1. Chapter 7 Network Security
A note on the use of these ppt slides:
We’re making these slides freely available to all (faculty, students, readers). They’re in powerpoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:
If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)
If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.
Thanks and enjoy! JFK / KWR
All material copyright
J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking: A Top Down Approach Featuring the Internet, 2nd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2002.
7: Network Security

2. Chapter 7: Network security
Foundations:
what is security?
cryptography
authentication
message integrity
key distribution and certification
Security in practice:
application layer: secure
transport layer: Internet commerce, SSL, SET
network layer: IP security
Firewalls
7: Network Security

3. 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
8: Network Security

4. Internet security threats
Packet sniffing:
broadcast media
promiscuous NIC reads all packets passing by
can read all unencrypted data (e.g. passwords)
e.g.: C sniffs B’s packets A C
src:B dest:A payload B
7: Network Security

5. Internet security threats
IP Spoofing:
can generate “raw” IP packets directly from application, putting any value into IP source address field
receiver can’t tell if source is spoofed
e.g.: C pretends to be B A C
src:B dest:A payload B
7: Network Security

6. Internet security threats
Denial of service (DOS):
flood of maliciously generated packets “swamp” receiver
Distributed DOS (DDOS): multiple coordinated sources swamp receiver
e.g., C and remote host SYN-attack A A C
SYN
SYN
SYN
SYN
SYN B
SYN
SYN
7: Network Security

7. Cryptography Principles
7: Network Security

8. 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
channel
data, control messages
secure
sender
secure
receiver
data
data
Trudy
8: Network Security

9. 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?
8: Network Security

10. The language of cryptography
Alice’s
encryption
key
Bob’s
decryption
key K A K B
encryption
algorithm
plaintext
ciphertext
decryption
algorithm
plaintext
symmetric key crypto: sender, receiver keys identical
public-key crypto: encryption key public, decryption key secret (private)
8: Network Security

11. Symmetric Key Cryptography
7: Network Security

12. 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
Q: How hard to break this simple cipher?:
brute force (how hard?)
other?
7: Network Security

13. Perfect cipher Definition: Example: one time pad Cons: size
Let C = E[M]
Pr[C=c] = Pr[C=c | M]
Example: one time pad
Generate random bits b1 ... bn
E[M1 ... Mn] = (M1  b1 ... Mn  bn )
Cons: size
Pseudo Random Generator
G(R) = b1 ... bn
Indistinguishable from random (efficiently)
7: Network Security

14. 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
7: Network Security

15. Symmetric key crypto: DES
DES operation
initial permutation
16 identical “rounds” of function application, each using different 48 bits of key
final permutation
7: Network Security

16. Block Cipher chaining How do we encode a large message Encoding:
Would like to guarantee integrity
Encoding:
Ci = E[Mi  Ci-1]
Decoding:
Mi = D[Ci]  Ci-1
Malfunctions:
Loss
Reorder/ integrity
7: Network Security

17. Cipher block chaining (CBC)
Break plaintext into blocks
XOR previous round ciphertext with new plaintext
Block 1 IV
DES
Cipher 2 3 4 +
7: Network Security

18. Cipher Block Chaining Mode
Cipher block chaining. (a) Encryption. (b) Decryption.
7: Network Security

19. DES performance DES relies on confusion and diffusion
not mathematically proven to be secure
More secure: 3DES (triple DES) with cipher block chaining (CBC)
3DES products for IP security*
~10 Mb/s in software
~100 Mb/s in hardware
Both parties must know secret key, and keep it secret
* source:
7: Network Security

20. Diffie-Hellman key exchange protocol
Goal: Allow strangers establish a shared secret key for later communication
Assume two parties (Alice and Bob) want to establish a secret key.
Alice and Bob agree on two large numbers, n and g
usually, these are publicly known, and have some additional conditions applied (e.g., n must be prime)
7: Network Security

21. Diffie-Hellman Key Exchange
Alice picks large x, Bob picks large y (e.g., 512 bits)
7: Network Security

22. Man in the middle attack
Eavesdropper can’t determine secret key (gxy mod n) from (gx mod n) or (gy mod n)
However, how does Alice and Bob know if there is a third party adversary in between?
7: Network Security

23. Exponentiation Compute gx mod n Expg,n (x) Assume x = 2y + b
Let z = Expg,n (y)
R=z2
If (b=1) R = g R mod n
Return R
Complexity: logarithmic in x
7: Network Security

