The document discusses various aspects of network security, including threats (passive and active), firewalls (their types, functions, and policies), and virtual private networks (VPNs) with a focus on tunneling protocols like PPTP and L2TP. It highlights the importance of firewalls in protecting networks from unauthorized access and describes the limitations and challenges associated with packet filtering. Additionally, the document explains the function of IPSec in securing data transfer across networks.

1. NETWORK SECURITY
By
VIKAS JAGTAP
Mobile:8087094711
Email: vikas27jagtap@gmail.com

2. Objective
Threats
Firewall
Packet-filtering firewalls
Firewall policies and rules
Common Problem with Packet Filtering
Virtual Private Networks
IPSec (Internet Protocol Security)
SSL (Secure Socket Layer)
Cryptography
Symmetric Key Signatures
Public key Signatures
The Birthday Attack
Summary

3. Threats
• THREATS are possible attacks.
• It includes
– The spread of computer viruses
– Infiltration and theft of data from external hackers
– Engineered network overloads triggered by malicious mass
e-mailing
– Misuse of computer resources and confidential information
by employees
– Unauthorized financial transactions and other kinds of
computer fraud conducted in the company's name
– Electronic inspection of corporate computer data by
outside parties
– Damage from failure, fire, or natural disasters

4. Threats
• Network Security threats fall into two
categories. Passive threats and Active
threats.
• Passive threats is also called as
eavesdropping, involve attempt by an
attacker to obtain information relating to a
communication.
• Active threats involves some modification
of the transmitted data or creation of false
transmissions.

5. • A network security threat is any potentially adverse
occurrence that can harm or interrupt the systems
using the network, or cause a monetary loss to an
organization.
• Once the threats are identified they are then ranked
according to their occurrence.
• The next slide summarizes the most common threats
to security.
Threats

6. Common Security Threats

7. What is a Firewall?
• A firewall is a method of achieving security between
trusted and untrusted networks
• The choice, configuration and operation of a firewall
is defined by policy, which determines the the
services and type of access permitted
• Firewall = policy+implementation
• Firewall = “zone of risk” for the trusted network
Gateway (DMZ)

8. Firewalls
• Firewalls are used to prevent intruders on
the Internet from making unauthorized
access and denial of service attacks to your
network.
• A firewall is a router, gateway, or special
purpose computer that examines packets
flowing into and out of the organization’s
network (usually via the Internet or
corporate Intranet), restricting access to
that network.

9. Types of firewalls
 Packet Filtering firewall
 Operate on transport and network layers of the
TCP/IP stack
 Application Gateways/Proxies
 Operate on the application protocol level
InternalNetwork
External
Network
PacketFilteringFirewall
ProxyClient
ActualServerProxyFirewall

10. Packet Filtering Firewall
 Operate on transport and network layers of
the TCP/IP stack
 Decides what to do with a packet
depending upon the following criteria:
 Transport protocol (TCP,UDP,ICMP),
 Source and destination IP address
 The source and destination ports
 ICMP message type/code
 Various TCP options such as packet size,
fragmentation etc

11. Packet Filtering
• 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 or with SYN bit set and ACK bit
unset.
– Prevents external clients from making TCP
connections with internal clients, but allows
internal clients to connect to outside.

12. Packet Filtering Firewall: Terminology
• Stateless Firewall: The firewall makes a decision on
a packet by packet basis.
• Stateful Firewall : The firewall keeps state
information about transactions (connections).
• NAT - Network Address translation
– Translates public IP address(es) to private IP
address(es) on a private LAN.
– We looked at this already (must be stateful)

13. Packet Filtering Firewall: Functions
• Forward the packet(s) on to the intended
destination
• Reject the packet(s) and notify the sender
(ICMP dest unreach/admin prohibited)
• Drop the packet(s) without notifying the
sender.
• Log accepted and/or denied packet
information
• NAT - Network Address Translation

14. Packet Filtering Firewall: Disadvantages
• Filters can be difficult to configure. It’s not
always easy to anticipate traffic patterns and
create filtering rules to fit.
• Filter rules are sometimes difficult to test
• Packet filtering can degrade router performance
• Attackers can “tunnel” malicious traffic through
allowed ports on the filter.

