bits
Table too big: instead use function that simulates a randomly permuted table
52
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
53
Why rounds in prototpe?
If only a single round, then one bit of input affects at most 8 bits of output.
In 2nd round, the 8 affected bits get scattered and inputted into multiple substitution boxes.
How many rounds? How many times do you need to shuffle cards Becomes less efficient as n increases
54
Encrypting a large message
Why not just break message in 64-bit blocks, encrypt each block separately? If same block of plaintext appears twice, will
give same cyphertext.
How about: Generate random 64-bit number r(i) for each
plaintext block m(i) Calculate c(i) = KS( m(i) r(i) ) Transmit c(i), r(i), i=1,2,… At receiver: m(i) = KS(c(i)) r(i) Problem: inefficient, need to send c(i) and r(i)
55
Cipher Block Chaining (CBC)
CBC generates its own random numbers Have encryption of current block depend on result of
previous block c(i) = KS( m(i) c(i-1) )
m(i) = KS( c(i)) c(i-1)
How do we encrypt first block? Initialization vector (IV): random block = c(0) IV does not have to be secret
Change IV for each message (or session) Guarantees that even if the same message is sent
repeatedly, the ciphertext will be completely different each time
Cipher Block Chaining cipher block: if
input block repeated, will produce same cipher text:
t=1m(1) = “HTTP/1.1” block
cipherc(1) = “k329aM02”
…
cipher block chaining: XOR ith input block, m(i), with previous block of cipher text, c(i-1) c(0) transmitted to receiver
in clear what happens in “HTTP/1.1”
scenario from above?
+
m(i)
c(i)
t=17m(17) = “HTTP/1.1”block
cipherc(17) = “k329aM02”
blockcipher
c(i-1)
57
Symmetric key crypto: DES
DES: Data Encryption Standard US encryption standard [NIST 1993] 56-bit symmetric key, 64-bit plaintext input Block cipher with cipher block chaining How secure is DES?
DES Challenge: 56-bit-key-encrypted phrase decrypted (brute force) in less than a day
No known good analytic attack making DES more secure:
3DES: encrypt 3 times with 3 different keys(actually encrypt, decrypt, encrypt)
58
Symmetric key crypto: DES
initial permutation 16 identical “rounds” of
function application, each using different 48 bits of key
final permutation
DES operation
Advanced Encryption Standard (AES)
needed a replacement
for 3DES
3DES was not reasonable for long
term use
NIST called for proposals for a
new AES in 1997
should have a security strength equal to or better
than 3DES
significantly improved efficiency
symmetric block cipher
128 bit data and 128/192/256 bit keys
selected Rijndael in November
2001
published as FIPS 197
60
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
Time to Break a Code
assuming 106 decryptions/µs
Symmetric Encryption Algorithms
Average Time Required for Exhaustive Key Search
64
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
Public-Key Encryption Structure
publicly publicly proposed proposed by Diffie by Diffie
and and Hellman in Hellman in
19761976
publicly publicly proposed proposed by Diffie by Diffie
and and Hellman in Hellman in
19761976
based on based on mathematimathemati
cal cal functionsfunctions
based on based on mathematimathemati
cal cal functionsfunctions
asymmetricasymmetric• uses two uses two
separate separate keyskeys
• public key public key and private and private keykey
• public key is public key is made public made public for others to for others to useuse
asymmetricasymmetric• uses two uses two
separate separate keyskeys
• public key public key and private and private keykey
• public key is public key is made public made public for others to for others to useuse
some form some form of protocol of protocol is needed is needed
for for distributiodistributio
nn
some form some form of protocol of protocol is needed is needed
for for distributiodistributio
nn
66
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-
67
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
69
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 cyphertext).
70
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
0000l000 12 24832 17
c m = c mod nd
17 481968572106750915091411825223071697 12
cd
encrypt:
decrypt:
Encrypting 8-bit messages.
More Efficient RSA
Modular exponentiation example 520 = 95367431640625 = 25 mod 35
A better way: repeated squaring Note that 20 = 2 10, 10 = 2 5, 5 = 2 2 +
1, 2 = 1 2 51= 5 mod 35 52= (51) 2 = 52 = 25 mod 35 55= (52) 2 51 = 252 5 = 3125 = 10 mod 35 510 = (55) 2 = 102 = 100 = 30 mod 35 520 = (510) 2 = 302 = 900 = 25 mod 35
No huge numbers and it’s efficient!CS
450/650 Lecture 9: RSA
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Does RSA Really Work?