24. Public Key Cryptography
7: Network Security

25. 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
encryption key public (known to all)
decryption key private (known only to receiver)
7: Network Security

26. Public key cryptography+
Bob’s public
key K B -
Bob’s private
key K B
plaintext
message, m
encryption
algorithm
ciphertext
decryption
algorithm
plaintext
message
K (m) B +
m = K (K (m)) B + -
8: Network Security

27. Public key encryption algorithms
Two inter-related requirements: . . 1
need d ( ) and e ( ) such that B B
d (e (m)) = m B 2
need public and private keys
for d ( ) and e ( ) . B
RSA: Rivest, Shamir, Adelson algorithm
7: Network Security

28. RSA: Choosing keys 1. Choose two large prime numbers p, q.
(e.g., 1024 bits each)
2. Compute n = pq, z = (p-1)(q-1)
3. Choose e (with e<n) that has no common factors
with z. (e, z are “relatively prime”).
4. Choose d such that ed-1 is exactly divisible by z.
(in other words: ed mod z = 1 ).
5. Public key is (n,e). Private key is (n,d). K B + K B -
8: Network Security

29. RSA: Encryption, decryption
0. Given (n,e) and (n,d) as computed above
1. To encrypt bit pattern, m, compute
c = m mod n e
(i.e., remainder when m is divided by n)
2. To decrypt received bit pattern, c, compute
m = c mod n d d
(i.e., remainder when c is divided by n)
m = (m mod n) e
mod n d
Magic
happens!
7: Network Security

30. RSA operation Sender Plaintext (P) Recv plaintext (P) Public key e
Private key d
E(P, e)
D(C, d)
Ciphertext (C)
Encryption & Decryption
E(P) = Pemod n
D(C) = Cdmod n
Security is based on difficulty of factoring large numbers (can’t factor n to obtain p and q)
7: Network Security

31. Bob chooses p=5, q=7. Then n=35, z=24.
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). e
c = m mod n e
letter m m
encrypt: l 12 17 c d
m = c mod n d c
letter
decrypt: 17 12 l
7: Network Security

32. RSA example An example of the RSA algorithm.
p = 3, q = 11, n = 33, z = 20, d = 7
e found by solving 7e = 1 (mod 20)--> 3
7: Network Security

33. RSA: Why x y mod n = x (y mod (p-1)(q-1) ) mod n m = (m mod n) e mod n
Number theory result:
IF pq = n, p and q primes then:
x y mod n = x (y mod (p-1)(q-1) ) mod n
(m e)d mod n = m (ed mod (p-1)(q-1)) mod n
But ed – 1 divisible by (p-1)(q-1) i.e., ed mod (p-1)(q-1) = 1
= m 1 mod n = m
Always there is a solution, and satisfies (Chinese Remainder)
7: Network Security

34. modified Diffie-Hellman Key Exchange
Encrypt 1 with Bob’s public key, 2 with Alice’s public key
Prevents man-in-the-middle attack
Actually, nonces and a third message are needed to fully complete this exchange (in a few slides)
7: Network Security

35. Authentication
7: Network Security

36. Protocol ap1.0: Alice says “I am Alice”
Authentication
Goal: Bob wants Alice to “prove” her identity to him
Protocol ap1.0: Alice says “I am Alice”
“I am Alice”
Failure scenario??
8: Network Security

37. 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”
8: Network Security

38. Authentication: another try
Protocol ap2.0: Alice says “I am Alice” in an IP packet
containing her source IP address
“I am Alice”
Alice’s
IP address
Failure scenario??
8: Network Security

39. Authentication: another try
Protocol ap2.0: Alice says “I am Alice” in an IP packet
containing her source IP address
Trudy can create
a packet “spoofing”
Alice’s address
“I am Alice”
Alice’s
IP address
8: Network Security

40. Authentication: another try
Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.
“I’m Alice”
Alice’s
IP addr
password
Failure scenario?? OK
Alice’s
IP addr
8: Network Security

41. Authentication: another try
Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.
Alice’s
IP addr
Alice’s
password
“I’m Alice”
playback attack: Trudy records Alice’s packet
and later
plays it back to Bob OK
Alice’s
IP addr
“I’m Alice”
Alice’s
IP addr
password
8: Network Security