15. Application Gateway (Proxy Server)
 Operate at the application protocol level. (Telnet,
FTP, HTTP)
 Filters packets on application data as well as on
IP/TCP/UDP fields
 Application Gateways “Understand” the protocol
and can be configured to allow or deny specific
protocol operations.
 Typically, proxy servers sit between the client and
actual service. Both the client and server talk to
the proxy rather than directly with each other.

16. Application gateways
• Example: allow select
internal users to
telnet outside.
host-to-gateway
telnet session
gateway-to-remote
host telnet session
application
gateway
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.

17. Application Gateway (Proxy Server): Disadvantages
• Requires modification to client software
application
• Some client software applications don’t
accommodate the use of a proxy
• Some protocols aren’t supported by proxy
servers
• Some proxy servers may be difficult to configure
and may not provide all the protection you
need.

18. Firewall Hardware/Software
• Dedicated hardware/software application such as
Cisco PIX Firewall which filters traffic passing
through the multiple network interfaces.
• A Unix or Windows based host with multiple
network interfaces, running a firewall software
package which filters incoming and outgoing traffic
across the interfaces.
• A Unix or Windows based host with a single
network interface, running a firewall software
package which filters the incoming and outgoing
traffic to the individual interface.

19. Firewall Architecture
In the real world, designs are far more complex
InternalNetwork
External
Network
BorderRouter
ExternalFirewall
WebServer
IDS
InternalFirewall
DMZ
InternalRouter
CoreSwitch
CoreSwitch
CoreSwitch
Modem

20. 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.

21. Firewall policies and rules
• Network service access policy (NSAP)
– Defines which services are to be explicitly
allowed or denied+ways in which these
services are to be used
• Firewall design policy (FDP)
– Defines how the firewall implements
restricted access and service filtering
specified by the NSAP
– FDP must be continuously updated with new
vulnerabilities

22. Packet Traversal in a Firewall
Packet receipt by firewall
Link layer filtering
Dynamic ruleset (state)
Packet legality checks
IP and port filtering
NAT/PAT (header
rewrite)
Packet reassembly
Application level analysis
Routing decision
Dynamic ruleset (state)
Packet sanity checks
IP and port filtering
Packet release
Packet flow
Packet may be dropped
Packet may be dropped
Stream may be dropped
Optional outbound filtering
Bypass
On
Match
Firewall policies and rules

23. Filtering and Policies
Filtering Overview
• Determines which packets to allow through
firewall
– Can apply to inbound and/or outbound traffic
– Filter by protocol, port, or packet content
– Allows certain traffic while denying others
• Application filtering
– Authentication and virus checking
• Dynamic packet filtering
– Opens ports as needed

24. Filtering and Policies
Application Filters
• Filters based on packet contents
• FTP
– Dynamically opens ports
• Intrusion detection
– DNS attacks
– POP3 buffer overflow attacks
• STMP
– Block spam, viruses, and dangerous code
• Streaming media
– Specify streaming media protocols

25. Filtering and Policies
Stateful Inspection
• Inspects traffic source and destination
• Also known as dynamic packet filtering
– Opens ports in response to user request
– Closes ports when communication ends
• Outgoing packets that request specific types of
incoming packets are tracked
– Only replies are let back in

26. Filtering and Policies
Firewall Policies Overview
• Network rules
– Determines how two networks are connected
• Firewall policy rules
– Access rules
– Publishing rules
• Outgoing requests
– Checks network rules
– Check access rules
• Incoming requests
– Checks publishing rules
– Checks Web chaining rules

27. Filtering and Policies
Configuring Policies
• Action
– Allow or deny
• Protocols
• Source and destination networks
• Users or groups
• Schedule
• Application filtering
– HTTP, RPC, FTP, SMTP, etc.
• Change order of rule

28. Filtering and Policies
Outgoing Access Requests
Route or
NAT?
Route or
NAT?

29. Common Problem with
Packet Filtering
1. Filters are difficult to configure
2. TCP and UDP source port are often omitted
from filtering criteria
3. Special handling of start-of-connection
packets is impossible
4. Tabular filtering rule structures are too
cumbersome
5. Testing and monitoring filters is difficult
6. RPC is very difficult to filter effectively

30. VPN
• What is a VPN?
• Common uses of VPNs
• Basic requirements of VPN
• Tunneling Basics
• Tunneling Protocols
• Tunnel Types

31. What is a VPN?
• Virtual Private
Network is a network in
which some of the parts
are connected using the
public Internet, but the
data sent across the
Internet is encrypted, so
the entire network is
virtually private.
• A technology that allows to send confidential data
securely over the internet