Given c = Me mod N we must show M = cd mod N = Med mod N
We’ll use Euler’s Theorem If x is relatively prime to N then x(N) mod N
=1 (n): number of positive integers less than n that
are relatively prime to n.• If p is prime then, (p) = p-1
74CS 450/650 Lecture 9: RSA
76
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!
77
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 ?
Public-Key Encryption
Confidentiality
Private-Key Encryption
Authentication
Requirements for Public-Key Crypto.
81
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)
Asymmetric Encryption Algorithms
RSA RSA (Rivest, (Rivest, Shamir, Shamir,
Adleman)Adleman)
RSA RSA (Rivest, (Rivest, Shamir, Shamir,
Adleman)Adleman)
developed in 1977
most adopted approach to public-key encryption
block cipher in which the
plaintext and ciphertext are
between 0 and n-1
Diffie-Diffie-Hellman Hellman
key key exchange exchange algorithmalgorithm
Diffie-Diffie-Hellman Hellman
key key exchange exchange algorithmalgorithm
enables two users to
securely reach agreement
about a shared secret
limited to the exchange of
the keys
Digital Digital Signature Signature Standard Standard
(DSS)(DSS)
Digital Digital Signature Signature Standard Standard
(DSS)(DSS)
provides only a digital
signature function with
SHA-1
cannot be used for encryption
or key exchange
Elliptic curve
cryptography (ECC)
Elliptic curve
cryptography (ECC)
security like RSA, but with much smaller
keys
Applications for Public-Key Cryptosystems
Symmetric vs AsymmetricSecret Key (Symmetric) Public Key
(Asymmetric)
Number of keys
1 per communication pair 2 per user
Protection of key
Must be kept secret One key must be kept secret; the other can be freely exposed
Best uses Cryptographic workhorse; secrecy and integrity of data single characters to blocks of data, messages, files
Key exchange, authentication
Key distribution
Must be out-of-band Public key can be used to distribute other keys
Speed Fast Slow; typically, 1000 times slower than secret key
85
Session keys
Exponentiation is computationally intensive
3DES is at least 100 times faster than RSA
Session key, KS
Bob and Alice use RSA to exchange a symmetric key KS
Once both have KS, they use symmetric key cryptography
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
87
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
88
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)
89
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.
Example: 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!
90
Hash Function Algorithms
MD5 hash function widely used (RFC 1321) computes 128-bit message digest in 4-step process MD6 computes 512-bit digest
SHA-1 is also used US standard [NIST, FIPS PUB 180-1]
160-bit message digest• Cryptanalysis attack announced in 2005
SHA-2 has 224, 256, 384, 512 bit digests• Chosen in 2001
SHA-3 has arbitrary digest size• Chosen in 2012
SHAOutput
size (bits)
Internal state size
(bits)
Block
size (bits
)
Max message size (bits)
Word
size (bits
)
Rounds
Operations
Collisions found
SHA-0
160 160 512 264 − 1 32 80+, and, or,
xor, rotYes
SHA-1
160 160 512 264 − 1 32 80+, and, or,
xor, rotNone
(251 attack)
SHA-2
256/224
256 512 264 − 1 32 64+, and, or, xor, shr, rot
None
512/384
512 1024 2128 − 1 64 80+, and, or, xor, shr, rot
None
91
92
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
93
HMAC
Popular MAC standard Addresses some subtle security flaws
1. Concatenates secret to front of message. 2. Hashes concatenated message3. Concatenates the secret to front of digest4. Hashes the combination again.
Message Authentication Using a One-Way Hash Function
95
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?
96
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
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 the message. But did she send it?
97
MACTransfer $1Mfrom Bill to Trudy
MACTransfer $1M fromBill to Trudy
Playback attack
MAC =f(msg,s)
“I am Alice”
R
MACTransfer $1M from Bill to Susan
MAC =f(msg,s,R)
Defending against playback attack: nonce
100
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 a 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
Digital Signature Properties
101
Unforgeable: Only the signer can produce his/her signature
Authentic: A signature is produced only by the signer deliberately signing the document
Digital Signature Properties
Non-Alterable: A signed document cannot be altered without invalidating the signature
Non-Reusable: A signature from one document cannot be moved to another document
Signatures can be validated by other users the signer cannot reasonably claim that he/she did
not sign a document bearing his/her signature
102
103
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)
104
large message
mH: Hashfunction H(m)
digitalsignature(encrypt)
Bob’s private
key K B-
+
Bob sends digitally signed message:
Alice verifies signature and integrity of digitally signed message:
KB(H(m))-
encrypted msg digest
KB(H(m))-
encrypted msg digest
large message
m
H: Hashfunction
H(m)
digitalsignature(decrypt)
H(m)
Bob’s public
key K B+
equal ?