42. Authentication: yet another try
Protocol ap3.1: Alice says “I am Alice” and sends her
encrypted secret password to “prove” it.
“I’m Alice”
Alice’s
IP addr
encrypted
password
Failure scenario?? OK
Alice’s
IP addr
8: Network Security

43. Authentication: another try
Protocol ap3.1: Alice says “I am Alice” and sends her
encrypted secret password to “prove” it.
Alice’s
IP addr
encrypted
password
“I’m Alice”
record
and
playback
still works! OK
Alice’s
IP addr
“I’m Alice”
Alice’s
IP addr
encrypted
password
8: Network Security

44. 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
K (R)
A-B
Alice is live, and only Alice knows key to encrypt nonce, so it must be Alice!
Failures, drawbacks?
8: Network Security

45. “send me your public key”
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
(K (R)) = R A - K +
K (R) A -
and knows only Alice could have the private key, that encrypted R such that
“send me your public key” K A +
(K (R)) = R A - K +
8: Network Security

46. sends m to Alice encrypted with Alice’s public key
ap5.0: security hole
Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice)
I am Alice
I am Alice R T
K (R) - R A
K (R) -
Send me your public key T K +
Send me your public key A K + T
K (m) +
Trudy gets T
m = K (K (m)) + - A
K (m) +
sends m to Alice encrypted with Alice’s public key A
m = K (K (m)) + -
8: Network Security

47. 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!
8: Network Security

48. Message Integrity (Signatures etc)
7: Network Security

49. 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 verify that Bob, and no one else, signed document.
Assumption:
eB(dB(m)) = dB(eB(m))
RSA
Simple digital signature for message m:
Bob decrypts m with his private key dB, creating signed message, dB(m).
Bob sends m and dB(m) to Alice.
7: Network Security

50. Digital Signatures (more)
Suppose Alice receives msg m, and digital signature dB(m)
Alice verifies m signed by Bob by applying Bob’s public key eB to dB(m) then checks eB(dB(m) ) = m.
If eB(dB(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 dB(m) to court and prove that Bob signed m.
7: Network Security

51. Message Digests
Computationally expensive to public-key-encrypt long messages
Goal: fixed-length,easy to compute digital signature, “fingerprint”
apply hash function H to m, get fixed size message digest, H(m).
Hash function properties:
Many-to-1
Produces fixed-size msg digest (fingerprint)
Given message digest x, computationally infeasible to find m such that x = H(m)
computationally infeasible to find any two messages m and m’ such that H(m) = H(m’).
7: Network Security

52. Hash Function Algorithms
MD5 hash function widely used.
Computes 128-bit message digest in 4-step process.
arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x.
SHA-1 is also used.
US standard
160-bit message digest
Internet checksum would make a poor message digest.
Too easy to find two messages with same checksum.
7: Network Security

53. MD5 for message integrity
Keyed MD5
assume sender, receiver share secret key k
sender: m + MD5(m + k)
(+ means concatenation in this notation)
receiver computes MD5(m + k) and sees if it matches
7: Network Security

54. Digital signature = signed message digest
Alice verifies signature and integrity of digitally signed message:
Bob sends digitally signed message:
large
message m
H: Hash
function
KB(H(m)) -
encrypted
msg digest
H(m)
digital
signature
(encrypt)
Bob’s
private
key
large
message m K B -
Bob’s
public
key
digital
signature
(decrypt) K B +
KB(H(m)) -
encrypted
msg digest
H: Hash
function +
H(m)
H(m)
equal ?
8: Network Security

55. MD5 with RSA signature MD5 with RSA signature
sender: m + E(MD5(m), private)
receiver: compare MD5(m) with D(checksum, public)
checksum
7: Network Security

56. Key Distribution Centers
7: Network Security

57. Problems with Public-Key Encryption
A way for Trudy to subvert public-key encryption.
7: Network Security

58. Trusted Intermediaries
Problem:
How do two entities establish shared secret key over network?
Solution:
trusted key distribution center (KDC) acting as intermediary between entities
Problem:
When Alice obtains Bob’s public key (from web site, , diskette), how does she know it is Bob’s public key, not Trudy’s?
Solution:
trusted certification authority (CA)
7: Network Security

