Part I  General network security concepts

New Security Concerns

Security threats

TCP/IP flaws

 

Part II  Current Security Technologies

1.      IPSec

2.      SSL

3.      Kerberos

4.      VPN

5.      Firewalls

6.      Certificates

7.      Router & Switch security

8.      .NET security solutions

 

Part III  Business Model and Infrastructure

Business model: e-Commerce business

Enterprise Security Model (Focus on DMZ  security)

Security policy

Proposed security solutions with .Net technology

 

Part IV  Simulation

Simulation - performance analysis

Attacking and hacking simulations using some hacking tools
ECE1800 project

 

Implementing Network Security of Intranets and Extranets

 

 

Introduction

 

In recent years, Intranets and Extranets have become more widely deployed in many organizations. Many of these organizations are now extending their Intranets to reach key customers and business partners via Extranets. Undoubtedly, Intranets and Extranets offer clear cost savings and ease of installation compared with the expensive leased line networks or WANs based on proprietary technology. In addition, they enable highly productive and cost effective new ways for organizations to communicate with their customers, and collaborate with their business partners. However, Intranets and Extranets build on the public Internet which uses TCP/IP protocol (IPV4) as their communication language. The success of the Internet is its open design and it is flexible, powerful and well-suited to communication between a variety of platforms with divergent capabilities. However, its success also causes security problem for information transmitted over it. TCP/IP was designed to be open and had no security consideration in mind at the beginning. There are a number of serious security flaws inherent in TCP/IP protocol suite, which can be used to launch the malicious attacks. The new security challenges have emerged and become more and more critical for its business employment.

 

In this report, we outline the new security concerns for a organization to deploy Intranets and Extranets. We take an in-depth look at some security flaws of TCP/IP protocol and current security threats. In order to implement the network security in a corporation's Intranet and Extranets, we introduce new security technologies and services. Based on these technologies, we design our security solutions for a security sensitive e-commerce corporation to implement Intranet and Extranet. Finally, we conduct some tests and simulations to verify our work.

 

 

1       Basic Security Concepts

 

In this part, we begin with some fundamental concepts of network security. We outline the main security risks for an organization to deploy Intranets and Extranets. We describe some security vulnerabilities in the TCP/IP protocol suite that gives attackers many opportunities for malicious attacks on Internet connected computers. We also present the methods and techniques that attackers use and our defence solutions.

 

1.1             New Security Concerns

 

Information is the key asset in most organizations. Companies gain a competitive advantage by knowing how to use their information. As Intranets and Extranets have become widely deployed by more and more organizations, the growth in network complexity has increased the potential risks to organizational confidential information. The threat comes from others who would like to acquire the information or limit business opportunities by interfering with normal business processes. Information security has become a new challenge and a critical problem to these security-sensitive organizaitons. Figure 1 illustrates the potential points of attack for an organization to employ the Intranet and Extranet.

 

 

 

Figure 1: Expanding Networks Increase Possible Points of Attack

 

 

Intranet and Extranet security breaches can take a variety of forms. For example:

Ø      The organizational information can be interrupted, intercepted, modified and fabricated when it is transmitted from its internal network to the customers, branch offices, partners and remote employees over the public network like Internet

Ø      An unauthorized person might gain access to a company's computer system

Ø      Users may share documents between geographically separated offices over the Internet or Extranet

Ø      Telecommuters accessing the corporate Intranet from their home computer can expose sensitive data as it is sent over the wire

Ø      Inside users (e.g. employees) authorized to use the system for one purpose might use it for another, etc

 

In our project, we assume the insiders in an organization are fully trustful. We will only consider the treats from outside and address mainly network security.

 

1.2 Security Goal

 

Organizational valuable and sensitive information should be secure while making it readily available. Information security can be defined as:

Information security = Confidentiality + Integrity + Availability + Authentication

The goal of our security policy is to ensure that each of the four fundamental components are adequately addressed:

 

Ø      Confidentiality: sometimes referred to as privacy, is the protection of information from unauthorized disclosure.  Most organizations must provide information to some individuals while blocking access for someone else. Only users who need to view sensitive information are authorized to do so. This can be achieved either by restricting access to the information or by encryption to ensure the intercepted information unreadable.

Ø      Integrity: refers to the ability to protect information, data or transmissions from unauthorized, uncontrolled or accidental alterations as it travels between senders and recipients. Data integrity is achieved by preventing unauthorized or improper changes to data, ensuring internal and external consistency. Digital signatures validate a user's identity, so that message recipient can be sure that message senders are who they claim they are. Digital signatures also provide strong evidence that the message has not been altered since it was signed.

Ø      Availability: refers to the resources on the organization network must always be accessible to people. This applies to everything from servers and services to the information stored in a particular database. An unauthorized user can compromise the system and fill mailboxes with unsolicited commercial e-mail or disable large portions of the Intranet. Limiting the number of entry points to the network and restricting the number of people with access to these resources are two key methods for ensuring that your resources remain available.

Ø      Authentication: refers to the process of verifying the claimed identity of an individual, station or originator and determine whether to grant a user permission to connect to the system.  ?????                                                  

 

Why do we need to protect our information? Where do these security threats come from? Because the Internet.  In a local LAN, where information is only limited to use in the internal network, the organizational information is fully secure if not considering the inside attacks. The birth of the Internet brought endless business opportunities to the e-Commerce companies. As these organizations are migrating to the Internet, the security threats also emerge since IP-based network data is wide open to tampering and eavesdropping. Let's first take a deep look at the vulnerabilities and flaws in TCP/IP protocol suite -- the language of the Internet.

 

 

1.3             TCP/IP Vulnerabilities

 

TCP/IP is the most widely used network communication standard that runs over the Internet. It’s a connectionless, best-effort delivery protocol. The transmission data is broken into packets, each packet with a destination address for the routing through the Internet. The sequence number in each packet is used to identify the order of the packets, and the TCP port number provides a mechanism to direct data to a specific application. The packets travel over the Internet through the different (best possible) ways and assemble when they the destination. Thus, the packets are easily to be incepted, modified and fabricated when they are transmitted over the public network. There are a number of security flaws in TCP/IP protocol suite. These security flaws incur some attacks like packet sniffing, IP spoofing, sequence number spoofing, routing attacks, source address spoofing, authentication attacks, and Denial of Service, etc.

 

 

1.3.1 Packet Sniffing

 

Packet Sinffing is the action of intercepting and reading network traffic that is being transmitted across a shared network communication channel. This is usually done by using utility software checking for interfaces working in promiscuous mode, or physical checking of all the connections. A common target for packet sniffers is user account and password. Since in a normal networking environment, the user accounts, passwords and data information for most protocols (such as HTTP, FTP, Telnet, SNMP, POP3, SMTP, IMAP) are sent across the network in plain-text. Once equipped to eavesdrop, it is not difficult for an attacker to capture packets that contain readable user account and password.

 

1.3.1.1 How does it work

 

Ethernet is the most popular computer network. Ethernet protocol works by sending packets to all networked computers on the same network segment. The packet header contains the proper address of the destination host. The network interface card (NIC) hardware in a networked computer receives every piece of packets that is transmitted across the network. Usually the network device driver software will only process incoming packets which contain the address of its host computer, or broadcast packets. However, the network adapter hardware can be configured to operate in an altered state, That is to be in promiscuous mode.

 

The promiscuous mode is the biggest security flaw of the Ethernet. In promiscuous mode, the network device can process all traffic transmitted across the network and forward the packets to its operating system, no matter whether these packets are addressed to itself or not. This option is used as a debugging tool for network administration. However, it could be misused by an attacker to eavesdrop the traffic transmitted over the network. Sniffing software just works by placing a system's network interface into promiscuous mode. After the network traffic is processed by the Network Interface Card, software mechanisms are used to filter the captured packet, extract and reconstruct the data portion of the packets, and to display in a readable format. Figure ??? (in part 4) shows the FTP basic authentication session was captured using network monitor.

 

1.3.1.2 Defences:

There are several ways to prevent this kind of attacks:

·        Network Segmentation -- A network segmentation can separate the network part with high-level security from the low-level security network part, and minimizes the amount of information that can be collected with a network sniffer or an analyzer which operate in promiscuous mode. Repeaters and passive hubs used for network segmentation do not provide security because the flow of data arrives to any of its interfaces. Instead, routers, switches and bridges can limit the flow of traffic, allow the traffic only go to its destination interface.

·        Encryption -- Encryption schemes can be used to prevent the contents from being read even though an attacker capture the packets. E.g. Public key encryption programs PGP uses various forms of encryption and combines messages with a simple packet format to provide a simple and efficient security mechanism for the transmission of electronic mail (E-mail).

·        Secure Socket Layer (SSL) -- SSL can be built into popular web browser and web servers. It allows encrypted web surfing, and is almost always used in e-commerce when users enter their credit card information.

 

 

1.3.2 IP Spoofing

 

The spoofing attack exploits the fact that IP does not perform a robust mechanism for authentication. It believes that a packet comes from where it claims. Since many systems (such as router access control lists) define which packets may and which packets may not pass based on the sender's IP address, this is a useful technique to an attacker, and is also a strong weakness of TCP/IP protocol. IP addresses can be configured in software, and it is usually easy to configure one address for a machine as if it is another. A packet simply claims to originate from a given address, and there is no way to know if it is or not.

 

To engage in IP spoofing, a hacker first uses a variety of techniques to find an IP address of a trusted port, which is permitted access through the packet-filtering router or firewall, then modifies IP address in the packet header on the external network to gain access to the internal network. It is possible to route packets through the packet-filtering router or firewall if they is not configured to filter incoming packets whose source address is in the local domain. It is possible to spoof even if no reply packets can reach the attacker.

 

Detection:

·        If monitor packets using network-monitoring software such as netlog, and find a packet on your external interface that has both its source and destination IP addressed in the internal network, the internal network is currently under attack.

·        Another way to detect IP spoofing is to compare the process accounting logs between systems on the internal network. If the IP spoofing attack has succeeded on one of the systems, the log entry on the victim machine shows a remote access; on the apparent source machine, there is no corresponding entry for initiating that remote access.

 

Defenses:

1)      Avoid reliance on address-based authentication and trust mechanisms;

2)      Disabling source routing;

3)      Using a screening router or firewall which can intelligently filter network packets based on configurable rules. This can avoid the following attack:

·        Inbound attacks that originate from external networks: implemented by configuring the router to discard incoming datagram with a source address belonging to the internal network.

·        Outbound attacks that originate inside of your own networks: implemented by discarding outgoing datagram with a source address from an external network.

 

1.3.3 TCP Sequence Number Attack

 

This kind of attack uses TCP sequence number prediction to construct a TCP packet sequence without ever receiving any responses from the server. This allows an attacker to spoof a trusted host on a local network.