Digital signature = signed message digest
105
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. -
106
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
107
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
108
Certification Authorities When Alice wants Bob’s public key:
gets Bob’s certificate (from 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+
Public-Key Certificates
CS 450/650 Lecture 12: Key
Exchange
109
Public Key Certificates
111
Certificates: summary
Primary standard X.509 (RFC 2459) Certificate contains:
Issuer name Entity name, address, domain name, etc. Entity’s public key Digital signature
• signed with issuer’s private key
Public-Key Infrastructure (PKI) Certificates and certification authorities Often considered “heavy”
X.509 Certificates
Public Key Infrastructure X.509
(PKIX)
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
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 )+
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 )+
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))-
mH( ). H(m )
compare
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
119
Could do something like PGP:
•Want certificate exchange part of protocol: handshake phase
H( ). KA( ).-
+
KA(H(m))-
m
KA-
m
KS( ).
KB( ).+
+
KB(KS )+
KS
KB+
Internet
KS
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
121
SSL: Secure Sockets Layer
Widely deployed security protocol Supported by almost all
browsers and web servers https Tens of billions $ spent per
year over SSL Originally designed by
Netscape in 1993 Number of variations:
TLS: transport layer security, RFC 2246
Available to all TCP applications Secure socket interface
Original goals: Had Web e-commerce
transactions in mind Encryption (especially
credit-card numbers) Web-server authentication Optional client
authentication Minimum hassle in doing
business with new merchant
Provides Confidentiality Integrity Authentication
122
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
123
Toy SSL: a simple secure channel
Handshake: Alice and Bob use their certificates and 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 a series of records
Connection Closure: Special messages to securely close connection
124
Toy: A simple handshake
MS = master secret EMS = encrypted master secret
hello
certificate
KB+(MS) = EMS
125
Toy: Key derivation
Considered bad to use same key for more than one cryptographic operation Use different keys for message authentication code
(MAC) and encryption
Four keys: Kc = encryption key for data sent from client to server
Mc = MAC key for data sent from client to server
Ks = encryption key for data sent from server to client
Ms = MAC key for data sent from server to client
Keys derived from key derivation function (KDF) Takes master secret and (possibly) some additional
random data and creates the keys
126
Toy: Data Records Why not encrypt data in constant stream as we
write it to TCP? Where would we put the MAC? If at end, no message
integrity until all data processed. For example, with instant messaging, how can we do
integrity check over all bytes sent before displaying?
Instead, break stream in series of records Each record carries a MAC Receiver can act on each record as it arrives
Issue: in record, receiver needs to distinguish MAC from data Want to use variable-length records
length data MAC
127
Toy: Sequence Numbers
Attacker can capture and replay record or re-order records
Solution: put sequence number into MAC: MAC = MAC(Mx, sequence||data) Note: no sequence number field
Attacker could still replay all of the records Use random nonce
128
Toy: Control information
Truncation attack: attacker forges TCP connection close segment One or both sides thinks there is less data
than there actually is. Solution: record types, with one type for
closure type 0 for data; type 1 for closure
MAC = MAC(Mx, sequence||type||data)
length type data MAC
129
Toy SSL: summary
hello
certificate, nonce
KB+(MS) = EMS
type 0, seq 1, datatype 0, seq 2, data
type 0, seq 1, data
type 0, seq 3, data
type 1, seq 4, close
type 1, seq 2, close
en
cryp
ted
bob.com
130
Toy SSL isn’t complete
How long are the fields?
What encryption protocols?