32. Common uses of VPNs
1. Remote Access Over the Internet
Fig. Using a VPN connection to connect a remote client to a private intranet

33. Common uses of VPNs Cont.
2. Connecting Networks Over the Internet – Site-to-Site VPN
Fig. Using dedicated or dial-up lines to connect a branch office to a
corporate LAN

34. Common uses of VPNs Cont
3. Connecting Computers over an Intranet
Fig. Using a VPN connection to connect to a secured or hidden network

35. Basic requirements of VPN
• User Authentication
• Address Management
• Data Encryption
• Key Management
• Multiprotocol Support

36. Tunneling Basics
• Tunneling is the transmission of data intended for
use only within a private, usually corporate
network through a public network in such a way
that the routing nodes in the public network are
unaware that the transmission is part of a private
network
Fig. Tunneling

37. Tunneling Three Primary Components:
1. Passenger protocol:
– which is the protocol you are encapsulating
– IP, IPX, NetBEUI, AppleTalk, Banyan VINES, CLNS,
DECnet, ...
2. Carrier protocol
– which is the tunnel protocol
– PPP, PPTP, L2F, L2TP, GRE, IPSec
3. Transport protocol
– which transports the tunnel (i.e. the encapsulated protocol) –
IP
Tunneling Basics cont.

38. Tunneling Protocols
1. Point to Point Tunneling Protocols ( Layer 2 )
2. Layer 2 Forwarding Protocols (Layer 2)
3. Layer 2 Tunneling Protocols ( Layer 2)
4. IPSec ( Layer 3 )

39. Tunneling Protocols
1. Point to Point Tunneling Protocols

40. Point-to-Point Tunneling Protocol
– Microsoft’s Implementation of VPN
– Data is first encapsulated inside PPP packets
– PPP packets are then encapsulated in GRE packets
and sent over the link
– Developed as an extension of the
Point-to-Point Protocol (PPP).
– PPTP tunnels or encapsulates, IP, IPX, or NetBEUI
protocols inside of PPP datagrams
– It uses Microsoft Point-to-Point Encryption (MPPE)

41. L2F: Layer 2 Forwarding Protocol
• The Layer 2 Forward protocol (L2F) is used to establish a
secure tunnel across a public infrastructure (such as the
Internet) that connects an ISP POP to an enterprise home
gateway. This tunnel creates a virtual point-to-point
connection between the user and the enterprise customer's
network.
• Layer Two Forwarding protocol (L2F) permits the tunneling of
the link layer (i.e., HDLC, async HDLC, or SLIP frames) of
higher-level protocols.
• L2F allows encapsulation of PPP/SLIP packets within L2F. The
ISP NAS and the Home gateway require a common
understanding of the encapsulation protocol so that SLIP/PPP
packets can be successfully transmitted and received across
the Internet.

42. Tunneling Protocols
2. Layer 2 Tunneling Protocols

43. Layer 2 Tunneling Protocols
• L2TP is an industry-standard Internet tunneling
protocol with roughly the same functionality as the
Point-to-Point Tunneling Protocol (PPTP).
• Like PPTP, L2TP encapsulates Point-to-Point Protocol (PPP)
frames, which in turn encapsulate IP, IPX, or NetBEUI
protocols
• With L2TP, the computer performs all security checks and
validations, and enables data encryption, which makes it
much safer to send information over nonsecure networks by
using the new Internet Protocol security (IPSec)
• In this case data transfer through a L2TP-enabled VPN is as
secure as within a single LAN at a corporate site

44. Encryption of an L2TP packet with IPSec ESP

45. L2TP packet with IPSec ESP
• L2TP is a combination of PPTP and Layer 2 Forwarding (L2F), a
technology proposed by Cisco Systems, Inc. L2TP represents the
best features of PPTP and L2F. L2TP encapsulates PPP frames to
be sent over IP, X.25, Frame Relay, or Asynchronous Transfer
Mode (ATM) networks. When configured to use IP as its datagram
transport, L2TP can be used as a tunneling protocol over the
Internet
• L2TP over IP internetworks uses UDP and a series of L2TP
messages for tunnel maintenance. L2TP also uses UDP to send
L2TP-encapsulated PPP frames as the tunneled data.
• In Windows 2000, IPSec Encapsulating Security Payload (ESP) is
used to encrypt the L2TP packet. This is known as L2TP/IPSec.