59. Key Distribution Center (KDC)
Alice,Bob need shared symmetric key.
KDC: server shares different secret key with each registered user.
Alice, Bob know own symmetric keys, KA-KDC KB-KDC , for communicating with KDC.
Alice communicates with KDC, gets session key R1, and KB-KDC(A,R1)
Alice sends Bob KB-KDC(A,R1), Bob extracts R1
Alice, Bob now share the symmetric key R1.
7: Network Security

60. Certification Authorities
Certification authority (CA) binds public key to particular entity.
Entity (person, router, etc.) can register its public key with CA.
Entity provides “proof of identity” to CA.
CA creates certificate binding entity to public key.
Certificate digitally signed by CA.
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
7: Network Security

61. X.509 is the standard for certificates
The basic fields of an X.509 certificate.
7: Network Security

62. Public-Key Infrastructures
PKIs are a way to structure certificates
(a) A hierarchical PKI. (b) A chain of certificates.
7: Network Security

63. Example revisited (solved with certificates)
Trudy presents Alice a certificate, purporting to be Bob, but Alice is unable to trace Trudy’s certificate back to a trusted root
Be wary if your browser warns about certs!
7: Network Security

64. Secure
7: Network Security

65. Secure
Alice wants to send confidential , m, to Bob.
KS( ) .
KB( ) + -
KS(m )
KB(KS ) m KS KB
Internet
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.
7: Network Security

66. Secure
Alice wants to send confidential , m, to Bob.
KS( ) .
KB( ) + -
KS(m )
KB(KS ) m KS KB
Internet
Bob:
uses his private key to decrypt and recover KS
uses KS to decrypt KS(m) to recover m
7: Network Security

67. Secure e-mail (continued)
Alice wants to provide sender authentication message integrity.
H( ) .
KA( ) - +
H(m )
KA(H(m)) m KA
Internet
compare
Alice digitally signs message.
sends both message (in the clear) and digital signature.
7; Network Security

68. Pretty good privacy (PGP)
Internet encryption scheme, a de-facto standard.
Uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described.
Provides secrecy, sender authentication, integrity.
Inventor, Phil Zimmerman, was target of 3-year federal investigation.
A PGP signed message:
---BEGIN PGP SIGNED MESSAGE---
Hash: SHA1
Bob:My husband is out of town tonight.Passionately yours, Alice
---BEGIN PGP SIGNATURE---
Version: PGP 5.0
Charset: noconv
yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJhFEvZP9t6n7G6m5Gw2
---END PGP SIGNATURE---
7: Network Security

69. Secure Socket layer (SSl)
7: Network Security

70. Secure sockets layer (SSL)
Server authentication:
SSL-enabled browser includes public keys for trusted CAs.
Browser requests server certificate, issued by trusted CA.
Browser uses CA’s public key to extract server’s public key from certificate.
Visit your browser’s security menu to see its trusted CAs.
PGP provides security for a specific network app.
SSL works at transport layer. Provides security to any TCP-based app using SSL services.
SSL: used between WWW browsers, servers for I-commerce (https).
SSL security services:
server authentication
data encryption
client authentication (optional)
7: Network Security

71. Tools  Internet options  Content  Certificates
Internet Explorer:
Tools  Internet options  Content  Certificates
7: Network Security

72. Internet Explorer: Error Message
7: Network Security

73. SSL (continued) Encrypted SSL session:
Browser generates symmetric session key, encrypts it with server’s public key, sends encrypted key to server.
Using its private key, server decrypts session key.
Browser, server agree that future msgs will be encrypted.
All data sent into TCP socket (by client or server) is encrypted with session key.
SSL: basis of IETF Transport Layer Security (TLS).
SSL can be used for non-Web applications, e.g., IMAP.
Client authentication can be done with client certificates.
7: Network Security

74. SSL basics Alice validates Bob’s key
RA, RB are “nonces” used with Premaster key to create session key
EB is Bob’s public key
7: Network Security

75. Other Security Layers
7: Network Security

76. Secure electronic transactions (SET)
designed for payment-card transactions over Internet.
provides security services among 3 players:
customer
merchant
merchant’s bank
All must have certificates.
SET specifies legal meanings of certificates.
apportionment of liabilities for transactions
Customer’s card number passed to merchant’s bank without merchant ever seeing number in plain text.
Prevents merchants from stealing, leaking payment card numbers.
Three software components:
Browser wallet
Merchant server
Acquirer gateway
See book for description of SET transaction.
7: Network Security