No negotiation Allow client and server to support different
encryption algorithms Allow client and server to choose together
specific algorithm before data transfer
131
SSL Cipher Suite
Cipher Suite Public-key algorithm Symmetric encryption algorithm MAC algorithm
SSL supports a variety of cipher suites
Negotiation: client and server must agree on cipher suite
Client offers choice; server picks one
132
Real SSL: Handshake (1)
Purpose1. Server authentication2. Negotiation: agree on crypto
algorithms3. Establish keys4. Client authentication (optional)
133
Real SSL: Handshake (2)
1. Client sends list of algorithms it supports, along with client nonce
2. Server chooses algorithms from list; sends back: choice + certificate + server nonce
3. Client verifies certificate, extracts server’s public key, generates pre_master_secret, encrypts with server’s public key, sends to server
4. Client and server independently compute encryption and MAC keys from pre_master_secret and nonces
5. Client sends a MAC of all the handshake messages6. Server sends a MAC of all the handshake messages
134
Real SSL: Handshaking (3)
Last 2 steps protect handshake from tampering
Client typically offers range of algorithms, some strong, some weak
Man-in-the middle could delete the stronger algorithms from list
Last 2 steps prevent this Last two messages are encrypted
135
handshake: ClientHello
handshake: ServerHello
handshake: Certificate
handshake: ServerHelloDone
handshake: ClientKeyExchangeChangeCipherSpec
handshake: Finished
ChangeCipherSpec
handshake: Finished
application_data
application_data
Alert: warning, close_notify
Real Connection
TCP Fin follow
Everythinghenceforthis encrypted
136
Real SSL: Handshaking (4)
Why the two random nonces? Suppose Trudy sniffs all messages
between Alice & Bob. Next day, Trudy sets up TCP connection
with Bob, sends the exact same sequence of records. Bob (Amazon) thinks Alice made two separate
orders for the same thing. Solution: Bob sends different random nonce
for each connection. This causes encryption keys to be different on the two days.
Trudy’s messages will fail Bob’s integrity check.
137
SSL Record Protocol
data
data fragment
data fragment
MAC MAC
encrypteddata and MAC
encrypteddata and MAC
recordheader
recordheader
record header: content type; version; length
MAC: includes sequence number, MAC key Mx
Fragment: each SSL fragment 224 bytes (~16 Kbytes)
138
SSL Record Format
contenttype
SSL version length
MAC
data
1 byte 2 bytes 3 bytes
Data and MAC encrypted (symmetric algo)
SSL Record Protocol Operation
140
Key derivation
Client nonce, server nonce, and pre-master secret input into pseudo random-number generator. Produces master secret
Master secret and new nonces inputed into another random-number generator: “key block”
Key block sliced and diced: client MAC key server MAC key client encryption key server encryption key client initialization vector (IV) server initialization vector (IV)
Website protocol support
141
Protocolversion
Websitesupport
Security
SSL 2.0 14.0% (−0.4%) Insecure
SSL 3.0 45.5% (−1.8%)Insecure
(POODLE Bites)
TLS 1.0 99.7% (±0.0%)Depends on cipher
and client mitigations
TLS 1.1 53.0% (+1.5%)Depends on cipher
and client mitigations
TLS 1.2 56.0% (+1.5%)Depends on cipher
and client mitigations
wikipedia
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
143
What is confidentiality at the network-layer?
Between two network entities: Sending entity encrypts the payloads of
datagrams. Payload could be: TCP segment, UDP segment, ICMP message,
OSPF message, and so on. All data sent from one entity to the
other would be hidden: Web pages, e-mail, P2P file transfers, TCP
SYN packets, and so on. That is, “blanket coverage”.
144
Virtual Private Networks (VPNs)
Institutions often want private networks for security. Costly! Separate routers, links, DNS
infrastructure.
With a VPN, institution’s inter-office traffic is sent over public Internet instead. But inter-office traffic is encrypted before
entering public Internet
145
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)
146
IPsec services
Data integrity Origin authentication Replay attack prevention Confidentiality
Two protocols providing different service models: AH ESP
147
IPsec Transport Mode
IPsec datagram emitted and received by end-system.
Protects upper level protocols
IPsec IPsec
148
IPsec – tunneling mode (1)
End routers are IPsec aware. Hosts need not be.
IPsec IPsec
149
IPsec – tunneling mode (2)
Also tunneling mode.
IPsecIPsec
Two protocols
Authentication Header (AH) protocol provides source authentication & data
integrity but not confidentiality
Encapsulation Security Protocol (ESP) provides source authentication, data
integrity, and confidentiality more widely used than AH
150
151
Four combinations are possible!