46. PPTP Compared to L2TP/IPSec
PPTP
1. Data encryption begins
after the PPP connection
process (and, therefore,
PPP authentication) is
completed
2. PPTP connections use
MPPE, a stream cipher
that is based on the
Rivest-Shamir-Aldeman
(RSA) RC-4 encryption
algorithm and uses 40,
56, or 128-bit encryption
keys. Stream ciphers
encrypt data as a bit
stream
L2TP/IPSec
1. Data encryption begins
before the PPP connection
process by negotiating an
IPSec security association
2. L2TP/IPSec connections
use the Data Encryption
Standard (DES), which is a
block cipher that uses
either a 56-bit key for DES
or three 56-bit keys for 3-
DES. Block ciphers encrypt
data in discrete blocks
(64-bit blocks, in the case
of DES).

47. PPTP Compared to L2TP/IPSec
PPTP
3. PPTP connections require only
user-level authentication
through a PPP-based
authentication protocol .
4. PPTP provides only per-packet
data confidentiality.
L2TP/IPSec
3. L2TP/IPSec connections require
the same user-level
authentication and, in addition,
computer-level authentication
using computer certificates.
4. IPSec provides per packet data
authentication (proof that the
data was sent by the authorized
user), data integrity (proof that
the data was not modified in
transit), replay protection
(prevention from resending a
stream of captured packets), and
data confidentiality (prevention
from interpreting captured
packets without the encryption
key).

48. PPTP Compared to L2TP/IPSec
PPTP
5. PPTP does not require a
certificate infrastructure
6. PPTP can be used by
computers running Windows
XP, Windows 2000, Windows
NT version 4.0, Windows
Millennium Edition (ME),
Windows 98, and Windows 95
with the Windows Dial-Up
Networking
7. PPTP clients and server can
be placed behind a network
address translator (NAT) if
the NAT has the appropriate
editors for PPTP traffic
L2TP/IPSec
5. L2TP/IPSec requires a certificate
infrastructure for issuing
computer certificates to the VPN
server computer
6. L2TP/IPSec can only be used with
Windows XP and Windows 2000
VPN clients.
7. L2TP/IPSec-based VPN clients or
servers cannot be placed behind
a NAT both support IPSec NAT
Traversal (NAT-T).

49. Internet Protocol security (IPSec)
• IPSec is a Layer 3 protocol standard that
supports the secured transfer of information
across an IP internetwork. In addition to its
definition of encryption mechanisms for IP
traffic, IPSec defines the packet format for an
IP over IP tunnel mode, generally referred to
as IPSec tunnel mode. An IPSec tunnel consists
of a tunnel client and a tunnel server, which
are both configured to use IPSec tunneling and
a negotiated encryption mechanism.

50. Internet Protocol security (IPSec)
• IPSec tunnel mode uses the negotiated security
method (if any) to encapsulate and encrypt entire
IP packets for secure transfer across a private or
public IP internetwork.
• The encrypted payload is then encapsulated again
with a plain-text IP header and sent on the
internetwork for delivery to the tunnel server.
• Upon receipt of this datagram, the tunnel server
processes and discards the plain-text IP header,
and then decrypts its contents to retrieve the
original payload IP packet. The payload IP packet
is then processed normally and routed to its
destination on the target network

51. Internet Protocol security (IPSec)
• IPSec provides machine-level authentication, as well
as data encryption.
• IPSec negotiates between your computer and its
remote tunnel server before an L2TP connection is
established, which secures both passwords and data
• Created to add Authentication, Confidentiality, and
Integrity to IP traffic

52. Internet Protocol security (IPSec)
• IPSec tunnel mode has the following features and limitations:
• It supports IP traffic only.
• It functions at the bottom of the IP stack; therefore,
applications and higher-level protocols inherit its behavior.
• It is controlled by a security policy—a set of filter-matching
rules. This security policy establishes the encryption and
tunneling mechanisms available, in order of preference, and the
authentication methods available, also in order of preference.
As soon as there is traffic, the two computers perform mutual
authentication, and then negotiate the encryption methods to
be used. Thereafter, all traffic is encrypted using the negotiated
encryption mechanism, and then wrapped in a tunnel header

53. IPsec
• Definition: (Webopedia)
– Short for IP Security, a set of protocols
developed by the IETF to support secure
exchange of packets at the IP layer. IPsec
has been deployed widely to implement
Virtual Private Networks (VPNs)

