ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Lecture 12 Network Security (2)

Khaled Mahbub, IICT, BUET, 2008 ICT 6621 : Advanced Networking

Lecture 12

Network Security (2)


Outline• Network Security Concepts

• Principles of cryptography

• Authentication

• Integrity

• Key Distribution and certification

• Security in different layers– E-Mail Security (Application)– Web Security

• SSL – The Secure Sockets Layer (Application-Transport)– Communication Security (Network/Data Link)

• IPSec• Firewalls


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 A obtains B’s

public key (from web site, e-mail, diskette), how does it know it is B’s public key, not C’s?

Solution:• trusted certification

authority (CA)


Key Distribution Center (KDC)• A and B need shared symmetric key.

• KDC: server shares different secret key with each registered user (many users)

• A and B know own symmetric keys, KA-KDC KB-KDC , for communicating with KDC.

KB-KDC

KX-KDC

KY-KDC

KZ-KDC

KP-KDC

KB-KDC

KA-KDC

KA-KDC

KP-KDC

KDC

A

BP


Key Distribution Center (KDC)Q: How does KDC allow B and A to determine shared

symmetric secret key to communicate with each other?

A and B communicate: using R1 as session key for shared symmetric encryption

A knows R1

B knows to use R1 to communicate with A

KDC generates R1

KB-KDC(A,R1)

KA-KDC(A,B)

KA-KDC(R1, KB-KDC(A,R1) )

AB

• Kerberos is an authentication service developed at MIT that uses symmetric key encryption techniques and a Key Distribution Center. The Kerberos Authentication Server (AS) plays the role of the KDC.


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”

B’s public

key K B+

B’s identifying informatio

n

digitalsignature(encrypt)

CA private

key K CA-

K B+

certificate for B’s public key,

signed by CAB


Certification Authorities• When A wants B’s public key:

– gets B’s certificate (from B’s Web page, e-mail message or elsewhere).

– apply CA’s public key to B’s certificate, get B’s public key

B’s public

key K B+

digitalsignature(decrypt)

CA public

key K CA+

K B+


A Certificate Contains

Field Description

version version number of X.509 specification (standard for Certification, developed by IETF)

serial number CA-issued unique identifier for a certificate

signature specifies the algorithm used by CA to "sign" this certificate

Issuer name identity of CA issuing this certificate

Validity period start and end of period of validity for certificate

Subject name identity of entity whose public key is associated with this certificate

Subject public key the subject's public key as well as an indication of the public key algorithm (and algorithm parameters) to be used with this key




• Authentication

• Integrity



• SSL – The Secure Sockets Layer (Application-Transport)

– Communication Security (Network/Data Link)• IPSec• Firewalls


Secure e-mail• A wants to send confidential e-mail, m, to B

KS( ).

KB( ).+

+ -

KS(m

)

KB(KS )+

m

KS

KS

KB+

Internet

KS( ).

KB( ).-

KB-

KS

mKS(m

)

KB(KS )+

A B

• A:– generates random symmetric private key, KS.

– encrypts message with KS

– also encrypts KS with B’s public key.

– sends both KS(m) and KB(KS) to B.


Secure e-mail• A wants to send confidential e-mail, m, to B

KS( ).

KB( ).+

+ -

KS(m

)

KB(KS )+

m

KS

KS

KB+

Internet

KS( ).

KB( ).-

KB-

KS

mKS(m

)

KB(KS )+

A B

• B:

– uses his private key to decrypt and recover KS

– uses KS to decrypt KS(m) to recover m


Secure e-mail• A wants to provide sender authentication, message integrity.

• “A” applies a hash function, H (e.g., MD5), to message m to obtain a message digest

• encrypts the result of the hash function with private key, to create a digital signature

• 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 )

compareA B

• “B” applies the hash function, H (e.g., MD5), to message m to obtain a message digest,

• applies A's public key, to signature

• compares the result of the operations


Secure e-mail• A wants to provide secrecy, sender authentication,

message integrity.

“A” uses three keys: her private key, B’s public key, newly created symmetric key

H( ). KA( ).-

+

KA(H(m))-

m

KA-

m

KS( ).

KB( ).+

+

KB(KS )+

KS

KB+

Internet

KS


Pretty Good Privacy (PGP)

• Internet e-mail encryption scheme, de-facto standard.• uses symmetric key cryptography, public key cryptography, hash

function, and digital signature as described.• provides secrecy, sender authentication, integrity.• inventor, Phil Zimmerman.