Transport mode with AH
Transport mode with ESP
Tunnel modewith AH
Tunnel modewith ESP
Most common andmost important
152
Security associations (SAs) Before sending data, a virtual connection is
established from sending entity to receiving entity.
Called “security association (SA)” SAs are simplex: for only one direction
Both sending and receiving entities maintain state information about the SA Recall that TCP endpoints also maintain state
information. IP is connectionless; IPsec is connection-oriented!
153
193.68.2.23200.168.1.100
172.16.1/24172.16.2/24
SA
InternetHeadquartersBranch Office
R1R2
Example SA from R1 to R2
R1 stores for SA 32-bit identifier for SA: Security Parameter Index (SPI) the origin interface of the SA (200.168.1.100) destination interface of the SA (193.68.2.23) type of encryption to be used (for example, 3DES with
CBC) encryption key type of integrity check (for example, HMAC with with MD5) authentication key
154
Security Association Database (SAD) Endpoint holds state of its SAs in a SAD, where
it can locate them during processing.
When sending IPsec datagram, R1 accesses SAD to determine how to
process datagram.
When IPsec datagram arrives to R2, R2 examines SPI in IPsec datagram, indexes SAD with SPI, and processes datagram accordingly.
Focus for now on tunnel mode with ESP
155
IPsec datagram
new IPheader
ESPhdr
originalIP hdr
Original IPdatagram payload
ESPtrl
ESPauth
encrypted
“enchilada” authenticated
paddingpad
lengthnext
headerSPISeq
#
156
What happens?
193.68.2.23200.168.1.100
172.16.1/24172.16.2/24
SA
InternetHeadquartersBranch Office
R1R2
new IPheader
ESPhdr
originalIP hdr
Original IPdatagram payload
ESPtrl
ESPauth
encrypted
“enchilada” authenticated
paddingpad
lengthnext
headerSPISeq
#
157
R1 converts original datagraminto IPsec datagram
Appends to back of original datagram (which includes original header fields!) an “ESP trailer” field.
Encrypts result using algorithm & key specified by SA. Appends to front of this encrypted quantity the “ESP
header, creating “enchilada”. Creates authentication MAC over the whole enchilada,
using algorithm and key specified in SA; Appends MAC to back of enchilada, forming payload; Creates brand new IP header, with all the classic IPv4
header fields, which it appends before payload.
158
Inside the enchilada:
ESP trailer: Padding for block ciphers ESP header:
SPI, so receiving entity knows what to do Sequence number, to thwart replay attacks
MAC in ESP auth field is created with shared secret key
new IPheader
ESPhdr
originalIP hdr
Original IPdatagram payload
ESPtrl
ESPauth
encrypted
“enchilada” authenticated
paddingpad
lengthnext
headerSPISeq
#
159
IPsec sequence numbers
For new SA, sender initializes seq. # to 0 Each time datagram is sent on SA:
Sender increments seq # counter Places value in seq # field
Goal: Prevent attacker from sniffing and replaying a packet
• Receipt of duplicate, authenticated IP packets may disrupt service
Method: Destination checks for duplicates But doesn’t keep track of ALL received packets;
instead uses a window
160
Security Policy Database (SPD)
Policy: For a given datagram, sending entity needs to know if it should use IPsec.
Needs also to know which SA to use May use: source and destination IP address;
protocol number. Info in SPD indicates “what” to do with
arriving datagram; Info in the SAD indicates “how” to do it.
Summary: IPsec services
Suppose Trudy sits somewhere between R1 and R2. She doesn’t know the keys. Will Trudy be able to see contents of
original datagram? How about source, dest IP address,
transport protocol, application port? Flip bits without detection? Masquerade as R1 using R1’s IP address? Replay a datagram?
161
162
Internet Key Exchange
In previous examples, we manually established IPsec SAs in IPsec endpoints:
Example SASPI: 12345Source IP: 200.168.1.100Dest IP: 193.68.2.23 Protocol: ESPEncryption algorithm: 3DES-cbcHMAC algorithm: MD5Encryption key: 0x7aeaca…HMAC key:0xc0291f…
Such manually keying is impractical for large VPN with, say, hundreds of sales people.
Instead use IPsec IKE (Internet Key Exchange)
163
IKE: PSK and PKI
Authentication (proof who you are) with either pre-shared secret (PSK) or with PKI (pubic/private keys and certificates).