54. Goal of IPsec
• Provides security services at IP layer
– Access control
– Integrity
– Data origin Authentication
– Rejection of replayed packets
– Confidentiality

55. IPsec Architecture
• Components
– Security Protocols
– Security Associations
– Key Management
– Algorithms for authentication and
encryption

56. Security Protocols
• Authentication Header (AH)
– Data Origin Authentication
– Anti-replay service
– Data Integrity
• Encapsulating Security Payload (ESP)
– Confidentiality
– Data Origin Authentication
– Anti-replay service
– Connectionless Integrity

57. Applications of IPSec
• IPSec provides the capability to secure
communications across a LAN, across
private and public WANs, and across
the Internet. Examples of its use
include:
– Secure branch office connectivity over the
Internet
– Secure remote access over the Internet

58. Applications of IPSec
• Establishment of extranet and intranet
connectivity with partners
• Enhancement of electronic commerce
security
• encrypt or authenticate all traffic at the
IP level

59. Applications of IPSec
• Using IPSec all distributed applications can
be secured,
– Remote logon,
– client/server,
– e-mail,
– file transfer,
– Web access
– etc.

60. Applications of IPSec

61. Where can IPSec be used
• These protocols can operate in
– networking devices,
• such as a router or firewall
– or they may operate directly on the
workstation or server.

62. How can IPSec be used
• Secure Communications between devices
– Workstation to Workstation
– Protection against data changes
• Accidental or Intentional
– Contents can be hidden
• Secure communicatoins through IPSec
tunnels

63. Benefits of IPSec
• The benefits of IPSec include:
– Strong security that can be applied to all
traffic crossing the perimeter.
– Transparent to applications.
– No need to change software on a user or
server system
• When IPSec is implemented in a router or
firewall

64. Benefits of IPSec
• The benefits of IPSec include:
– IPSec can be transparent to end users.
– There is no need to train users on
security mechanisms
– IPSec can provide security for individual

65. The Scope of IPSec
• IPSec provides three main facilities
– An authentication-only function,
• Referred to as Authentication Header (AH)
– Acombined authentication/ encryption function
• Called Encapsulating Security Payload (ESP)
– A key exchange function.
• IKE (ISAKMP / Oakley)

66. The Scope of IPSec
• Both authentication and encryption are
generally desired,
– (1) assure that unauthorized users do not
penetrate the virtual private network
– (2) assure that eavesdroppers on the Internet
cannot read messages sent over the virtual
private network.
• Because both features are generally
desirable, most implementations are
likely to use ESP rather than AH.

67. Tunnel Types
Tunnels can be created in various ways.
• Voluntary tunnels: A user or client computer can
issue a VPN request to configure and create a
voluntary tunnel. In this case, the user’s computer
is a tunnel endpoint and acts as the tunnel client.
• Compulsory tunnels: A VPN-capable dial-up access
server configures and creates a compulsory tunnel.
With a compulsory tunnel, the user’s computer is
not a tunnel endpoint. Another device, the dial-up
access server, between the user’s computer and
the tunnel server is the tunnel endpoint and acts
as the tunnel client.

68. RADIUS
• The Remote Authentication Dial-in User Service (RADIUS)
protocol is a popular method for managing remote user
authentication and authorization.
• RADIUS is a lightweight, UDP-based protocol.
• RADIUS servers can be located anywhere on the Internet and
provide authentication (including PPP PAP, CHAP, MS-CHAP, MS-
CHAP v2, and EAP) and authorization for access servers such as
NASes and VPN servers.
• RADIUS servers can provide a proxy service to forward
authentication requests to distant RADIUS servers. For example,
many ISPs have joined consortia to allow roaming subscribers to
use local services from the nearest ISP for dial-up access to the
Internet. These roaming alliances take advantage of the RADIUS
proxy service. If an ISP recognizes a user name as being a
subscriber to a remote network, the ISP uses a RADIUS proxy to
forward the access request to the appropriate network.

69. Hardware Software Requirements for
Installing VPN
• Recommended hardware are 450-MHz
Pentium III with at least 256 megabytes
of RAM
• Your server will need to have two
network cards. One card will connect to
the Internet and the other will connect to
the local area network
• Software include server software like
windows 2000 server and client software
like windows 98,windows 2000
professional etc.