 

The connection-oriented transport control protocol (TCP) uses a 3-way handshake to establish the connection before transmitting data between the server and client. Telnet is just such an application. When a Telnet session is started, the application layer will request TCP as its transport service in order to insure reliability of the connection.

 

The client sends a connection request to the server by setting the SYN bit and selecting an initial sequence number ISNc, the server acknowledges the request by setting the ACK bit and its own sequence number ISNs, and the client acknowledges the acknowledges by setting the ACK bit. After the 3-way handshake, data transmission can take place. The 3-way handshake can be shown as the following Figure:

 

                    

 

    

 

Figure 2: TCP Three-way Handshake

 

 

Before data transmission, the Client must first receive the sequence number ISNy from the Server. The 3-way handshake protocol can be easily exploited by the attacker.  By monitoring a network connection, a hacker can record the exchange of sequence numbers and predict the next sequence number.

 

The sequence number attack works this way: In this case, the attacker could send the following sequence to impersonate trusted Client (C):

 

 

 

 

 

               Figure 3. TCP Sequence Number  Attack

 

                 

 

 

Even though the message 2 from the Server (S) to the Client(C) didn't go to the Attacker(X ), the attacker was able to know the contents, and could send data at the right time.

In some systems, for example, Berkeley systems, the initial sequence number variable is incremented by a constant amount once per second, and by half that amount each time a connection is initiated. Thus, if the attacker initiates a legitimate connection and observes the ISNs used, he can calculate, ISNs used on the next connection attempt.

 

Defences

 

1)      Randomize the increment of the sequence number attacks, make it difficult to guess or calculate

2)      Use a cryptographic algorithm (or device) for ISNs generation, such as DES.

 

 

1.3.4 Denial-of-Service

 

Denial of service occurs when a hostile entity uses a critical service of the computer system in such a way that no service or severely degraded service is available to others. The DoS attack is simple: send more requests to the machine than it can handle. DoS cause datagram to be discarded before final delivery, effectively blocks the communication path. Some DoS attacks are: SYN Flooding attack, The Ping of Death (ICMP attack), E-mail Bombing, Spoofing Attack DNS, Windows Nuke (newk)OBB Attack on Port 139, etc.

 

Here we briefly describe the SYN Flooding attack and the Ping of Death (ICMP attack):

 

1.3.4.1 SYN Flooding Attack

 

SYN attacks take advantage of a flaw in TCP "three-way handshake"(See Figure 1). In a normal connection, the client sends a SYN message requesting the connection to the server, the server acknowledges the request with SYN/ACK message, the client then sends ACK message acknowledging the approval. The connection between the client and the server is then open, and the service-specific data can be exchanged between the client and the server. The problem arises at the point where the server system has sent an acknowledgment (SYN-ACK) back to the client but has not yet received the ACK message (it is called "half-open connection"). When the server receives the SYN request from the client, it must keep track of the partially opened connection in a "listen queue" for a period of time (typically 75 seconds). The server has built in its system memory a data structure describing all pending connections. This data structure is of finite size, and it can be made to overflow by intentionally creating too many half-open connections.

In DoS attack, an attacker can send a bundle of SYN requests whose source address are set to a routable but unreachable (false return address), which is easily be done by IP spoofing. The final ACK messages will never be sent to the victim server. The server's queue is filled up with pending connections. When the queue limit is reached, TCP drops all new incoming requests until time-out and close the connections. However, the attacker continues sending a new patch of IP-spoofed packets requesting new connections faster than the server can expire the pending connections, and the process begins again. The service is disabled indefinitely.

 

 

 

Figure 4: SYN Flood Attack

 

Defences:

§         Deploying system operating patches: several vendors have released operating system patches to compensate and react to SYN attacks

§         Not running the visible-to-the-world servers at a level too close to its capacity.

§         Using packet filtering to guard against attacks by checking the pattern of information or request, prevent obviously forged packets from entering into the network address space.

§         Using Proxy Server to protect the main server. A proxy server stands between both the client and the server during the connection. A proxy server acts as the "man in the middle" so that there is no direct contact between a client and the server.

 

Denial of service is very easy to launch, but difficult (sometimes impossible) to track. It is becoming a greater problem for the Internet in recent years. It grows at a rate about 50% per year greater than the rate of growth of Internet hosts, although the total number of incidents was small.

 

 

1.3.4.2 Smurf Attack

 

The smurf attack is another kind of DoS attack. It names after the source code employed to launch the attack (smurf.c). The smurf attack employs forged ICMP echo request packets and the direction of those packets to IP network broadcast address. Figure ??? illustrates how a smurf attack works.

 

 

 

 

Figure 5: Smurf Attack

 

 

The attacker (128.100.153.9) issues the ICMP ECHO_REQUEST to the broadcast address of the intermediary network (the network mask is 102.100.36.255). The attacker spoofs the source address using the IP address (207.125.64.39) of the system it wishes to target. When the intermediate network receives the packet with the falsified source address, they respond. The targeted victim system then receives flooding echo replies from all systems on the intermediary network. This flood can overwhelm the targeted victim’s network. Both the intermediate and victim’s networks will see degraded performance, and eventually results in unavailable for the service.

 

 

1.3.4.3 The Ping of Death (ICMP Attack)

 

This attack is also known as a "Ping Flood" attack. It emploited a flaw in many venfors' implementations of ICMP. ICMP is part of the IP protocol using the IP datagram to deliver messages.  PING is a TCP/IP command that simply sends out an IP packet to a specified IP address or host name to see if there is a response from the address or host. Normally a PING (echo request) packet's size is about 32 to 64 bytes. However, the attacker can send a constant stream of over-sized forged PING packets to the target system. In many cases, this flood of traffic can cause an overflow in system's internals, and result in system crashes.                                                                                                                                                                                                                      

Defenses:

§         Blocking ICMP packets on the network firewalls to prevent the traffic from effecting the internal system.

 

 

 

1.3.5 IP Session Hijacking

 

IP Session Hijacking is an action that the attacker takes over the control of the client's session (e.g Telnet). The hijacking attack is usually launched after the user's authentication is complete. An attacker first need to attack the connection by either closing it or messing up the user's SEQ/ACK. Then the attacker takes over the session without being noticed by the user.

 

Session hijacking is a higher level attack. Since when a TCP link is established between the client and the server after the client authenticated itself.  You can not be sure that it will be the same person for the rest of the session. There are techniques to take over the connection. These tools send a message to the client to cut the connection so that the attacker can communicate via the same TCP link to the server.

 

The following are some techniques that an attacker usually use to launch an IP Session Hijacking attack:

 

1.3.5.1 Using reset (RST)

TCP segments have flags which indicate the status of the packet. RST is one flag that tells the receiving TCP module to abort the connection because of some abnormal condition. To be accepted, only the sequence number has to be correct since there is no ACK in a RST packet. When A and B are in the connection, the attacker H watches the traffic between A and B, and calculates the sequence number for A's next sent packet from B's ACK's packet. When B is waiting for A's response, H launch a forged RST packet to B as if it comes from A.

 

1.3.5.2 Using FIN to close a connection

Another flag in TCP segments is FIN, which tells the receiver that the sender does not have any more data to send. This flag is used when closing a connection in a normal legal way. This works almost the same as the former one. Instead of sending RST packet, it sends the FIN packet. The attacker H can pretend to be either A or B, send a FIN packet to the other host, and close the connection between A and B.

 

1.3.5.3 SEQ/ACK mess up

Since TCP separates good and forged packets by their SEQ/ACK numbers, i.e. B trusts the packets from A because of its correct SEQ/ACK numbers. So, if there is a way to mess up A's SEQ/ACK, B would stop believing A's real packet. The attacker H could then impersonate to be A, but using correct SEQ/ACK numbers to connect with B. To mess up A's SEQ/ACK numbers, the attacker simply insert a data packet with correct SEQ/ACK number for B at the right time. Host B would accept the packet, and update ACK numbers. When A continues to send packets to B, the real packets would be dropped.

 

Defences:

This can be solved by using encryption scheme. In this case, the attacker can still take over the session, but he can't see anything because the session is encrypted. The attacker don't have the needed cryptographic key to decrypt the data stream, therefore, be unable to do anything with the hijacked session. In addition, a scheme that authenticates the data’s source throughout the transmission is also needed.

 

 

1.3.6 Source Routing Attack

 

The biggest security hole in TCP/IP protocol is IP source routing. Briefly, IP source routing is an option that can be used to specify a direct route to a destination and return path back to the origination. The route can involve the use of other routers or hosts that normally would not be used to forward packets to the destination. This means if the originator of the connection wishes to specify a particular path for some reason, replies may not reach the originator if a different path is followed. The attacker can then exploits this flaw and use any IP source address desired, including that of a trusted machine on the target's local network. Any facilities available to such machines become available to the attacker.

The following example shows how this can be used such that an attacker's system could masquerade as the trusted client of a particular server is as follows:

1. The attacker would change his host's IP address to match that of the trusted client;

2. The attacker would then construct a source route to the server that specifies the direct path the IP packets should take to the server and should take from the server back to the attacker's host, using the trusted client as the last hop in the route to the server;

3. The attacker sends a client request to the server using the source route;

4. The server accepts the client request as if it came directly from the trusted client and returns a reply to the trusted client;

5. The trusted client, using the source route, forwards the packet on to the attacker's host.

 

Defences

It is rather hard to defend against this sort of attack. The best idea would be for the gateways into the local network to refuse source routing protocol. And also be configured to reject external packets that claim to be from the internal local network.

 

Man-in-the-Middle Attack

 

The "man in the middle" is a rogue program that intercepts all communication between the client and a server with which the client is attempting to communicate via SSL. The rogue program intercepts the legitimate keys that are passed back and forth during the SSL handshake, substitutes its own, and makes it appear to the client that it is the server, and to the server that it is the client.

The encrypted information exchanged at the beginning of the SSL handshake is actually encrypted with the rogue program's public key or private key, rather than the client's or server's real keys. The rogue program ends up establishing one set of session keys for use with the real server, and a different set of session keys for use with the client. This allows the rogue program not only to read all the data that flows between the client and the real server, but also to change the data without being detected.

2       New Technologies

 

Though there are a variety of network attacks, there also exist a variety of security technologies available to address these security holes and provide security services over IP-based netowrk. In this part, we will first briefly give an introduction of these security technologies that we will cover in the next part for deriving our solution to the corporation's Intranet and Extranet. These technologies include IPSec, VPN, SSL (now TLS), Kerberos, Firewall, Certificate, etc.

 

2.1 IPSec (Internet Protocol Security)

 

IPSec is a security framework of open standards designed by Internet Engineering Task Force (IETF) to secure private communications over IP network. IPSec applies at the IP layer and thus offering protection for IP and all upper layer protocols. Security Services provided by IPSec are data authentication, confidentiality, integrity and replay prevention. The greatest advantage of IPSec is it completely transparent for the applications.

