Chapter 8: Network Security

1
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

2
Chapter 8 roadmap

• 8.1 What is network security?
• 8.2 Principles of cryptography
• 8.3 Message integrity
• 8.4 Securing e-mail
• 8.5 Securing TCP connections SSL
• 8.6 Network layer security IPsec
• 8.7 Securing wireless LANs
• 8.8 Operational security firewalls and IDS

3
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

Alice
Bob
data, control messages
channel
secure sender
secure receiver
data
data
Trudy
4
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)

5
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

6
Chapter 8 roadmap

• 8.1 What is network security?
• 8.2 Principles of cryptography
• 8.3 Message integrity
• 8.4 Securing e-mail
• 8.5 Securing TCP connections SSL
• 8.6 Network layer security IPsec
• 8.7 Securing wireless LANs
• 8.8 Operational security firewalls and IDS

7
The language of cryptography
Alices encryption key
Bobs decryption key
encryption algorithm
decryption algorithm
ciphertext
plaintext
plaintext

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

8
Symmetric key cryptography

• substitution cipher substituting one thing for
  another
• monoalphabetic cipher substitute one letter for
  another

plaintext abcdefghijklmnopqrstuvwxyz
ciphertext mnbvcxzasdfghjklpoiuytrewq
E.g.
Plaintext bob. i love you. alice
ciphertext nkn. s gktc wky. mgsbc
Key the mapping from the set of 26 letters to
the set of 26 letters

• Q How hard to break this simple cipher?
• brute force (how hard?) or other?

9
Polyalphabetic encryption

• n monoalphabetic cyphers, M1,M2,,Mn
• Cycling pattern
• e.g., n5, 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

10
Symmetric key cryptography
encryption algorithm
decryption algorithm
ciphertext
plaintext
plaintext message, m
K (m)
A-B

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

A-B
11
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

12
Stream Ciphers
pseudo random
keystream generator
key
keystream

• 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)

13
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

14
Block ciphers

• How many possible mappings are there for k3?
• 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 k64
• 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

15
Prototype function
From Kaufman et al
8-bit to 8-bit mapping
16
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

17
Symmetric key crypto DES

• initial permutation
• 16 identical rounds of function application,
  each using different 48 bits of key
• final permutation

18
Symmetric key crypto DES

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

19
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

20
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

21
Public key cryptography

Bobs public key
K
B
-
Bobs private key
K
B
encryption algorithm
decryption algorithm
plaintext message
plaintext message, m
ciphertext
22
Public key encryption algorithms
Requirements
.
.

-

• need K ( ) and K ( ) such that

B
B

given public key K , it should be impossible to
compute private key K
B
-
B
RSA Rivest, Shamir, Adleman algorithm
23
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 eltn) 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).
24
RSA Encryption, decryption
0. Given (n,e) and (n,d) as computed above
2. To decrypt received bit pattern, c, compute
d
(i.e., remainder when c is divided by n)
Magic happens!
c
25
RSA example
Bob chooses p5, q7. Then n35, z24.
e5 (so e, z relatively prime). d29 (so ed-1
exactly divisible by z.
e
m
m
letter
encrypt
l
12
1524832
17
c
letter
decrypt
17
12
l
481968572106750915091411825223071697
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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 (ab) mod n
• (a mod n) - (b mod n) mod n (a-b) mod n
• (a mod n) (b mod n) mod n (ab) mod n
• Thus
• (a mod n)d mod n ad mod n
• Example x14, n10, d2(x mod n)d mod n 42
  mod 10 6xd 142 196 xd mod 10 6

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RSA Why is that
Useful number theory result If p,q prime and n
pq, then
(using number theory result above)
(since we chose ed to be divisible by (p-1)(q-1)
with remainder 1 )
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RSA another important property
The following property will be very useful later
use public key first, followed by private key
use private key first, followed by public key
Result is the same!
29
Why is RSA Secure?
Why
?

• Follows directly from modular arithmetic
• (me mod n)d mod n med mod n mde mod n
• (md mod n)e mod n

• Suppose you know Bobs 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.

30
Still Need Secret Session keys

• Exponentiation is computationally intensive
• DES 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

31
Chapter 8 roadmap

• 8.1 What is network security?
• 8.2 Principles of cryptography
• 8.3 Message integrity
• 8.4 Securing e-mail
• 8.5 Securing TCP connections SSL
• 8.6 Network layer security IPsec
• 8.7 Securing wireless LANs
• 8.8 Operational security firewalls and IDS

32
Message Integrity

• Bob receives msg from Alice, wants to ensure
• message originally came from Alice
• message not changed since sent by Alice
• Cryptographic Hash
• takes input m, produces fixed length value, H(m)
• e.g., as in Internet checksum
• computationally infeasible to find two different
  messages, x, y such that H(x) H(y)
• equivalently given m H(x), (x unknown), can
  not determine x.
• note Internet checksum fails this requirement!