70. Connect to the VPN
• From “My Network Places” – Right-Click –
“Properties” – “Create New Connection”

71. Windows 2000 server installation
• Once you have Windows 2000 Server
installed, go to Start | Programs |
Administrative Tools | Routing And
Remote Access to pull up the RRAS
Microsoft Management Console

72. Windows 2000 server installation

73. Windows 2000 server installation

74. Introduction to SSL
• SSL- Developed by Netscape Communication
• SSL – accepted universally on the World Wide Web
for AUTHENTICATED and ENCRYPTED
communication between clients and servers
• IETF standard called Transport Layer Security is
based on SSL
• SSL protocol runs above TCP/IP and below higher
level protocols such as HTTP
• Uses TCP/IP to authenticate itself to an SSL
enabled client

75. What does SSL actually do?
• Fragments messages to be transmitted into
manageable blocks
• Compresses the data
• Encrypts and transmits the data
• Received data is decrypted
• Verified, decompressed
• Reassembled and transmitted to higher layers

76. SSL in TCP/IP Protocol Stack

77. SSL Record Layer
 Receives uninterrupted data from upper
layers
 Fragmentation / Reassemble data
 Compresses/Decompress data
 Encrypt/Decrypt and verification of data

78. SSL Handshake Protocol
 Maintains information about
the current state and next
state called the pending
state
 Once the handshake is
complete, the two parties
have shared secrets used to
encrypt records and compute
keyed messages
authentication codes on their
contents.
 Maintains the handshake
state information of the client
and server and ensures that
the protocol state machines
of client and server work
consistently

79. SSL Record Protocol
 Receives uninterrupted
data from upper layers
 Fragmentation /
Reassemble data
 Compresses/Decompress
data
 Encrypt/Decrypt and
verification of data

80. Design:Secure Library Class Diagram

81. http connection sequence diagram

82. https connection sequence diagram

83. Differencebetween http and https
• http
– Stateless protocol
– Non secure connection
– Non Secure Sockets
• https
– Session based
protocol
– Secure connection
– Secure Sockets

84. How/Why Gateways use SSL
• SSL designed to provide security between client and server and
avoid man-in-the-middle attack
• SSL considers a proxy server as a middleman
• Gateways act as clients and authenticate servers. Client
authentication is not possible.
• Gateway/proxy can internally authenticate client within the
firewall
• Packet Filtering by allowing specific ports for specific traffic.
443 in case of SSL
• SSL can works with gateways that support SOCKS. SOCKS is a
networking proxy protocol that enables hosts on one side of a
SOCKS server to gain full access to hosts on the other side of the
SOCKS server without requiring direct IP-reach ability. SOCKS is
often used as a network firewall, redirecting connection requests
from hosts on opposite sides of a SOCKS server. The SOCKS
server authenticates and authorizes requests, establishes a
proxy connection, and relays data between hosts.

85. Gateways and SSL
• Proxy Server should
support SOCKS to support
SSL
• With SOCKS, DNS is the
responsibility of the client
• SSL tunneling, DNS is the
responsibility of the proxy
• Proxy Server can spoof
mock on behalf of internal
client. Makes connection
faster

86. SSL brief
• Secure Sockets Layer (SSL) is a technique used on
the Web that operates between the application and
transport layers.
• SSL combines symmetric encryption with digital
signatures. SSL has four steps:
– Negotiation: browser and server first agree on
the encryption technique they will use (e.g.,
RC4, DES).
– Authentication: the server authenticates itself
by sending its digital signature to the browser.
– Symmetric Key Exchange: browser and server
exchange sym. keys used to encrypt outgoing
messages.
– Sym. Key Encryption w/ Dig. Signatures:
encrypted messages are then sent that include
digital signatures.

87. Cryptography

88. The Scenario

89. The Scenario
Alice

90. Bob
The Scenario

91. Oscar
The Scenario

92. Nosy Neighbor
Sender Recipient
Insecure Channel
The Scenario

93. Nosy Neighbor
Sender Recipient
Insecure Channel
The Scenario

94. Nosy Neighbor
Sender Recipient
Insecure Channel
The Scenario

95. Nosy Neighbor
Sender Recipient
The Scenario
THE INTERNET

96. Nosy Neighbor
Sender Recipient
THE INTERNET
The Scenario

97. Contents
• What is cryptography?
• Symmetric Key
• Asymmetric Key
• How it works
• Encryption Algorithms/Standards

98. What is Cryptography?
Cryptography is the science of using
mathematics to encrypt and decrypt data.
Cryptography enables you to store sensitive
information or transmit it across insecure
networks (like the Internet) so that it cannot
be read by anyone except the intended
recipient.