 

2.1.1 IPSec Overview

 

IPSec relies on two mechanisms (or protocols), AH (Authentication Header) and ESP (Encapsulating Security Payload). The parameters necessary to the use of these protocols are managed by security associations (SA), an association containing the parameters used to protect a given part of the traffic. SAs are stored in the Security Association Database (SAD) and are managed using the IKE (Internet Key Exchange) protocol. The protection offered by IPSec is based on choices defined in the Security Policy Database (SPD). This database allows to decide, for each packet, if it will be afforded some security services, will be authorized to pass by or will be rejected.

 

IPSec has two modes: transport mode, which protects only the transported data, and tunnel mode, which also protects the IP header. IPSec can be used either on a terminal host or on a security gateway, which allows for both link-by-link and end-to-end security. IPSec can thus be used, in particular, for the creation of virtual private networks (VPNs) or for the protection of remote accesses.

 

2.1.2 How it works

 

Ø      Outbound traffic:

When the IPSec "layer" receives data to be sent, it starts by consulting the policy database (SPD) to determine what processing is required for the packet. If the packet must be afforded security services, the IPSec engine recovers the characteristics of the corresponding SA(s) and consults the SA database (SAD). If the necessary SA already exists, it is used to process the traffic in question. If not, IPSec calls IKE to establish a new SA with the necessary characteristics.

 

Ø      Inbound traffic:

When the IPSec "layer" receives a packet from the network, it examines the header to determine if this packet was afforded IPSec protection and if so what are the SA references. It then consults the SAD to determine the parameters to use for checking and/or the deciphering of the packet. Once the packet is checked and/or deciphered, the SPD is consulted to determine if the required IPSec processing was applied. If the received packet is a "normal" IP packet, the SPD makes it possible to know if it can nevertheless bypass IPSec.

 

 

 

 

 

 

2.1.3 Business deployments

 

By placing IPSec-enabled hardware at different points in the network---routers, firewalls, hosts, as BITW "crypto boxes"--- different security deployments can be realized. End-to-end security can be achieved by deploying IPSec-enabled stacks on hosts. A VPN can be constructed by IPSec-enabled routers protecting traffic between protected subnets. Three basic configurations are possible:

 

Figure ??. Various possible configurations depending on the equipment implementing IPSec

 

The first situation when two distant private network are to be connected using an unreliable network such as the Internet. In such a case, a virtual private network (VPN) is established between the security gateways.

 

The second situation corresponds to the case where mobile users are to securely access the Intranet. The unreliable network can be the Internet, the telephone network ...

 

Lastly, in the third situation, two parties wish to communicate in a secure way but do not have any confidence in the network that separates them.

 

The disadvantage of end-to-end security is that various applications such as QoS solutions, traffic shapping, firewalling and traffic monitoring, which require the ability to "inspect" or modify a transient packet will be unable to make the decisions that they are supposed to make. In addition, Network Address Translation (NAT) will also fail to modify a packet that has been secured.

 

There are also more complex configurations where several security associations, possibly affording different security services:

 

 

 

 

 

                              -Figure ??? Examples of double uses of IPSec

 

In the above example, the first association can ensure the security services required by the external security policy (authentication and confidentiality for example), and the second SA can ensure the services required by the internal security policy (authentication of the terminal host for example).

 

2.2 VPN (Virtual Private Network)

 

2.2.1 VPN Overview

 

A Virtual Private Network (VPN) is a collection of technologies that creates secure connections between two locations over the Internet or any insecure network that uses the TCP/IP protocol suite for communication. It usually achieves this by employing some combination of tunneling, encryption, authentication, access control, and auditing. It provides a virtual “tunnel” through the Internet or other public networks in a manner that provides the same security and features formerly available only in private networks (See Figure 2.2.1). 

 

VPN technology also allows the branch offices or business partners to connect to the corporation over a public network, while still maintaining secure communications. The VPN connection across the Internet logically operates as a Wide Area Network (WAN) link between the sites. It also allows the employees working at home or on the road to connect in a secure fashion to a remote corporate server using the routing infrastructure provided by the Internet. From the user’s perspective, the VPN is a point-to-point connection between the user’s computer and a corporate server.

 

 

Figure 2.2.1: Virtual Private Network

 

2.2.2 Tunneling Concept

 

Tunneling is a method of using an internetwork infrastructure to transfer data for one network over another network. The data to be transferred (or payload) can be the frames (or packets) of another protocol. Instead of sending a frame as it is produced by the originating node, the tunneling protocol encapsulates the frame in an additional header. The additional header provides routing information so that the encapsulated payload can traverse the intermediate internetwork.

The encapsulated packets are then routed between tunnel endpoints over the internetwork. The logical path through which the encapsulated packets travel through the internetwork is called a tunnel. Once the encapsulated frames reach their destination on the internetwork, the frame is unencapsulated and forwarded to its final destination. Tunneling includes this entire process (encapsulation, transmission, and unencapsulation of packets).

Figure : Tunneling

New tunneling technologies have been introduced in recent years. These newer technologies include:

Ø      Point-to-Point Tunneling Protocol (PPTP)

PPTP is a Layer 2 protocol that uses a TCP connection for tunnel maintenance and Generic Routing Encapsulation (GRE) encapsulated PPP frames for tunneled data. The payloads of the encapsulated PPP frames can be encrypted and/or compressed. Figure ?? shows how a PPTP packet is assembled prior to transmission.

 

 

Ø      Layer 2 Forwarding (L2F)

L2F is a runnel technology proposed by Cisco. It is a transmission protocol that allows dial-up access servers to frame dial-up traffic in PPP and transmit it over WAN links to an L2F server (a router). The L2F server then unwraps the packets and injects them into the network. Unlike PPTP and L2TP, L2F has no defined client. L2F functions in compulsory tunnels only.

 

Ø      Layer 2 Tunneling Protocol (L2TP)

L2TP is a combination 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 can also be used directly over various WAN media (such as Frame Relay) without an IP transport layer.

 

 

 

 

Ø      IP Security Protocol (IPSec)

 

IPSec is a Layer 3 protocol standard that supports the secured transfer of information across an IP network. 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.

IPSec Tunnel Mode uses the negotiated security method to encapsulate and encrypt entire IP packets for secure transfer across a private or public IP network. The encrypted payload is then encapsulated again with a plain-text IP header and sent on the network 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.

 

2.2.3 How Tunneling Works

 

For Layer 2 tunneling technologies, a tunnel is similar to a session. Both of the tunnel endpoints must agree to the tunnel and must negotiate configuration variables, such as address assignment or encryption or compression parameters. In most cases, data transferred across the tunnel is sent using a datagram-based protocol. A tunnel maintenance protocol is used as the mechanism to manage the tunnel.

Layer 3 tunneling technologies generally assume that all of the configuration issues have been handled out of band, often by manual processes. For these protocols, there may be no tunnel maintenance phase. For Layer 2 protocols (PPTP and L2TP), however, a tunnel must be created, maintained, and then terminated.

Once the tunnel is established, tunneled data can be sent. The tunnel client or server uses a tunnel data transfer protocol to prepare the data for transfer. For example, when the tunnel client sends a payload to the tunnel server, the tunnel client first appends a tunnel data transfer protocol header to the payload. The client then sends the resulting encapsulated payload across the internetwork, which routes it to the tunnel server. The tunnel server accepts the packets, removes the tunnel data transfer protocol header, and forwards the payload to the target network.

 

 

 

2.4 SSL (Secure Socket Layer)

 

SSL is a security scheme proposed by Netscape Communication Corporation for providing a secure channel between two application hosts. SSL can provide data encryption, server authentication, message integrity, and optional client authentication for a TCP/IP connection. 

 

SSL is the technology used to encrypt and decrypt messages sent between the browser and server. By encrypting the data, you protect messages from being read while they are transferred across the Internet. SSL encrypts a message from the browser, then sends it to the server. When the message is received by the server, SSL decrypts it and verifies that it came from the correct sender (a process known as authentication).

      SSL consists of software installed on both the browser and server.  Several companies, including Verisign, SSL.com, and Equifax offer SSL encryption and authentication tools. Verisign's digital certificates, are already installed in most recent versions of the major browsers.
      

Digital certificates are used by the SSL security protocol to encrypt, decrypt, and authenticate data. The certificate contains the owner's company name and other specific information that allows recipients of the certificate to identify the certificate's owner. The certificate also contains a public key used to encrypt the message being transported across the Internet. SSL uses two kinds of certificates: root certificates and server certificates. Root certificates are installed on the browser, and server certificates exist on the Web server. A root certificate tells the browser that you will accept certificates signed by the owner of the root certificate. A server certificate is installed on the Web server. It works much like the root certificate and is in charge of encrypting the messages sent to browsers and decrypting messages received from browsers.
 

How does it work

 

SSL uses the RSA public key cryptography, which is widely used for authentication

and encryption in the computer industry.  The public key encryption is a technique that uses two asymmetric keys for encryption and decryption. Each pair of keys consists of a public key and a private key. The public key is made public by distributing it widely. The private key is never distributed; it is always kept secret.  Data that is encrypted with the public key can be decrypted only with the private key.  Conversely, data encrypted with the private key can be decrypted only with the public key.

 

SSL handles the scrambling of messages so that only the intended recipient can read it. The encryption/decryption process goes something like this:

1. The user browses to the secure Web server's site.

2. The user's SSL secured session is started and a unique public key is created for the browser (using the certificate authority's root certificate).

3.A message is encrypted and then sent from the browser using the server's public key. The message is scrambled during the transmission so that nobody who intercepts the message can read it.

4.The message is received by the Web server and is decrypted using the server's private key.

      The process of SSL encryption relies upon two keys: the server's public key and private key. The private key only exists on the Web server itself and is used by the Web server to encrypt and decrypt secure messages. The public key exists on any client computer that has installed a root certificate for that Web server. Once the public key is installed, the user can send encrypted messages to and decrypt messages received from the Web server. Figure ? shows this process. Just to be extra safe, the keys are discarded once the transaction's session ends.

Figure ??? Asymmetrical Encription using SSL

      SSL doesn't prevent the message from being intercepted. However, it does make the message useless to the rogue interceptor. In other words, someone could capture the message on its way to the secure Web server, but could not decrypt it because they do not have the server's private key.

      The encryption process can be either symmetric or asymmetric. Symmetric encryption uses a single key by both parties to encrypt and decrypt secure messages. The problem is that the key itself has to be passed along as part of the conversation. With asymmetric encryption, the keys are never transported over the public network, there is never a risk of them being stolen by a attacker

.