---BEGIN PGP SIGNED MESSAGE---Hash: SHA1B: Hello there, the journey was very

good one… , A---BEGIN PGP SIGNATURE---Version: PGP 5.0Charset: noconvyhHJRHhGJGhgg/

12EpJ+lo8gE4vB3mqJhFEvZP9t6n7G6m5Gw2

---END PGP SIGNATURE---

A PGP signed message:

-----BEGIN PGP MESSAGE-----

Version: PGP 5.0

u2R4d+/jKmn8Bc5+hgDsqAewsDfrGdszX68liKm5F6Gc4sDfcXyt

RfdSlOjuHgbcfDssWe7/K=lKhnMikLo0+l/BvcX4t==Ujk9PbcD4

Thdf2awQfgHbnmKlok8iy6gThlp

-----END PGP MESSAGE

A secret PGP message:




• Authentication

• Integrity



• SSL – The Secure Sockets Layer (Application-Transport)– Communication Security (Network/Data Link)

• IPSec• Firewalls


Secure Sockets Layer (SSL)

• transport layer security to any TCP-based application using SSL services.

• used between Web browsers, servers for e-commerce (https).

• security services:– server authentication

– data encryption

– client authentication (optional)

• 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.

• Do it yourself: check your browser’s security menu to see its trusted CAs.


SSL

Encrypted SSL session:• Browser generates

symmetric session key, encrypts it with server’s public key, sends encrypted key to server.

• Using private key, server decrypts session key.

• Browser, server know session key– All data sent into TCP

socket (by client or server) encrypted with session key.

• 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. (if needed)




• Authentication

• Integrity






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 (authentication, integrity)– encapsulation security payload

(ESP) protocol (authentication, integrity, secrecy)

• For both AH and ESP, source, destination handshake:– create network-layer

logical channel called a security association (SA)

• Each SA unidirectional.• Uniquely determined by:

– security protocol (AH or ESP)

– source IP address– 32-bit connection ID


Authentication Header (AH) Protocol

• provides source authentication, data integrity, no confidentiality

• AH header inserted between IP header, data field.

• protocol field: 51

• intermediate routers process datagrams as usual

AH header includes:• connection identifier (Security

Parameter Index, SPI)• authentication data: source-

signed message digest calculated over original IP datagram.

• next header field: specifies type of data (e.g., TCP, UDP, ICMP)

IP header data (e.g., TCP, UDP segment)AH header


ESP Protocol

• provides secrecy, host authentication, data integrity.

• next header field is in ESP trailer.

• data, ESP trailer encrypted.

• ESP authentication field is similar to AH authentication field.

• Protocol = 50.

IP header TCP/UDP segmentESP

headerESP

trailerESP

authent.

encryptedauthenticated




• Authentication

• Integrity






Firewalls• isolates organization’s

internal net from larger Internet, allowing some packets to pass, blocking others.

administerednetwork

publicInternet

firewallFirewalls: Whyprevent denial of service attacks:

– flooding: attacker establishes many bogus TCP connections, no resources left for “real” connections.

prevent illegal modification/access of internal data.– e.g., attacker replaces CIA’s homepage with something else

allow only authorized access to inside network (set of authenticated users/hosts)


Firewalls

• Typical configuration of firewall:– Two routers that

do packet filtering. These are standard routers equipped with some extra functionality.

– An application gateway that operates at the application level.

administered

network

publicInternet

Packet filteringrouter

Packet filteringrouter

Application gateway


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?


Packet Filtering• Example 1: block incoming and outgoing datagrams with

IP protocol field = 17 and with either source or destination 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.


Application Gateways• 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

destination host. Gateway relays data between 2 connections3. Router filter blocks all telnet connections not originating from

gateway.

host-to-gatewaytelnet session

gateway-to-remote host telnet session

applicationgateway

router and filter


Limitations of Firewalls and Gateways

• IP spoofing: router can’t know if data “really” comes from claimed source

• if multiple applications need special treatment, each has own application 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.


Reading Material• Chapter 7 – text3 (Kurose)

• Chapter 8 – text2 (Tanenbaum)


Notice• Mid term 2 exam marks are available at,

http://teacher.buet.ac.bd/khaledmahbub/ANT_Exam_Marks.html

Bonus Marks:– Old Marking Scheme

• Mid Term 1 30%• Mid Term 2 30%• Final Exam 40%

– New Marking Scheme• Mid Term 1 25%• Mid Term 2 25%• Attendance 10%• Final Exam 40%

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ICT 6621 : Advanced NetworkingKhaled Mahbub, IICT, BUET, 2008 Lecture 12 Network Security (2)

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