33
Internet checksum poor crypto hash function

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

But given message with given hash value, it is
easy to find another message with same hash
value
message
ASCII format
message
ASCII format
I O U 9 0 0 . 1 9 B O B
49 4F 55 39 30 30 2E 31 39 42 4F 42
I O U 1 0 0 . 9 9 B O B
49 4F 55 31 30 30 2E 39 39 42 4F 42
B2 C1 D2 AC
B2 C1 D2 AC
different messages but identical checksums!
34
Message Digests
large message m
H Hash Function

• Computationally expensive to public-key-encrypt
  long messages
• Goal fixed-length, easy- to-compute digital
  fingerprint
• apply hash function H to m, get fixed size
  message digest, H(m).

H(m)

• Hash function properties
• many-to-1
• But given message digest x H(m), its
  infeasible to find m that H(m) H(m)
• Data integrity cannot replace m with m

35
Message Authentication Code
(shared secret)
s
(message)
s
(shared secret)
36
HMAC

• Popular MAC standard
• Addresses some subtle security flaws
• Concatenates secret to front of message.
• Hashes concatenated message
• Concatenates the secret to front of digest
• Hashes the combination again.

37
MACs in practice

• MD5 hash function widely used (RFC 1321)
• computes 128-bit MAC in 4-step process.
• arbitrary 128-bit string x, appears difficult to
  construct msg m whose MD5 hash is equal to x
• recent (2005) attacks on MD5
• SHA-1 is also used
• US standard NIST, FIPS PUB 180-1
• 160-bit MAC
• Partial attack

38
Digital Signatures

• cryptographic technique analogous to hand-written
  signatures.
• sender (Bob) digitally signs document,
  establishing he is document owner/creator.
• verifiable, nonforgeable recipient (Alice) can
  prove to someone that Bob, and no one else
  (including Alice), must have signed document

39
Digital Signatures

• simple digital signature for message m
• Bob signs m by encrypting with his private key
  KB, creating signed message, KB(m)

-
-
Bobs private key
Bobs message, m
(m)
Dear Alice Oh, how I have missed you. I think of
you all the time! (blah blah blah) Bob
Bobs message, m, signed (encrypted) with his
private key
public key encryption algorithm
40
Digital Signatures (more)
-

• suppose Alice receives msg m, digital signature
  KB(m)
• Alice verifies m signed by Bob by applying Bobs
  public key KB to KB(m) then checks KB(KB(m) )
  m.
• if KB(KB(m) ) m, whoever signed m must have
  used Bobs 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.

-
41
Digital signature signed MAC

• Alice verifies signature and integrity of
  digitally signed message

Bob sends digitally signed message
H(m)
Bobs private key
Bobs public key
equal ?
42
Authentication

• Goal Bob wants Alice to prove her identity to
  him

Protocol ap1.0 Alice says I am Alice
I am Alice
Failure scenario??
43
Authentication

• Goal Bob wants Alice to prove her identity to
  him

Protocol ap1.0 Alice says I am Alice
in a network, Bob can not see Alice, so Trudy
simply declares herself to be Alice
I am Alice
44
Authentication another try
Protocol ap2.0 Alice says I am Alice in an IP
packet containing her source IP address
Failure scenario??
45
Authentication another try
Protocol ap2.0 Alice says I am Alice in an IP
packet containing her source IP address
Trudy can create a packet spoofing Alices
address
46
Authentication another try
Protocol ap3.0 Alice says I am Alice and sends
her (encrypted) secret password to prove it.
Failure scenario??
47
Authentication another try
Protocol ap3.0 Alice says I am Alice and sends
her (encrypted) secret password to prove it.
Alices password
Alices IP addr
Im Alice
playback attack Trudy records Alices packet and
later plays it back to Bob
48
Authentication yet another try
Goal avoid playback attack
Nonce number (R) used only once in-a-lifetime
ap4.0 to prove Alice live, Bob sends Alice
nonce, R. Alice must return R, encrypted with
shared secret key
I am Alice
R
Alice is live, and only Alice knows key to
encrypt nonce, so it must be Alice!
Failures, drawbacks?
49
Two-way Authentication
1 Request from Alice 2 4 Challege (a nonce)
from Bob Alice, resp 3 5 Response from Alice
Bob, resp (authenticated afterwards)

50
What if we do it in three steps?

• Alice request and challenges Bob first
• Bob responses (and get authenticated) and
  challenges Alice next
• Alice responses (is she authenticated?)