 

 

 

 

 

 

 

SHTTP (Secure HTTP)

 

SHTTP is the scheme designed by Enterprise Integration Technologies (EIT). It is a higher level protocol that only works with the HTTP protocol, but is potentially more extensible than SSL.   S-HTTP is backwards compatible with HTTP. It is designed to incorporate different cryptographic message formats into WWW browsers and servers. This will include PEM, PGP, and PKCS-7. Non S-HTTP browsers/servers should be able to communicate with S-HTTP without a discernible difference, unless they request protected documents 

SHTTP provides a wide variety of mechanisms to provide for confidentiality, authentication, and integrity. SHTTP is not tied to any particular cryptographic

system, key infrastructure, or cryptographic format. 

 

                                                                                                                                                                                Kerbros

 

Kerberos was developed by MIT as a solution to the network security problems. It is designed to provide strong authentication for client/server applications by using secret-key cryptography.

Password based authentication is not secure for use over the public network, since passwords sent across the network can be intercepted and used by eavesdroppers to impersonate the user. In addition to the security concern, password based authentication is inconvenient; users need to enter a password each time they access a network service.

The Kerberos protocol uses strong cryptography so that a client can prove its identity to a server (and vice versa) across an insecure network connection. After a client and server has used Kerberos to prove their identity, they can also encrypt all of their communications to assure privacy and data integrity. Thus Kerberos basically provides entity authetication in a centralized environment (authentication between both principles comes from their respective trusts in the central server) and provides a shared secret key agreement mechanism.

 

Overall Functionality

 

At a basic level, the Kerveros protocol brings four entities on the scene: the client, the server with which the client wishes to communicate, the Autehtication server (AS) and the Ticket Granting Server(TGS). These last two, which usuallty reside on the same host, make up the Key Distribution Center (KDC). The whole system is based on the use of tickets, which are issued by the KDC and enable the clients to authenticate when connecting to servers.

The AS contains a database of all the principles) clients and servers) for which it is responsible in its ream. For each such principal, the AS stores information such as the principal’s name and secret key. Hence the AS shares a different secret key with each principal located in its realm. Authentication between the client and the server takes place in three steps. First the client obtains a special Ticket Granting Tag (TGT) from the AS. Next the client obtains a ticket from the TGS for use with the server (this exchange between the client and the TGS is authenticated thanks to the TGT). Finally , the client authenticates with the server using the ticket obtained from the TGS. The reason for the TGS is that it avoids using the client’s secret key each time a ticket is required for a server. The client uses the session secret key generated by the AS to authenticate when tickets will be obtained.

 

 

 

 

 

Symmetric Encryption vs. Asymmetric Encryption (Private Key vs. Public Key)

Symmetric, or private-key, encryption (also known as conventional encryption) is based on a secret key that is shared by both communicating parties. The sending party uses the secret key as part of the mathematical operation to encrypt (or encipher) plain text to cipher text. The receiving party uses the same secret key to decrypt (or decipher) the cipher text to plain text. Examples of symmetric encryption schemes are the RSA RC4 .

Asymmetric, or public-key, encryption uses two different keys for each user: one is a private key known only to this one user; the other is a corresponding public key, which is accessible to anyone. The private and public keys are mathematically related by the encryption algorithm. One key is used for encryption and the other for decryption, depending on the nature of the communication service being implemented.

Public key encryption technologies allow digital signatures to be placed on messages. A digital signature uses the sender’s private key to encrypt some portion of the message. When the message is received, the receiver uses the sender’s public key to decipher the digital signature to verify the sender’s identity.

Certificates

With symmetric encryption, both sender and receiver have a shared secret key. The distribution of the secret key must occur (with adequate protection) prior to any encrypted communication. However, with asymmetric encryption, the sender uses a private key to encrypt or digitally sign messages, while the receiver uses a public key to decipher these messages. The public key can be freely distributed to anyone who needs to receive the encrypted or digitally signed messages. The sender needs to carefully protect the private key only.

To secure the integrity of the public key, the public key is published with a certificate. A certificate (or public key certificate) is a data structure that is digitally signed by a certificate authority (CA)—an authority that users of the certificate can trust. The certificate contains a series of values, such as the certificate name and usage, information identifying the owner of the public key, the public key itself, an expiration date, and the name of the certificate authority. The CA uses its private key to sign the certificate. If the receiver knows the public key of the certificate authority, the receiver can verify that the certificate is indeed from the trusted CA and, therefore, contains reliable information and a valid public key. Certificates can be distributed electronically (through Web access or email), on smart cards, or on floppy disks.

Public key certificates provide a convenient, reliable method for verifying the identity of a sender. IPSec can optionally use this method for end-to-end authentication. Remote access servers can use public key certificates for user authentication.

Extensible Authentication Protocol (EAP)

Most implementations of PPP provide very limited authentication methods. EAP is an IETF-proposed extension to PPP that allows for arbitrary authentication mechanisms for the validation of a PPP connection. EAP was designed to allow the dynamic addition of authentication plug-in modules at both the client and server ends of a connection. This allows vendors to supply a new authentication scheme at any time. EAP provides the highest flexibility in authentication uniqueness and variation.

 

Digital Signature

 

As more and more e-Commerce companies try to increase the security of transactions on the Internet and intranets, more and more use is being made of digital certificates, making them a key component of Internet security.

 

How it works

 

A digital certificate can be thought of as the digital equivalent of an employee badge or a driver's license. The certificate identifies its owner to someone who needs proof of the bearer's identity. The private cryptographic key corresponding to a certificate also can be used to digitally sign documents before they are distributed; correspondents or business partners then use a copy of the digital certificate to confirm the sender's digital identity.

Digital certificates are often used to establish a user's identity for electronic transactions, including browsing Web sites, engaging in electronic commerce, signing E-mail and remotely accessing network resources.

For example, a Web browser and a Web server using the SSL (Secure Sockets Layer) protocol authenticate each other with digital certificates to ensure that each party is who he or she claims to be. The public key contained in a Web server's digital certificate also is used by browsers to encrypt data sent back to the server.

Similarly, the SET (Secure Electronic Transaction) protocol for electronic commerce requires that a digital certificate be issued for each credit card, and that both merchants and customers have digital certificates to prove their identity. (In the case of customers, these are the digital certificates issued for their credit cards.)

Digital certificates also play a major role in guaranteeing digital signatures for such purposes as verifying the authenticity of E-mail. A sender can generate a digital signature for a message using a private key, but recipients of the signed message need the sender's corresponding public key to verify the digital signature. Obtaining a copy of the sender's digital certificate is one way of doing this.

Corporations also can issue digital certificates to their employees, making it possible to control access to network resources based on those certificates. This eliminates the need to remember log-on names and passwords for each workgroup server, printer and other resource. And when employees are on the road, they can use their digital certificates to identify themselves to the corporate firewall when trying to access the company's network.

 

 

3 Business Model and Security implementation

 

In this part, we will take an E-commerce company (we call it the Corporation in the following) as our implementation scenario, and derive security solution by employing the technologies we mentioned in Part 2.

 

3.1 E-commerce Business Requirements

 

Most organizations use Internet-based services to provide enhanced communications and a cost-saving means of automating business processes. Typically an E-commerce (e.g the Corporation) company has network components as show below in Case 1 in Figure 3.1. On one hand, the Corporation has some servers (e.g. SQL server) containing sensitive organizational information, such as financial data or proprietary technology. All these data should be as secure as possible and make sure no unauthorized person can get to it. On the other hand, The Corporation would like to employ inexpensive Internet to communication with its branch offices, distributed business partners and the remote employees. It provides some business services for its customers to access over the Internet to its web server, the ftp server and the e-mail server. It also provides the wireless messaging services over the Internet for the mobile users.

 

 

 

Figure 3.1 Network Infrastructure without Security Consideration

 

 

 

 

 

3.2 Security Infrastructure

 

3.2.1 Network Topology Security

 

When developing a secure Intranet an Extranet, the first concern should be network topology security. The internal network in Case 1 is a typical Ethernet with a hub connecting all the machines. The problem of it is that any device can monitor communications between two machines on the same network segment. As we mentioned above, the Corporation has some confidential information contained in the servers need to keep fully secure. This can be achieved by limiting unauthorized users accessing to them. One good way is segmenting the Internal network from both security and performance point of view.

 

Thus, the Corporation's internal network is segmented into two subnet parts as Case 2 in Figure 3.2 shows: One subnet is the trusted network which contains the confidential organization resources like financial data, proprietary technology or acquisition and merger information, etc. Another is publicly opened and less trusted DMZ (Demilitarized Zone) part, which allows web browse, file transfer and mobile access.

 

 

Figure 2: Segmentation of the Internal Network

 

 

 

In this scenario, A firewall is used here between the Corporation and the Internet to separate the untrusted network (defined as the External network) from the Corporation's trusted network and DMZ part (defined as the Internal network). For this deployment, all the internal system is protected by the firewall from the Internet-based attacks. The router is used between the DMZ and the trusted internal network to provide the second-line protection of the trusted network because the router has the ability to filter out certain traffic. Segmenting networks effectively prevents packets from traversing the entire network, thus reduces the amount of information that can be collected with a network monitor or an analyzer through physical accessing to a segment.

 

The DMZ part lies between the trusted network and the untrusted public network, and serves as an additional protection for the trusted internal network behind the firewall. The problem of this solution is that all the traffic to the trusted network must travel through the DMZ part, which causes overloaded traffic in the DMZ part. The performance of the network is greatly reduced, especially when we apply the security solutions like packet filtering, access control and encryption. In addition, it increases the difficulty for firewall to control the traffic.

 

In order to achieve the better network performance while considering security, we derive our second solution of Case 3 as show in Figure 3.3:

 

 

 

 

 

In this scenario, we completely separate the trusted network and DMZ part. This solution solves the problems in Case 2 and has the following advantages:

Ø      Balance the traffic between two subnets, thus unloading the traffic of the DMZ subnet. The traffic going to the trusted network can be routed directly, no need to pass through the DMZ;

Ø      Since each subnet has its different security policy, it is better for firewall to separate them. It is easy for firewall to control the traffic and also easy to be configured;

Ø      Greatly increase the network performance when a variety of security technologies are implemented.

 

Because of the above three reasons, we prefer to choose the Case 3 as our security infrastructure solution.

 

 

 

 

 

 

Based on this scenario, we design our security policies for the Corporation as following:

Ø      Internal users can access the entire network, including the trusted network, DMZ and the Internet

Ø      External users can access only the DMZ network (a network that provides Mail, FTP and HTTP services)

Ø      Any traffic coming into the corporate the trusted network must be securely authenticated and encrypted

Ø      Allow the traffic from the trusted network to the DMZ part, but block the traffic initiated from DMZ to the trusted netwok

Ø      Remote employ and branch office or business partner must use VPN or IPSec to access to the corporation trusted network

Ø      Authentication??