With PSK, both sides start with secret: then run IKE to authenticate each other and to
generate IPsec SAs (one in each direction), including encryption and authentication keys
With PKI, both sides start with public/private key pair and certificate. run IKE to authenticate each other and obtain
IPsec SAs (one in each direction). Similar with handshake in SSL.
165
Summary of IPsec
IKE message exchange for algorithms, secret keys, SPI numbers
Either the AH or the ESP protocol (or both) The AH protocol provides integrity and source
authentication The ESP protocol additionally provides
encryption IPsec peers can be two end systems, two
routers/firewalls, or a router/firewall and an end system
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
Wireless Security Overview
concerns for wireless security are similar to those found in a wired environment
security requirements are the same: confidentiality, integrity, availability,
authenticity, accountability most significant source of risk is the underlying
communications medium
Wireless LAN Security
Wired Equivalent Privacy (WEP) algorithm 802.11 privacy
Wi-Fi Protected Access (WPA) set of security mechanisms that eliminates most
802.11 security issues based on the current state of the 802.11i
standard Robust Security Network (RSN)
final form of the 802.11i standard Wi-Fi Alliance certifies vendors in compliance
with the full 802.11i specification under WPA2
170
WEP Design Goals
Symmetric key crypto Confidentiality Station 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
171
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
172
Stream cipher and packet independence Recall design goal: each packet separately
encrypted If for frame n+1, use keystream from where
we left off for frame n, then each frame is not separately encrypted Need to know where we left off for packet n
WEP approach: initialize keystream with key + new IV for each packet:
keystreamgeneratorKey+IVpacket keystreampacket
173
WEP encryption (1) Sender calculates Integrity Check Value (ICV) over data
four-byte hash/CRC for data integrity Each side has 104-bit shared key Sender creates 24-bit initialization vector (IV), appends
to key: gives 128-bit key Sender also appends keyID (in 8-bit field) 128-bit key inputted into pseudo random number
generator to get keystream data in frame + ICV is encrypted with RC4:
Bytes of keystream are XORed with bytes of data & ICV IV & keyID are appended to encrypted data to create
payload Payload inserted into 802.11 frame
encrypted
data ICVIV
MAC payload
KeyID
174
WEP encryption (2)
IV (per frame)
KS: 104-bit secret
symmetric key k1
IV k2IV k3
IV … kNIV kN+1
IV… kN+1IV
d1 d2 d3 … dN
CRC1 … CRC4
c1 c2 c3 … cN
cN+1 … cN+4
plaintext frame data
plus CRC
key sequence generator ( for given KS, IV)
802.11 header IV
&
WEP-encrypted data plus ICV
Figure 7.8-new1: 802.11 WEP protocol New IV for each frame
175
WEP decryption overview
Receiver extracts IV Inputs IV and shared secret key into pseudo
random generator, gets keystream XORs keystream with encrypted data to
decrypt data + ICV Verifies integrity of data with ICV
Note that message integrity approach used here is different from the MAC (message authentication code) and signatures (using PKI).
encrypted
data ICVIV
MAC payload
KeyID
176
End-point authentication w/ nonce
Nonce: number (R) used only once –in-a-lifetime
How: 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!
177
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.
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 k2
IV k3IV …
Next time IV is used, Trudy can decrypt!
Attack on Integrity Check Value 1. Hacker intercepts WEP-encrypted packets2. Hacker flips bits in intercepted packet and
recalculates ICV 3. Hacker sends modified frame to AP with known
IV4. AP calculate ICV: ICV correct so AP accepts frame5. AP forwards frame6. L3 device decrypt frame, reject it and sends
predictable response7. AP encrypts response and sends it8. Hacker uses response (known plaintext) to
derive key from stream cipher.179
Attack on RC4
over all possible RC4 keys, the statistics for the first few bytes of output keystream are strongly non-random leaking information about the key
If the long-term key and nonce are concatenated to generate RC4 key, the key can be discovered by analysing a large number of encrypted messages 24-bit IV is not long enough to prevent
repetition on a busy network• there is a 50% probability the same IV will repeat
after 5000 packets
180
Wi-Fi Protected Access (WPA)
Flaws in WEP known since January 2001 flaws include weak encryption, static encryption
keys, lack of key distribution method.