77. IPsec: Network Layer Security
Network-layer secrecy:
sending host encrypts the data in IP datagram
TCP and UDP segments; ICMP and SNMP messages.
Network-layer authentication
destination host can authenticate source IP address
Two principle protocols:
authentication header (AH) protocol
encapsulation security payload (ESP) protocol
For both AH and ESP, source, destination handshake:
create network-layer logical channel called a service agreement (SA)
Each SA unidirectional.
Uniquely determined by:
security protocol (AH or ESP)
source IP address
32-bit connection ID
7: Network Security

78. ESP Protocol Provides secrecy, host authentication, data integrity.
Data, ESP trailer encrypted.
Next header field is in ESP trailer.
ESP authentication field is similar to AH authentication field.
Protocol = 50.
7: Network Security

79. Authentication Header (AH) Protocol
AH header includes:
connection identifier
authentication data: signed message digest, calculated over original IP datagram, providing source authentication, data integrity.
Next header field: specifies type of data (TCP, UDP, ICMP, etc.)
Provides source host authentication, data integrity, but not secrecy.
AH header inserted between IP header and IP data field.
Protocol field = 51.
Intermediate routers process datagrams as usual.
7: Network Security

80. IPsec authentication (AH)
The IPsec authentication header in transport mode for IPv4.
HMAC stands for Hashed Message Authentication Code
7: Network Security

81. IPsec encryption (ESP)
(a) ESP in transport mode (for end-to-end IPsec). (b) ESP in tunnel mode (used in VPNs).
7: Network Security

82. Access Control in the Network
7: Network Security

83. Firewalls Two firewall types: packet filter application gateways
To prevent denial of service attacks:
SYN flooding: attacker establishes many bogus TCP connections. Attacked host alloc’s TCP buffers for bogus connections, none left for “real” connections.
To prevent illegal modification of internal data.
e.g., attacker replaces CIA’s homepage with something else
To prevent intruders from obtaining secret info.
firewall
isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others.
Two firewall types:
packet filter
application gateways
7: Network Security

84. Packet Filtering
Internal network is connected to Internet through a router.
Router manufacturer provides options for filtering packets, based on:
source IP address
destination IP address
TCP/UDP source and destination port numbers
ICMP message type
TCP SYN and ACK bits
Example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23.
All incoming and outgoing UDP flows and telnet connections are blocked.
Example 2: Block inbound TCP segments with ACK=0.
Prevents external clients from making TCP connections with internal clients, but allows internal clients to connect to outside.
7: Network Security

85. Filter-based firewalls
Rest of the Internet
Local site
Firewall
Sit between site and rest of Internet, filter packets
Enforce site policy in a manageable way
e.g. pass (*,*, , 80 ), then drop (*, *, *, 80)
Rules may be added dynamically to pass new connections
Sometimes called a “ level 4” switch
Acts like a router (accepts and forwards packets)
But looks at information up to TCP port numbers (layer
7: Network Security

86. Proxy-Based Firewalls
Solves more complex policy problems
Example:
I want internal company to be able to access entire web server
I want outside world only to access public pages
Firewall only lets proxy
connect to web server
Remote
company W eb
user
server
Internet
Internal
Proxy
Firewall
Company net
server
Random
external
Use proxies outside firewall
user
7: Network Security

87. Proxy-based firewalls
Run proxies for Web, mail, etc. just outside firewall
In the “ de-militarized zone” DMZ
External requests go to proxies, only proxies connect inside
External user may (“classical model”) or may not (“transparent model”) know this is happening
Proxies filter based on application semantics
7: Network Security

88. Virtual Private Networks
Using firewalls and IPsec encryption to provide a “leased-line” like connection over the Internet
(a) A leased-line private network. (b) A virtual private network.
7: Network Security

89. Application gateways
host-to-gateway
telnet session
gateway-to-remote
host telnet session
application
gateway
router and filter
Filters packets on application data as well as on IP/TCP/UDP fields.
Example: allow select internal users to telnet outside.
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.
7: Network Security

90. 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.
7: Network Security

91. Network Security (summary)
Basic techniques…...
cryptography (symmetric and public)
authentication
message integrity
…. used in many different security scenarios
secure
secure transport (SSL)
IP sec
Firewalls
7: Network Security