 

 

Ø      The router is used between DMZ and the trusted subnet can be configured to allow users from the trusted subnet access to DMZ, but don't permit some traffic from DMZ go through the trusted subnet.

     

Ø      The firewall should follow the principle "That which is not Expressly Permitted is prohibited" deny any service unless it is implicitly permitted", i.e. the firewall denies all services by default, unless they are explicitly permitted.

Ø      The basic rules of the firewall:

§         Allow all outgoing packets from the internal network except the packets with a source address from an external network (prevent IP Spoofing);

§         Allow incoming packets with port 80 (HTTP), port 25 (SMTP), port 110 (POP3), port 20-21 (FTP), port 53 (DNS), VPN, IPSec and SSL service ports go to the DMZ, but block the packets with a source address belong to the internal network (prevent IP Spoofing);

§         Allow ICMP packets, and block ICMP nukes;

§         Allow broadcast packets;

§         Allow ARP and RARP;

Ø      Network Address Translation (NAT)

NAT is applied in this scenario to enable the Corporation to maintain unregistered IP addressing schemes and provide Internet access to all internal hosts utilizing a single corporation IP address, thus conceal the internal IP address from the untrusted Internet.

 

 

Remote Access VPN Solution

 

To provide remote users with the ability to connect to the internal network, regardless of their locations, the corporation must deploy a secure remote access solution. The solution must allow roaming or remote clients to connect to LAN resources, and the solution must allow remote offices to connect to each other to share resources and information (LAN-to-LAN connections). In addition, the solution must ensure the privacy and integrity of data as it traverses the Internet.

Ø      Remote User Access

Building up a remote access VPN connection provides remote access to corporate resources over the public Internet, while still maintaining privacy of information. Figure 2 shows a VPN used to connect a remote user to a corporate intranet.

Figure 2: Using a VPN to connect a remote client to a private LAN

Rather than making a long distance (or 1-800) call to a corporate or outsourced Network Access Server (NAS), the user calls a local ISP. Using the connection to the local ISP, the VPN software creates a virtual private network between the dial-up user and the corporate VPN server across the Internet.

Ø      Connecting two remote sites

In order to implement the secure communication between the corporation and its branch offices or business partners, VPNs is used to connect the local area networks at two remote sites. The Corporation gateway that act as a VPN server connect to a local ISP with a dedicated line, while branch offices or business partner can connect to the local ISP either using dedicated link or a dial-up link.

 

 

Figure 3: Using a VPN to connect two remote sites

 

IPSec VPN Employment

2.2.4. 1 IPsec client software

A dial VPN consists of an IPsec gateway and a PC equipped with an IPsec client. The IPsec gateway is located between the corporate office and for the Internet. The IPsec gateway is often a part of the firewall. It permits users that are connected to the Internet to reach resources on the corporate network in a very secure way. The PC can use the Internet at the same time that it is connected to the corporate network. Only packets destined for the corporate network are encrypted via IPsec. 

2.2.4.2 Router with IPsec software

Instead of each PC being equipped with IPsec client software the encryption can also be done in the router that is connected to the public network. The advantage is that you don’t need to change the configuration of the PC’s, the only thing that is necessary is to configure the IP address of the IPsec gateway and the destination IP addresses that are behind the gateway.

 

2.2.4.3 LAN-to-LAN VPNs

When using IPsec in tunnel mode it allows for two private networks to communicate over the public Internet in a very secure way. Both private networks can use private IP addresses, these IP addresses are not visible on the Internet.

Firewall

For a firewall to function as the company desires, the network service access policy should exist prior to the implementation of the firewall. The policy must be realistic and sound. A realistic policy provides a balance between protecting the network from known risks on the one hand and providing users reasonable access to network resources on the other. If a firewall system denies or restricts services, only a strong network service access policy will prevent the firewall's access controls from being modified or circumvented on an ad hoc basis. A sound, management-backed policy can provide this defense against user resistance.

 

A router is a network traffic-managing device that sits in between sub-networks and routes traffic intended for, or emanating from, the segments to which it's attached. Naturally, this makes them sensible places to implement packet filtering rules, based on your security polices that you've already developed for the routing of network traffic.

 

 

A firewall insulates a private network from a public network using carefully established controls on the types of request they will route through to the private network for processing and fulfillment. For example, an HTTP request for a public Web page will be honored, whereas an FTP request to a host behind the firewall may be dishonored. Firewalls typically run monitoring software to detect and thwart external attacks on the site, and are needed to protect internal corporate networks. Firewalls appear primarily in two flavors; application level gateways and proxy servers. Other uses of firewalls include technologies such as Virtual Private Networks that use the Internet to tunnel private traffic without the fear of exposure

Defining firewalls

A slightly more specific definition of a firewall comes from William Cheswick and Steven Bellovin, two engineers with AT&T who wrote the classic Firewalls and Internet Security (Addison Wesley, 1994). They based the book on their experience developing a firewall to protect AT&T connections to the Internet. Cheswick and Bellovin define a firewall as a collection of components or a system placed between two networks and possessing the following properties:

* All traffic from inside to outside, and vice-versa, must pass through it;
* Only authorized traffic, as defined by the local security policy, is allowed to pass through it; and
* The system itself is highly resistant to penetration.

Put simply, a firewall is a mechanism used to protect a trusted network from an untrusted network, usually while still allowing traffic between the two. Typically, the two networks in question are an organization's internal (trusted) network and the (untrusted) Internet. However, nothing in the definition of a firewall ties the concept to the Internet. We traditionally define the Internet as the worldwide network of networks that uses TCP/IP for communications. We define an internet as any connected set of networks. Although many firewalls are currently deployed between the Internet and internal networks, there are good reasons for using firewalls in any internet, or intranet, such as a company's WAN. There will be more about this use of firewalls later in this chapter.

Another approach to firewalls views them as both policy and the implementation of that policy in terms of network configuration. Physically, a firewall comprises one or more host systems and routers, plus other security measures such as advanced authentication in place of static passwords. As shown in Figure 1.1, a firewall may consist of several different components, including filters, or screens, that block transmission of certain classes of traffic, and a gateway, which is a machine or set of machines relaying services between the internal and external networks by means of proxy applications. The intermediate area occupied by the gateway we often refer to as the demilitarized zone (DMZ). These terms will all be explained in more detail, starting with traffic.

Firewalls as Filters

When TCP/IP sends data packets on their merry way, the packets seldom go straight from the host system that generated them to the client that requested them. Along the way they normally pass through one or more routers. In this, TCP/IP transmissions differ from LAN communications, which broadcast over a shared wire.

To look at how TCP/IP routes packets, and how this allows sites to filter for security, let us first examine old-fashioned LAN communications. Suppose five PCs reside on a LAN. If PC #2 wants to send some data to PC #4, it shouts out over the network and hopes that PC #4 hears it. The other three systems on the same wire will also hear the same data. This is true of both Ethernet and Token Ring, the two most widely used LAN protocols. This method of communication, in which a number of computers share the same wiring, increases efficiency, limits distance and scope. It also limits the number of computers that can talk on the same wire.

Early efforts to enable computers to communicate with each other over long distances used telephone lines and switches to connect calls from one specific computer to another in a remote location (the X.25 protocol was developed for this). A connection between two computers might pass through several switches until it reached its final destination. When LANs emerged it made sense for all the computers on one LAN to have access to the machine that had access to the remote connection, thus creating a WAN. LAN protocols, however, were incompatible with X.25, and the machine hosting the connection to the WAN tended to get overworked.

Next came a special type of switch called a router, which could take over the work of making external connections, and could also convert LAN protocols, specifically IP, into WAN protocols. Routers have since evolved into specialized computers. The typical router is about the same size as a VCR, although smaller models and rackmounted units for major interconnections have entered the market.

Basically, routers look at the address information in TCP/IP packets and direct them accordingly. Data packets transmitted over the Internet from the Web browser on a PC in Florida to a Web server in Pennsylvania will pass through numerous routers along the way, each of which makes decisions about where to direct the traffic. Figure 1.3 shows the traceroute program in action, listing the path the data takes.

Figure 1.3: The traceroute program shows the path Internet data takes.

Suppose the Web browser is on a PC on a LAN with a PPP connection to an Internet Service Provider (ISP). A router, or a computer acting as a router, will likely direct the packets out from the LAN to the ISP. Routers at the ISP will send the data to a backbone provider, which will route it, often in several hops, to the ISP that serves the machine that hosts the Web site.

Routers make their routing decisions based on tables of data and rules. It is possible to manipulate these rules by means of filters so that, for example, only data from certain addresses may pass through the router. In effect, this turns a router that can filter packets into an access-control device, or firewall. If the router can generate activity logs, this further enhances its value as a security device. We will discuss how this works in more detail in the next chapter.

Firewalls as Gateways

Internet firewalls are often referred to as secure Internet gateways. Like the gates in a medieval walled city, they control access to and from the network.

In firewall parlance, a gateway is a computer that provides relay services between two networks. A firewall may consist of little more than a filtering router as the controlled gateway. Traffic goes to the gateway instead of directly entering the connected network. The gateway machine then passes the data, in accordance with access-control policy, through a filter, to the other network or to another gateway machine connected to the other network.

In some configurations, called dual-homed gateways, one computer containing two network connectors acts as the gateway. Alternatively, a pair of machines can create a miniature network referred to as the DMZ (see Figure 1.4). Typically, the two gateways will have more open communication through the inside filter than the outside gateway has to other internal hosts. The outside filter can be used to protect the gateway from attack, while the inside gateway is used to guard against the consequences of a compromised gateway [Ches94].

Figure 1.4: The use of gateways.

Firewalls as Control Points

By concentrating access control, firewalls become a focal point for the enforcement of security policy. Some firewalls take advantage of this to provide additional security services, including traffic encryption and decryption. In order to communicate in encryption mode, the sending and receiving firewalls must use compatible encrypting systems. Current standards efforts in encryption and key management have begun to allow different manufacturers' firewalls to communicate securely, but these efforts have a ways to go before the customer can assume compatibility. Firewall-to-firewall encryption is thus used for secure communication over the public Internet between known entities with prior arrangement, rather than for any-to-any connections. Nevertheless it is a powerful feature, enabling the creation of virtual private networks (VPN) as a lower-cost alternative to a leased line or a value-added network (VAN).

Verifying the authenticity of system users is another important part of network security, and firewalls can perform sophisticated authentication, using smart cards, tokens and other methods. Firewalls can also protect other external network connections, such as remote dial-in. A company can apply the same traffic-restricting protections, enhanced by authentication.

Internal Firewalls

While the phenomenal growth of Internet connections has understandably focused attention on Internet firewalls, modern business practices continue to underscore the importance of internal firewalls. Mergers, acquisitions, reorganizations, joint ventures and strategic partnerships all place additional strains on security as the scope of the network's reach expands. Someone outside the organization may suddenly need access to some, but not all, internal information. Multiple networks designed by different people, according to different rules, must somehow trust each other. In these circumstances, firewalls play an important role in enforcing access-control policies between networks and protecting trusted networks from those that are untrusted.