51
Reflection Attack

• Trudy gets challenged (steps 1 and 2) in session
  one.
• Trudy fools Bob to answer his own challenge (in
  steps 3 and 4) in session two.
• Trudy can now proceed with the first session.

52
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
R
and knows only Alice could have the private key,
that encrypted R such that
send me your public key
53
Ap5.0 Security Hole

• 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 its Bobs public key.
• Pizza Store verifies signature then delivers
  four pizzas to Bob.
• Bob doesnt even like Pepperoni

54
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
R
Send me your public key
Send me your public key
Trudy gets
sends m to Alice encrypted with Alices public key
55
ap5.0 security hole

• Man (woman) in the middle attack Trudy poses as
  Alice (to Bob) and as Bob (to Alice)

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

56
Public Key Certification

• public key problem
• When Alice obtains Bobs public key (from web
  site, e-mail, diskette), how does she know it is
  Bobs public key, not Trudys?
• solution
• trusted certification authority (CA)

57
Certification Authorities

• Certification Authority (CA) binds public key to
  particular entity, E.
• E registers its public key with CA.
• E provides proof of identity to CA.
• CA creates certificate binding E to its public
  key.
• certificate containing Es public key digitally
  signed by CA CA says This is Es public key.

Bobs public key
CA private key
certificate for Bobs public key, signed by CA
-
Bobs identifying information
58
Certification Authorities

• when Alice wants Bobs public key
• gets Bobs certificate (Bob or elsewhere).
• apply CAs public key to Bobs certificate, get
  Bobs public key

Bobs public key
CA public key

59
Trusted Intermediaries

• Symmetric key problem
• How do two entities establish shared secret key
  over network?
• Solution
• trusted key distribution center (KDC) acting as
  intermediary between entities

• Public key problem
• When Alice obtains Bobs public key (from web
  site, e-mail, diskette), how does she know it is
  Bobs public key, not Trudys?
• Solution
• trusted certification authority (CA)

60
Chapter 8 roadmap

• 8.1 What is network security?
• 8.2 Principles of cryptography
• 8.3 Message integrity
• 8.4 Securing e-mail
• 8.5 Securing TCP connections SSL
• 8.6 Network layer security IPsec
• 8.7 Securing wireless LANs
• 8.8 Operational security firewalls and IDS

61
Secure e-mail

• Alice wants to send confidential e-mail, m, to
  Bob.

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

62
Secure e-mail

• Alice wants to send confidential e-mail, m, to
  Bob.

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

63
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.

64
Secure e-mail (Put it all together)PGP Phil
Zimmerman

• Alice wants to provide secrecy, sender
  authentication, message integrity.

Alice uses three keys her private key, Bobs
public key, newly created symmetric key Q What
does Bob have to do?
65
Chapter 8 roadmap

• 8.1 What is network security?
• 8.2 Principles of cryptography
• 8.3 Message integrity
• 8.4 Securing e-mail
• 8.5 Securing TCP connections SSL
• 8.6 Network layer security IPsec
• 8.7 Securing wireless LANs
• 8.8 Operational security firewalls and IDS

66
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
• Provides
• Confidentiality
• Integrity
• Authentication

• 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
• Available to all TCP applications
• Secure socket interface

67
SSL and TCP/IP

• SSL provides application programming interface
  (API)
• to applications
• C and Java SSL libraries/classes readily
  available

68
Could do something like PGP
KS
m
m
Internet
KS

• But want to send byte streams interactive data
• Want a set of secret keys for the entire
  connection
• Want certificate exchange part of protocol
  handshake phase

69
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

70
Toy A simple handshake
hello
certificate
KB(MS) EMS

• MS master secret
• EMS encrypted master secret

71
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

72
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
73
Toy Sequence Numbers

• Attacker can capture and replay record or
  re-order records
• Solution put sequence number into MAC
• MAC MAC(Mx, sequencedata)
• Note no sequence number field
• Attacker could still replay all of the records
• Use random nonce

74
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, sequencetypedata)

length
type
data
MAC
75
Toy SSL summary
bob.com
encrypted
76
Toy SSL isnt 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

77
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