99. Symmetric key
• Alice and Bob agree on an encryption
method and a shared key.
• Alice uses the key and the encryption
method to encrypt (or encipher) a
message and sends it to Bob.
• Bob uses the same key and the related
decryption method to decrypt (or
decipher) the message.

100. Symmetric (Shared-Key) Cryptosystems
Nosy Neighbor
Sender Recipient
Insecure Channel

101. Symmetric Key Cryptography
EncryptionEncryption
““The quickThe quick
brown foxbrown fox
jumps overjumps over
the lazythe lazy
dog”dog”
““AxCv;5bmEseTfid3)fAxCv;5bmEseTfid3)f
GsmWe#4^,sdgfMwirGsmWe#4^,sdgfMwir
3:dkJeTsY8Rs@!3:dkJeTsY8Rs@!
q3%”q3%”
““The quickThe quick
brown foxbrown fox
jumps overjumps over
the lazythe lazy
dog”dog”
DecryptionDecryption
Plain-text inputPlain-text input Plain-text outputPlain-text outputCipher-textCipher-text
Same keySame key
(shared secret)(shared secret)

102. Symmetric Pros and Cons
• Strength:
– Simple and really very fast (order of
1000 to 10000 faster than asymmetric
mechanisms)
•Super-fast (and somewhat more
secure) if done in hardware (DES,
Rijndael)
• Weakness:
– Must agree the key beforehand
– Securely pass the key to the other party

103. Asymmetric ( Public Key)
• Knowledge of the encryption key
doesn’t give you knowledge of the
decryption key
• Receiver of information generates a
pair of keys
– Publish the public key in a directory
• Then anyone can send her messages
that only she can read

104. Asymmetric ( Public Key)
• Alice generates a key value (usually a
number or pair of related numbers) which
she makes public.
• Alice uses her public key (and some
additional information) to determine a
second key (her private key).
• Alice keeps her private key (and the
additional information she used to
construct it) secret.

105. Asymmetric Key
• Bob (or Carol, or anyone else) can use
Alice’s public key to encrypt a message
for Alice.
• Alice can use her private key to decrypt
this message.
• No-one without access to Alice’s private
key (or the information used to construct
it) can easily decrypt the message.

106. Asymmetric (Public-Key) Cryptosystems
Nosy Neighbor
Sender Recipient
Insecure Channel
Bob’s
Public Key
My
Private Key
Bob’s
Public Key

107. Asymmetric ( Public Key)
EncryptionEncryption
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jumps overjumps over
the lazy dog”the lazy dog”
““Py75c%bn&*)9|Py75c%bn&*)9|
fDe^bDFaq#xzjFr@g5=fDe^bDFaq#xzjFr@g5=
&nmdFg$5knvMd’rkveg&nmdFg$5knvMd’rkveg
Ms”Ms”
““The quickThe quick
brown foxbrown fox
jumps overjumps over
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DecryptionDecryption
Clear-text InputClear-text Input Clear-text OutputClear-text OutputCipher-textCipher-text
DifferentDifferent keyskeys
Recipient’s publicRecipient’s public
keykey
Recipient’sRecipient’s
private keyprivate key
privatprivat
ee
publicpublic

108. Public Key Pros and Cons
• Weakness:
– Extremely slow
– Susceptible to “known ciphertext” attack
– Problem of trusting public key
• Strength
– Solves problem of passing the key
– Allows establishment of trust context between
parties

109. How it works?
• A cryptographic algorithm, or cipher, is a
mathematical function used in the encryption and
decryption process
• A cryptographic algorithm works in combination
with a key — a word, number, or phrase — to
encrypt the plaintext. The same plaintext encrypts
to different cipher text with different keys.
• The security of encrypted data is entirely
dependent on two things: the strength of the
cryptographic algorithm and the secrecy of the key.