Consider a manufacturing company that has, over time, developed separate networks within the sales, marketing, payroll, accounting and production departments. Although users in one department may wish to access certain other networks, it is probably unnecessary and undesirable for all users to have access to all networks. Consequently, when connecting the networks, the organization may choose to limit the connection, either with packet-filtering routers or with a more complex firewall.

In a WAN that must offer any-to-any connectivity, other forms of application-level security can protect sensitive data. However, segregating the networks by means of firewalls greatly reduces many of the risks involved; in particular, firewalls can reduce the threat of internal hacking--that is, unauthorized access by authorized users, a problem that consistently outranks external hacking in information-security surveys. By adding encryption to the services performed by the firewall, a site can create very secure firewall-to-firewall connections. This even enables wide-area networking between remote locations over the Internet. By using authentication mechanisms on the firewall, it is possible to gain a higher level of confidence that persons outside the firewall who request data from inside the firewall--for example, salespersons on the road needing access to an inventory database--are indeed who they claim to be.

Figure 1.5: Firewall to firewall encryption

Firewalls and Policy

Diagrams of the various configurations of filters and gateways help when planning a firewall defense, but the system administrator must not lose sight of the broader definition of a firewall as an implementation of security policy. A firewall is an approach to security; it helps implement a larger security policy that defines the services and access to be permitted. In other words, a firewall is both policy and the implementation of that policy in terms of network configuration, host systems and routers, as well as other security measures such as advanced authentication in place of static passwords.

Firewall design policy is a lower-level policy that describes how the firewall will actually go about restricting the access and filtering the services as defined in the network service access policy. We will examine both levels of policy in the following sections.

 

Firewall Design Policy

The firewall design policy is specific to the firewall and defines the rules used to implement the network service access policy. The company must design the policy in relation to, and with full awareness of, issues such as the firewall's capabilities and limitations, and the threats and vulnerabilities associated with TCP/IP. As mentioned earlier, firewalls generally implement one of two basic design policies:

1. Permit any service unless it is expressly denied; or
2. Deny any service unless it is expressly permitted.

Firewalls that implement the first policy (the permissive approach) allow all services to pass into the site by default, with the exception of those services that the service-access policy has identified as disallowed. Firewalls that implement the second policy (the restrictive approach) deny all services by default, but then pass those services that have been identified as allowed. This restrictive second policy follows the classic access model used in all areas of information security.

The permissive first policy is less desirable, since it offers more avenues for circumventing the firewall. With this approach, users could access new services not currently addressed by the policy. For example, they could run denied services at non-standard TCP/UDP ports that are not specifically mentioned by the policy.

This is where firewall design comes in. Certain firewalls can implement either a permissive or a restrictive design policy. A company can also choose to locate those systems requiring services that should not be passed through the firewall on screened subnets, separated from other site systems. Some use this approach for Web servers, which are partially shielded by packet filtering but are not sheltered behind the firewall. (If the Web server calls information from, or feeds data to, internal database systems, then that connection between the Web server and the internal machines should be well protected.)

 

 

 

 

 

Security Policy

 

The drawback of the Firewall is that it presents a single point of failure. Firewall generally acts as a choke point, a single point through which all incoming and outgoing network traffic is passed. If the firewall is configured wrong or is down, then the whole internal network is compromised. This concern can be addressed by building some redundancy into the choke point. Since router has the function of filtering traffic and access control, it can be a reasonable consideration.

 

In order to connect from the LAN to the outside world, a proxy is often installed on the Firewall machine.  A proxy is a small program that can see both sides of the firewall. Requests for information from the Web server are intercepted by the proxy, forwarded to the server machine, and the response forwarded back to the requester.  A proxy server mediates traffic between a protected network and the Internet.  Many proxies contain extra logging or support for user authentication.    Since proxies must "understand" the application protocol being used, they can also  implement protocol specific security (e.g., an FTP proxy might be configurable to permit incoming FTP and block outgoing FTP).  Another way of contacting the outside world from behind a firewall is allowing the firewall to pass requests for port 80 that are bound to or returning from the WWW server machine. This has the effect of poking a small hole in the dike through which the rest of the world can send and receive requests to the WWW server machine. 

 

 

 

 

 

 

 

 

 

 

 

 

 

Use network switches instead of network hubs. Using hubs, when a packet arrives at one port of the hub, it is copied to the other ports, so that all segments of the LAN can see all packets. With switches, the packets are only forwarded to the destination port, not to all ports. This also improves network throughput.

 

 

Router

 

Routers are often used to connect two or more networks. They control the flow of data packets on a network and determine the best way to reach the appropriate destination. In our corporate network, we use a router to separate the network segments. We can configure it to route the traffic based on predetermined rules to deny or block unauthorized traffic. For example, we can use filtering commands to limit certain protocols (e.g.snmp) or employ access lists to control the IP addresses that are allowed through. The router provides the second-line of defense for the trusted network.

 

Password

 

Password is the most widely used scheme to authenticate and identify users to the system. Most system or network use password as the only means of authentication and identification. In normal case, passwords are transmitted in plain-text over the public network, for example, when you access a system over a network using Telnet, FTP or rlogin. Passwords can be intercepted and displayed in plain-text using packet sniffer. Figure??? shows the FTP transmission process using a network monitor.

 

The highlight line exposes the packet containing the username and password.

 

 

 

Even if a password is encrypted before transmission, it still can be captured and retransmitted at a later time referred as "reply attack". It can also be cracked by some password cracking software. LophtCrack2.5 is one of such password crackers that can decipher the password in the password file. LophtCrack2.5 uses both Dictionary attack and Brute-force attack to calculate the password. It first dumps a password file of usernames and password, and extracts the password cryptographic hashes, then encrypts each word in a dictionary or every possible combination of letter, number and special characters with the same algorithms used to creat the encrypted password. Every key???It compares each encrypted word against password hash in the password file to find a match. When a match is found, a password is found. Figure ??? shows our testing result using LophtCrack2.5 to crack our system password. It takes  hours 

 

Defenses:

Ø      Making a password longer with a mix of letters, numbers and special characters. The longer the password, the more secure.

Ø      Using one-time password scheme, like smart card or token card.

Ø      Employing Kerberos scheme.

 

 

It is as important to have a secure Web server as it is to maintain a secure mail and database system.

 

if you are concerned that your data is being intercepted and read,consider getting a Secure Socket Layer (SSL) server. Examples of SSL servers include the Netscape Secure Server and the Oracle Secure Server. SSL servers encrypt data before sending it, which makes it almost impossible for anyone to intercept and read the communication going back and forth between your server and the client on the other end.

 

Any browser that can accept SSL will decrypt your information as it is sent back. Your server has a public key and a private key, and your browser has a public key and a private key, and when the browser makes a connection with a server, the two exchanges keys. In this sense, your browser and your server mesh by talking to one another cryptically.

 

You can do something similar with mail. PGP (Pretty Good Privacy) was developed for Internet mail by Phil Zimmerman a few years ago and is another option for maintaining security on an intranet. It can be used for any text transmission over the Internet.

PGP functions in much the same way as browsers do in that PGP exchanges keys with an individual or a site on the Internet.

The information is encrypted before it is sent and is then decrypted on the receiving end. Should someone intercept it in transmission, the information wouldn't be useful.

 

 

 

 

 

 

 

 

 

The Trusted Network is the network that internal employees use when at the office or via a secure controlled dial-in mechanism.

A DMZ (Demilitarized Zone) is an isolated network placed as a buffer area between a company's Trusted Network and the Nontrusted Network. It can be used to hide the design and configuration of the Trusted Network. The DMZ prevents outside users form gaining direct access to the Trusted Network:

§         Filter and manage DoS attacks

§         Scan e-mail messages for virus, content and size

§         Passive eavesdropping /packet sniffing

§         Application-layer attack

§         Port scans

§         Limit access to the Trusted Network via a single protocol

§         IP address spoofing

One of the most common rules is that a single protocol cannot transverse the DMZ. This means if the client is entering into the DMZ via http on port 80, he can't continue into the trusted network on the same port and protocol.

The DMZ is also used to control outbound traffic via proxy servers and filter servers:

§         Control e-mail messages based on destination, size and content

§         Scan for virus going out of the DMZ

§         Monitor and limit access to unauthorized access sites or web sites

 

 

 

Network Security Policy

 

The Network Security Policy identifies the threats against which protection is required, and defines the required level of protection. The Network Security Policy will itself contain several different policies, for example a Network Service Access

Policy and System Specific Policies.

 

The Network Security Policy will be based on a security strategy such as Least Privilege, Defence In Depth, Choke Point,

Weakest Link ,Fail Safe Stance etc. These and other strategies are discussed in chapter 4. The role of the security strategy can be illustrated with a small example :

 

Strategy 1 : Everything is forbidden unless explicitly permitted.

 

Strategy 2 : Everything is permitted unless explicitly forbidden.(11)

 

Implementations of both of these strategies can be found in organisations. They adopt philosophically opposing views of how to

implement security.

 

Some understanding of the services available on the Internet, and the risks these present, is required before an effective

network security policy can be developed.

 

 

Port Scanning

 

A port scanner is a program that listens to well-known port numbers to detect service running on a system that can be exploited to break into the system. If the port is listening, then the scan will succeed, otherwise the port isn’t reachable. The hacker could access those open ports with some type of  application. They could possibly even flood those ports with requests from several sources. Some port scanning attack can be detected by monitoring the system log files using intrusion detection software. However, some scanning programs use a SYN scan, are difficult to detect. These program employs a SYN packet to create a half-open connection that the victim system can't logged. Figure ??? shows a port scanner run on an address: 

The results showed that ports ?? are open. 

 

Ø      ?????

 

Identificaiton: is simply the process of identifying one's self to another entity or determing the identity of the individual or entity with whom you are communicating.

 

Access Control( Authorization): refers to the ability to control the level of access that individuals or entities have to a network or system and how much information they can receive.

 Non-repudiation: the ability to prevent individuals or entities from denying (repudiating) that information, data or files were sent or received or that information or files were accessed or altered, when in fact they were. This capability is crucial to e-commerce.