In April 2003, the Wi-Fi Alliance introduced an interoperable security protocol known as WiFi Protected Access (WPA). WPA was designed to be a replacement for WEP
networks without requiring hardware replacements.
WPA provides stronger data encryption (weak in WEP) and user authentication (largely missing in WEP)
WPA Security Enhancements
WPA includes Temporal Key Integrity Protocol (TKIP) and 802.1x mechanisms The combination of these two mechanisms provides
dynamic key encryption and mutual authentication
TKIP adds the following strengths to WEP: Per-packet key construction and distribution: WPA automatically generates a new unique encryption
key periodically for each client• This avoids the same key staying in use for weeks or months as
they do with WEP
Message integrity code: guard against forgery attacks 48-bit initialization vectors, use one-way hash function
instead of XOR
802.11i: improved security
numerous (stronger) forms of encryption possible
provides key distribution
uses authentication server separate from access point
AP: access point AS:Authentication server
wirednetwork
STA:client station
1 Discovery ofsecurity capabilities
3
STA and AS mutually authenticate, togethergenerate Master Key (MK). AP servers as “pass through”
2
3 STA derives Pairwise Master Key (PMK)
AS derivessame PMK, sends to AP
STA, AP use PMK to derive Temporal Key (TK) used for message encryption, integrity
4
802.11i: four phases of operation
Elements of IEEE 802.11i
IEEE 802.11i Phases of Operation
IEEE802.11i
Phasesof Operation
802.1X Access Control
MPDU Exchange
authentication phase consists of three phases: connect to AS
• the STA sends a request to its AP that it has an association with for connection to the AS;
• the AP acknowledges this request and sends an access request to the AS
EAP exchange• authenticates the STA and AS to each other
secure key delivery• once authentication is established, the AS generates
a master session key and sends it to the STA
IEEE 802.11i
Key Hierarchies
Phases of Operation
Pseudorandom Function
wirednetwork
EAP TLSEAP
EAP over LAN (EAPoL)
IEEE 802.11
RADIUS
UDP/IP
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)
Robust Security Network via 802.1X
Robust Security Network via 802.1X
Robust Security Network via 802.1X PMK – Pairwise Master Key
Sent from the AS to the Authenticator Both the Supplicant and Authenticator now
have the same PMK PMK is permanent for the entire session
• Must generate a Pairwise Transient Key for encryption of data.
– Done using 4-way handshake
Robust Security Network via 802.1X 4-Way Handshake
Confirm that the client holds the PMK Confirm that the PMK is correct and up-to-date Create pairwise transient key (PTK) from the
PMK Install the pairwise encryption and integrity
keys into IEEE 802.11 Transport the group temporal key (GTK) and
GTK sequence number from Authenticator to Supplicant and install the GTK and GTK sequence number in the STA and, if not already installed, in the AP
Confirm the cipher suite selection
Robust Security Network via 802.1X
Robust Security Network via 802.1X PTK (Pairwise Transient Key – 64 bytes)
16 bytes of EAPOL-Key Confirmation Key (KCK)– Used to compute MIC on WPA EAPOL Key message
16 bytes of EAPOL-Key Encryption Key (KEK) - AP uses this key to encrypt additional data sent (in the 'Key Data' field) to the client (for example, the RSN IE or the GTK)
16 bytes of Temporal Key (TK) – Used to encrypt/decrypt Unicast data packets
8 bytes of Michael MIC Authenticator Tx Key – Used to compute MIC on unicast data packets transmitted by the AP
8 bytes of Michael MIC Authenticator Rx Key – Used to compute MIC on unicast data packets transmitted by the station
Last two only used when TKIP is used.
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
Firewalls
isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others.
firewall
administerednetwork
publicInternet
firewall
Firewalls: Why
prevent denial of service attacks: SYN flooding: attacker establishes many bogus TCP
connections, no resources left for “real” connectionsprevent illegal modification/access of internal data.
access to sensitive dataallow only authorized access to inside network
set of authenticated users/hoststhree types of firewalls:
stateless packet filters stateful packet filters application gateways
Stateless 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?
Stateless packet filtering: example
example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23. all incoming, 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.• no servers!
Policy Firewall Setting
No outside Web access. Drop all outgoing packets to any IP address, port 80
No incoming TCP connections, except those for institution’s public Web server only.
Drop all incoming TCP SYN packets to any IP except 130.207.244.203, port 80
Prevent Web-radios from eating up the available bandwidth.