110. Encryption Algorithms/ Standards
Symmetric
• DES (Data Encryption Standard) is still the most popular
– Keys very short: 56 bits
– Brute-force attack took 3.5 hours on a machine costing
US$1m in 1993. Today it is done real-time
– Triple DES (3DES) more secure, but better options about
– Just say no, unless value of data is minimal
• IDEA (International Data Encryption Standard)
– Deceptively similar to DES, and “not” from NSA
– 128 bit keys
• RC2 & RC5 (by R. Rivest)
– RC2 is older and RC5 newer (1994) - similar to DES and
IDEA
• Blowfish, Twofish
– B. Schneier’s replacement for DES, followed by Twofish,
one of the NIST competition finalists

111. Rijndael (AES)
• Standard replacement for DES for US government, and,
probably for all of us as a result…
– Winner of the AES (Advanced Encryption Standard)
competition run by NIST (National Institute of
Standards and Technology in US) in 1997-2000
– Comes from Europe (Belgium) by Joan Daemen and
Vincent Rijmen. “X-files” stories less likely (unlike
DES).
• Symmetric block-cipher (128, 192 or 256 bits) with
variable keys (128, 192 or 256 bits, too)
• Fast and a lot of good properties, such as good immunity
from timing and power (electric) analysis
• Construction, again, deceptively similar to DES (S-boxes,
XORs etc.) but really different

112. RC4
• Symmetric
– Fast, streaming encryption
• R. Rivest in 1994
– Originally secret, but “published” on sci.crypt
• Related to “one-time pad”, theoretically most
secure
• But!
• It relies on a really good random number
generator
– And that is the problem
• Nowadays, we tend to use block ciphers in
modes of operation that work for streams

113. RSA

114. RSA, DSA, ElGamal, ECC
Asymmetric
– Very slow and computationally expensive – need a computer
– Very secure
• Rivest, Shamir, Adleman – 1978
– Popular and well researched
– Strength in today’s inefficiency to factorise into prime numbers
– Some worries about key generation process in some implementations
• DSA (Digital Signature Algorithm) – NSA/NIST thing
– Only for digital signing, not for encryption
– Variant of Schnorr and ElGamal sig algorithm
• ElGamal
– Relies on complexity of discrete logarithms
• ECC (Elliptic Curve Cryptography)
– Really hard maths and topology
– Improves RSA (and others)

115. MD5, SHA
• Hash functions – part of the digital signature
• Goals:
– Not reversible: can’t obtain the message from its hash
– Hash much shorter than original message
– Two messages won’t have the same hash
• MD5 (R. Rivest)
– 512 bits hashed into 128
– Mathematical model still unknown
– Recently (July 2004) broken, do not use on its own
• SHA (Secure Hash Algorithm)
– US standard based on MD5
– SHA-0 broken (July 2004), SHA-1 probably too weak (partly
broken), use SHA-256 at least

116. Diffie-Hellman, “SSL”, Certs
• Methods for key generation and exchange
• DH is clever since you always generate a
new “key-pair” for each asymmetric
session
– STS, MTI, and certs make it even safer
• Certs (certificates) are the most common
way to exchange public keys
– Foundation of Public Key Infrastructure (PKI)
• SSL uses a protocol to exchange keys
safely

117. RSA
• Factoring large composite numbers is a
“hard” problem
– If we have two “large” primes p and q, it is
“hard” to recover p and q if all we know is
n, where n = pq
• If b is relatively prime to (p-1)(q-1), then
xb
mod n is a trapdoor one-way function
– To feasibly compute its inverse requires
knowledge of p and q

118. The Birthday Attack
• A birthday attack refers to a class of brute-force
attacks, which gets its name from the surprising
result that the probability that two or more people
in a group of 23 share the same birthday is greater
than 1/2; such a result is called a birthday paradox.
• Mathematically, if some function, when supplied
with a random input, returns one of k equally-likely
values, then by repeatedly evaluating the function
for different inputs, we expect to obtain the same
output after about 1.2k^1/2.
• For the above birthday paradox, replace k with 365.

119. The Birthday Attack
• Birthday attacks are often used to find collisions of
hash functions
• To avoid this attack, the output length of the hash
function used for a signature scheme can be
chosen large enough so that the birthday attack
becomes computationally infeasible.

120. Conclusions

121. Summary
Introduction
Threats
Firewall
Packet-filtering firewalls
Firewall policies and rules
Common Problem with Packet Filtering
Virtual Private Networks
IPSec (Internet Protocol Security)
SSL (Secure Socket Layer)
Cryptography
Symmetric Key Signatures
Public key Signatures
The Birthday Attack

122. .
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