 

 

 

 

 

 

Certificates  The oldest form of security, is to ask for a password. A password is a classic 'what you know' type of security. Of course, the problem is that anyone can access your information if he knows the password. A certificate (or public-key certificate, or Digital ID)  is a 'What you know and whay you have' type of security. In order to access information you need to have a specific file in your disc, that will authenticate you. Those files are encrypted, to provice a high level of security.  There are many standarts of certificates. The most popular one is X.509v3 (by ITU). A X.509v3 certificate holds the following information :  Name and indentifiying information (organization, for example) of the certificate holder (CH) The public key of the CH Issuer's name : The name of the company that issued the certificate (VerSign, Netscape, etc) Issuer's digital signature Expiration date Serial number The issuer is an entity that attests to the identity of the holder of the certificate. The issuer is usually an external company (like VeriSign) that all it does it to verify the identity.  Certificates are very usefull when extra security is needed. It's your Digital ID, and can be used to indentify you in cyberspace (your electronic network). The certificate proves several important services :  Real-Time encryption over SSL SIngle user login : You can log in once, through your browser (if it supports certificates, that is) and the browser will use that login every time a certificate is needed  Secure E-mail (Over S/MIME). Strong authentication. Certificates work in both ways. If you connect to some server, you can view it's certificate so you'll be sure to whom you're talking. The new browsers also have client-side certificates. So the server can know who YOU are.    Secure E-Mail (S/MIME) S/MIME is a new standard for secure E-Mail. It is an open standard (The specifications are open for all, which means many companies can issue a S/MIME compatiable E-Mail client), which is used for encrypted, signed mail.  S/MIME has these basic features :  Encryption Authentication (Digital signatures) Cross-Platform messaging Tamper detection (it uses a secure hashing function to detect message tampering) The main advantage of S/MIME is it's interoperability, the fact that it's an open standard, and it has a good chance of become the De-Facto standard for secure E-Mail.

 

 Developing Security Polices and Controls

A company's security plan consists of security policies. Security policies give specific guidelines for areas of responsibility, and consist of plans that provide steps to take and rules to follow to implement the policies.

Policies should define what you consider valuable, and should specify what steps should be taken to safeguard those assets. Policies can be drafted in many ways. One example is a general policy of only a few pages that covers most possibilities. Another example is a draft policy for different sets of assets, including e-mail policies, password policies, Internet access policies, and remote access policies.

Two common problems with organizational policies are:

1.                  The policy is a platitude rather than a decision or direction.

2.                  The policy is not really used by the organization. Instead it is a piece of paper to show to auditors, lawyers, other organizational components, or customers, but it does not affect behavior.

A good risk assessment will determine whether good security policies and controls are implemented. Vulnerabilities and weaknesses exist in security policies because of poor security policies and the human factor, as shown in the following diagram. Security policies that are too stringent are often bypassed because people get tired of adhering to them (the human factor), which creates vulnerabilities for security breaches and attacks.

For example, specifying a restrictive account lockout policy increases the potential for denial of service attacks. Another example is implementing a security keypad on the server room door. Administrators may get tired of entering the security PIN number and stop the door from closing by using a book or broom, thereby bypassing the security control. Specifying restrictive password policy can actually reduce the security of the network. For example, if you require passwords longer than seven characters, most users have difficulty remembering them. They might write their passwords down and leave them where an intruder can find them.

To be effective, policy requires visibility. Visibility aids implementation of policy by helping to ensure policy is fully communicated throughout the organization. This is achieved through the plan of each policy that is a written set of steps and rules. The plan defines when, how, and by whom the steps and rules are implemented. Management presentations, videos, panel discussions, guest speakers, question/answer forums, and newsletters increase visibility. If the organization has computer security training and awareness, it is possible to effectively notify users of new policies. It also can be used to familiarize new employees with the organization's policies.

Computer security policies should be introduced in a manner that ensures that management's unqualified support is clear, especially in environments where employees feel inundated with policies, directives, guidelines, and procedures. The organization's policy is the vehicle for emphasizing management's commitment to computer security and making clear their expectations for employee performance, behavior, and accountability.

People like confidentiality and privacy, however attackers can eavesdrop or steal information that is sensitive to a person or organization. If a company comes up with a new innovative product and would like to store the ideas on a computer system, it is going to want protection for that the data on the system and the transferring of data from one system to another. Networks and data communication channels are often insecure, subjecting messages transmitted over the channels to passive and active threats. With a passive threat, an intruder intercepts messages to view the data. This intrusion is also known as eavesdropping. With an active threat, the intruder modifies the intercepted messages. An effective tool for protecting messages against the active and passive threats inherent in data communications is cryptography.

Cryptography is the science of mapping readable text, called plaintext, into an unreadable format, called ciphertext, and vice versa. The mapping process is a sequence of mathematical computations. The computations affect the appearance of the data, without changing its meaning.

To protect a message, an originator transforms a plaintext message into ciphertext. This process is called encryption as shown in following flow diagram. The ciphertext is transmitted over a network or data communications channel. If the message is intercepted, the intruder only has access to the unreadable ciphertext. Upon receipt, the message recipient transforms the ciphertext into its original plaintext format. This process is called decryption.

The mathematical operations used to map between plaintext and ciphertext are cryptographic algorithms. Cryptographic algorithms require the text to be mapped, and at a minimum, require some value that controls the mapping process. This value is called a key. Given the same text and the same algorithm, different keys produce different mappings.

Cryptography is used to provide the following services: authentication, integrity, non-repudiation, and secrecy. In an e-mail message, for example, cryptography provides:

·                     Authentication. Allows the recipient of a message to validate its origin. It prevents an imposter from masquerading as the sender of the message.

·                     Integrity. Assures the recipient that the message was not modified en route. Note that the integrity service allows the recipient to detect message modification, but not to prevent it.

·                     Non-repudiation. There are two types of non-repudiation service. Non-repudiation with proof of origin provides the recipient assurance of the identity of the sender. Non-repudiation with proof of delivery provides the sender assurance of message delivery.

·                     Secrecy. Also known as confidentiality, prevents disclosure of the message to unauthorized users.

Public Key Infrastructures

Public key cryptography can play an important role in providing needed security services including confidentiality, authentication, digital signatures, and integrity. Public key cryptography uses two electronic keys: a public key and a private key. These keys are mathematically related, but the private key cannot be determined from the public key. The public key can be known by anyone while the owner keeps the private key secret.

A Public Key Infrastructure (PKI) provides the means to bind public keys to their owners and helps in the distribution of reliable public keys in large heterogeneous networks. Public keys are bound to their owners by public key certificates. These certificates contain information such as the owner's name and the associated public key and are issued by a reliable certification authority (CA). Digital certificates, also called Digital IDs, are the electronic counterparts to driver licenses, passports, or membership cards. A digital certificate can be presented electronically to prove your identity or your right to access information or services online. Digital certificates are used not only to identify people, but also to identify Web sites (crucial to secure e-business) and software that is being sent over the Web. Digital certificates bring trust and security when you are communicating or doing business on the Internet.

A PKI is often composed of many CAs linked by trust paths. The CAs may be linked in several ways. They may be arranged hierarchically under a "root CA" that issues certificates to subordinate CAs. The CAs can also be arranged independently in a network. This makes up the PKI architecture.

Digital Signatures

Electronic transactions are becoming increasingly important. Many companies offering online services and e-commerce would like to have mechanisms in place to increase confidence in electronic transactions. When a buyer buying a product from a seller hands a bank check (bill of exchange) to the seller he or she has to sign the check verifying his or her identity and making the transaction legal.

The widespread use of PKI technology to support digital signatures can help increase confidence in electronic transactions. For example, the use of a digital signature allows a seller to prove that goods or services were requested by a buyer and therefore demand payment. The use of a PKI allows parties without prior knowledge of each other to engage in verifiable transactions.

For example, a buyer interested in purchasing goods electronically would need to obtain a public key certificate from a CA. The process of obtaining a certificate from a CA is to generate a public-private key pair. The buyer sends the public key with valid information about the company to a registration authority (RA), and asks for a certificate. The RA verifies the buyer's identity based on the information provided and vouches for the identity of the buyer to a CA, who would then issue the certificate.

The newly certified buyer can now sign electronic purchase orders for the goods. The goods vendor receiving the purchase order can obtain the buyer's certificate and the certificate revocation list (CRL) for the CA that issued the buyer's certificate, check that the certificate has not been revoked, and verify the buyer's signature. By verifying the validity of the certificate, the vendor ensures receipt of a valid public key for the buyer; by verifying the signature on the purchase order, the vendor ensures the order was not altered after the buyer issued it.

Once the validity of the certificate and the signature are established, the vendor can ship the requested goods to the buyer with the knowledge that the buyer ordered the goods. This transaction can occur without any prior business relationships between the buyer and the seller.

Secure Sockets Layer

Secure Sockets Layer (SSL) is a protocol that protects data sent between Web browsers and Web servers. SSL also ensures that the data came from the Web site it is supposed to have originated from and that no one tampered with the data while it was being sent. Any Web site address that starts with "https" has been SSL-enabled.

SSL provides a level of security and privacy for those wishing to conduct secure transactions over the Internet. SSL protocol protects HTTP transmissions over the Internet by adding a layer of encryption. This ensures that your transactions are not subject to "sniffing" by a third party.

SSL provides visitors to your Web site with the confidence to communicate securely via an encrypted session. For companies wishing to conduct serious e-commerce, such as receiving credit card numbers or other sensitive information, SSL is a must. Web users can tell when they've reached an SSL-protected site by the "https" designation at the start of the Web page's address. The "s" added to the familiar HTTP—the Hypertext Transfer Protocol—stands for secure.

Companies that want to conduct business via the Internet through and using the capabilities of SSL need to contact a certificate authority, such as VeriSign Inc., which is a third-party organization that confirms a company is indeed what it claims to be. Once that is complete, the company can set up its Web servers for SSL connections. Users don't have to do anything to trigger an SSL connection. The client portion of SSL is built into the Web browser.

Secure E-mail

Standard Internet e-mail is usually sent as plaintext over networks. Intruders can monitor mail servers and network traffic to obtain sensitive information.

There are currently two actively proposed methods for providing secure e-mail security services: Pretty Good Privacy (PGP) and Secure/Multipurpose Internet Mail Extensions (S/MIME). These services typically include authentication of the originator and privacy for the data. They can also provide a signed receipt from the recipient. At the core of these capabilities is the use of public key technology and large-scale use of public keys requires a method of certifying that a given key belongs to a given user.

PGP is a military grade encryption scheme available to all computer users. It works using paired sets of keys. The public key can be used to encode a message that can only be decoded with the matching private key. Likewise, e-mail "signed" with a private key can be verified as authentic with its matching public key.

S/MIME is the same cryptographic method used for secure e-mail, adopted by every major e-mail vendor in the industry. S/MIME uses public key cryptography to digitally sign and encrypt each message sent between trading partners. This ensures that not only can the message not be read, but also that the message came only from the sender and was not altered in transport.

Authentication

Modern computer systems provide a service to multiple users and require the ability to accurately identify the user making a request. In traditional systems, the user's identity is verified by checking a password typed during login; the system records the identity and uses it to determine what operations may be performed. The process of verifying the user's identity is called authentication. Password-based authentication is not suitable for use on computer networks. Passwords sent across the network can be intercepted and subsequently used by eavesdroppers to impersonate the user.