Drop all incoming UDP packets - except DNS and router broadcasts.
Prevent your network from being used for a smurf DoS attack.
Drop all ICMP packets going to a “broadcast” address
(eg 130.207.255.255).
Prevent your network from being tracerouted
Drop all outgoing ICMP TTL expired traffic
Stateless packet filtering: more examples
actionsourceaddress
destaddress
protocolsource
portdestport
flagbit
allow222.22/1
6outside of222.22/16
TCP > 1023 80any
allowoutside
of222.22/1
6
222.22/16TCP 80 > 1023 ACK
allow222.22/1
6outside of222.22/16
UDP > 1023 53 ---
allowoutside
of222.22/1
6
222.22/16UDP 53 > 1023 ----
deny all all all all all all
Access Control Lists ACL: table of rules, applied top to bottom to incoming
packets: (action, condition) pairs
Stateful packet filtering stateless packet filter: heavy handed tool
admits packets that “make no sense,” • e.g., dest port = 80, ACK bit set, even though no TCP
connection established:
actionsource
addressdest
addressprotocol
sourceport
destport
flagbit
allow outside of222.22/16
222.22/16TCP 80 > 1023 ACK
stateful packet filter: track status of every TCP connection track connection setup (SYN), teardown (FIN)
can determine whether incoming, outgoing packets “makes sense” timeout inactive connections at firewall:
no longer admit packets
actionsourceaddress
destaddress
protosource
portdestport
flagbit
check conxion
allow 222.22/16outside of222.22/16
TCP > 1023 80any
allow outside of222.22/16
222.22/16TCP 80 > 1023 ACK √
allow 222.22/16outside of222.22/16
UDP > 1023 53 ---
allow outside of222.22/16
222.22/16UDP 53 > 1023 ---- √
deny all all all all all all
Stateful packet filtering
ACL augmented to indicate need to check connection state table before admitting packet
Circuit-Level Gateway
circuit level proxy sets up two TCP connections, one between itself and
a TCP user on an inner host and one on an outside host
relays TCP segments from one connection to the other without examining contents
security function consists of determining which connections will be allowed
typically used when inside users are trusted may use application-level gateway inbound
and circuit-level gateway outbound lower overheads
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-gatewaytelnet session
gateway-to-remote host telnet session
applicationgateway
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.
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.
Distributed Distributed Firewall Firewall
ConfiguratiConfigurationon
Intrusion detection systems
packet filtering: operates on TCP/IP headers only no correlation check among sessions
IDS: Intrusion Detection System deep packet inspection: look at packet
contents• e.g., check character strings in packet against
database of known virus, attack strings examine correlation among multiple packets
• port scanning• network mapping• DoS attack
Intrusion Detection Systems
host-based IDS monitors the characteristics of a single host for
suspicious activity network-based IDS
monitors network traffic and analyzes network, transport, and application protocols to identify suspicious activity
comprises three logical components: sensors - collect data analyzers - determine if intrusion has occurred user interface - view output or control system
behavior
Webserver
FTPserver
DNSserver
applicationgateway
Internet
demilitarized zone
internalnetwork
firewall
IDS sensors
Intrusion detection systems
multiple IDSs: different types of checking at different locations
NISD Sensor Deployment Example
IDS Principles
assume intruder behavior differs from legitimate users
overlap in behaviors causes problems false positives false negatives
Intrusion Detection Techniques signature detection
at application, transport, network layers; unexpected application services, policy violations
anomaly detection denial of service attacks, scanning, worms
when a sensor detects a potential violation it sends an alert and logs event related info used by analysis module to refine intrusion detection
parameters and algorithms security administration can use this information to
design prevention techniques
Unified Threat Management
Honeypot decoy systems designed to:
lure a potential attacker away from critical systems collect information about the attacker’s activity encourage the attacker to stay on the system long enough for
administrators to respond filled with fabricated information that a legitimate user of the
system wouldn’t access resource that has no production value
incoming communication is most likely a probe, scan, or attack
outbound communication suggests that the system has probably been compromised
once hackers are within the network, administrators can observe their behavior to figure out defenses
Honeypot Deployment
Network Security (summary)
Basic techniques…... cryptography (symmetric and public) message integrity end-point authentication
…. used in many different security scenarios secure email secure transport (SSL) IP sec 802.11
Operational Security: firewalls and IDS