Verifying the identity of someone or something is important. Administrators do not want unauthorized users or imposters to impersonate users. Administrators want to be able to verify that whoever is logging on to a system is who they say they are. Microsoft Windows 2000 supports two types of authentication protocols: Kerberos authentication protocol and NTLM authentication protocol. Kerberos authentication protocol is the default authentication protocol for computers running Windows 2000. NTLM authentication protocol is provided for backward compatibility with other Microsoft operating systems. In this section we are going to outline the various features of each protocol and the application of each protocol.

Kerberos Authentication

Kerberos is designed to provide strong authentication for client/server applications by using secret-key cryptography. The Kerberos protocol uses strong cryptography so that a client can prove its identity to a server (and vice versa) across an insecure network connection. Kerberos is a trusted third-party authentication system, whose main purpose is to allow people and processes (known to Kerberos as principals) to prove their identity in a reliable manner over an insecure network. Instead of transmitting secret passwords in the clear, where they may be intercepted and read by unauthorized parties, principals obtain special Kerberos vouchers (known as session tickets) from Kerberos, which they can use to authenticate themselves to each other. The session ticket lasts only for the session while a user is logged on.

Kerberos authentication requires the existence of a trusted network entity that acts as an authentication server for clients and servers requesting authentication information. This authentication server is known the key distribution center (KDC). It has access to a database consisting of a list of users and client services, their default authentication parameters, their secret encryption keys, and other data. Authentication is typically a one-way process. This is the process by which a service authenticates the client. An advantage of Kerberos over NTLM is that it allows for mutual authentication, where the client authenticates the service.

Kerberos authentication occurs when special authentication model messages, session tickets, are passed among client applications, server applications, and one or more KDCs. Client processes acting on behalf of users authenticate themselves to servers by means of the session ticket. The KDC generates tickets, which are sent to the requesting client processes. Kerberos maintains a set of secret keys, one for every entity to be authenticated within a particular realm (a realm is the Protocols equivalent of a Windows 2000 domain) or domain. A client presents a ticket to the server as evidence that the principal is who it claims to be. The ticket presented to the server "proves" that a KDC authenticated the client.

Kerberos streamlines the process of logging on and accessing resources as opposed to NTLM. In Kerberos authentication, the computer first contacts the KDC for authentication to the network. Then, when the user is ready to access a resource for the first time, the computer contacts the KDC for a session ticket to access the resource. On each subsequent attempt, the computer can simply contact the resource directly, using the same ticket, without having to go to a domain controller first. In this way unnecessary communication with the domain controller is eliminated. This new process allows users to log on faster and gain access to network resources more quickly.

NTLM Authentication

In NTLM authentication, to avoid revealing passwords directly over an untrusted network, a challenge-response system is used. At its simplest, the server sends the user some sort of challenge, which would typically be some sort of random string. The user would then compute a response, usually some function based on both the challenge and the password. This way, even if an intruder captures a valid challenge-response pair, it will not help the intruder gain access to the system since future challenges are likely to be different and thus require different responses.

In Microsoft Windows NT, the client contacts a primary domain controller (PDC) or a backup domain controller (BDC) to log on to the domain. Then, when the client is ready to establish a session with a particular resource, such as a printer share, it will contact server that maintains the resource. The server, in turn, will contact the domain controller that maintains the resource in order to give it the client's required credentials or access token. NTLM is used in Windows 2000 for backward compatibility with other Windows products such as Windows NT. NTLM is also used with the Telnet service in Windows 2000 so users do not transmit their passwords in clear text to the Telnet service. The Telnet service is only implemented on Windows 2000 when Services for Unix is installed.

Smart Cards

Smart Cards are typically credit card type cards that contain a small amount of memory and sometimes a processor. Since smart cards contain more memory than a typical magnetic stripe and can process information, they are being used in security situations where these features are a necessity. They can be used to hold system logon information such as the user's private key along with other personal information on the user including passwords. In a typical smart card logon environment, the user is required to insert his or her smart card into a reader device connected to the computer. Then, the software uses the information stored on the smart card for authentication. When paired with a password and/or a biometric identifier, the level of security is increased. For example, requiring the user to simply enter a password for logon is less secure than having them insert a smart card and enter a password. File encryption utilities which use the smart card as the key to the electronic lock is another security use of smart cards.

Secure Code

Electronic software distribution over any network involves potential security problems. Software can contain programs such as viruses and Trojan horses. To help address some of these problems, you can associate digital signatures with the files. A digital certificate is a means of establishing identity via public key cryptography; code signed with a digital certificate verifies the identity of the publisher and ensures that the code has not been tampered with after it was signed. Certificates and object signing establish identity and let the user make decisions about the validity of a person's identity. When the user executes the code for the first time, a dialog box comes up. The dialog box provides information on the certificate and a link to the certificate authority.

Microsoft developed the Microsoft Authenticode technology, which enables developers and programmers to digitally sign software. Before software is released to the public or internal to the organization, developers can digitally sign the code. If the software is modified after digitally signing the software, the digital signature becomes invalid. In Internet Explorer, you can specify security settings that prevent users form downloading and running unsigned software from any security zone. Internet Explorer can be configured to automatically trust certain software vendors and authorities so that software and other information is automatically accepted.

Technologies to Secure Network Connectivity

Businesses and other organizations use the Internet because it provides useful services. Organization could choose to support or not support Internet-based services based on a business plan or an information technology strategic plan. In other words, organizations should analyze their business needs, identify potential methods of meeting the needs, and consider the security ramifications of the methods along with cost and other factors.

Most organizations use Internet-based services to provide enhanced communications between business units, or between the business and its customers, or provide a cost-savings means of automating business processes. Security is a key consideration—a single security incident can wipe out any cost savings or revenue provided by Internet connectivity.

Some of the ways to protect the organization from outside intrusions include firewalls and virtual private networks (VPN).

Firewalls

Many organizations have connected or want to connect their private LANs to the Internet so that their users can have convenient access to Internet services. Since the Internet as a whole is not trustworthy, their private systems are vulnerable to misuse and attack. A firewall is a safeguard that one can use to control access between a trusted network and a less trusted one. A firewall is not a single component; it is a strategy for protecting an organization's Internet-reachable resources. A firewall serves as the gatekeeper between the untrustworthy Internet and the more trustworthy internal networks.

The main function of a firewall is to centralize access control. If outsiders or remote users can access the internal networks without going through the firewall, its effectiveness is diluted. For example, if a traveling manager has a modem connected to his office computer that he or she can dial into while traveling, and that computer is also on the protected internal network, an attacker who can dial into that computer has circumvented the firewall. If a user has a dial-up Internet account with a commercial ISP, and sometimes connects to the Internet from his or her office computer via modem, he or she is opening an unsecured connection to the Internet that circumvents the firewall. Firewalls provide several types of protection:

·                     They can block unwanted traffic.

·                     They can direct incoming traffic to more trustworthy internal systems.

·                     They hide vulnerable systems that cannot easily be secured from the Internet.

·                     They can log traffic to and from the private network.

·                     They can hide information such as system names, network topology, network device types, and internal user IDs from the Internet.

·                     They can provide more robust authentication than standard applications might be able to do.

As with any safeguard, there are trade-offs between convenience and security. Transparency is the visibility of the firewall to both inside users and outsiders going through a firewall. A firewall is transparent to users if they do not notice or stop at the firewall in order to access a network. Firewalls are typically configured to be transparent to internal network users (while going outside the firewall); on the other hand, firewalls are configured to be non-transparent for outside network coming through the firewall. This generally provides the highest level of security without placing an undue burden on internal users.

Types of firewalls include packet filtering gateways, application gateways, and hybrid or complex gateways.

Packet Filtering Gateways

Packet filtering firewalls use routers with packet filtering rules to grant or deny access based on source address, destination address, and port. They offer minimum security but at a very low cost, and can be an appropriate choice for a low-risk environment. They are fast, flexible, and transparent. Filtering rules are not often easily maintained on a router, but there are tools available to simplify the tasks of creating and maintaining the rules.

Filtering gateways do have inherent risks, including:

·                     The source and destination addresses and ports contained in the IP packet header are the only information that is available to the router in making decision whether or not to permit traffic access to an internal network.

·                     They do not protect against IP or DNS address spoofing.

·                     An attacker will have a direct access to any host on the internal network once access has been granted by the firewall.

·                     Strong user authentication isn't supported with some packet filtering gateways.

·                     They provide little or no useful logging.

Application Gateways

An application gateway uses server programs (called proxies) that run on the firewall. These proxies take external requests, examine them, and forward legitimate requests to the internal host that provides the appropriate service. Application gateways can support functions such as user authentication and logging.

Because an application gateway is considered as the most secure type of firewall, this configuration provides a number of advantages to the medium-high risk site:

·                     The firewall can be configured as the only host address that is visible to the outside network, requiring all connections to and from the internal network to go through the firewall.

·                     The use of proxies for different services prevents direct access to services on the internal network, protecting the enterprise against insecure or badly configured internal hosts.

·                     Strong user authentication can be enforced with application gateways.

·                     Proxies can provide detailed logging at the application level.

Hybrid or Complex Gateways

Hybrid gateways combine two or more of the above firewall types and implement them in series rather than in parallel. If they are connected in series, then the overall security is enhanced; on the other hand, if they are connected in parallel, then the network security perimeter will be only as secure as the least secure of all methods used. In medium to high-risk environments, a hybrid gateway may be the ideal firewall implementation.

Virtual Private Networks and Wide Area Networks

Many organizations have local area networks and information servers spread across multiple locations. When organization-wide access to information or other LAN-based resources is required, leased lines are often used to connect the LANs into a Wide Area Network. Leased lines are relatively expensive to set up and maintain, making the Internet an attractive alternative for connecting physically separate LANs.

The major shortcoming to using the Internet for this purpose is the lack of confidentiality of the data flowing over the Internet between the LANs, as well as the vulnerability to spoofing and other attacks. Virtual private networks use encryption to provide the required security services. Typically encryption is performed between firewalls, and secure connectivity is limited to a small number of sites.

One important consideration when creating virtual private networks is that the security policies in use at each site must be equivalent. A VPN essentially creates one large network out of what were previously multiple independent networks. The security of the VPN will essentially fall to that of the lowest common denominator—if one LAN allows unprotected dial-up access, all resources on the VPN are potentially at risk.

Remote Access

Increasingly, businesses require remote access to their information systems. This may be driven by the need for traveling employees to access e-mail, sales people to remotely enter orders, or as a business decision to promote telecommuting. By its very nature, remote access to computer systems adds vulnerabilities by increasing the number of access points.

Dial-in

Typically the remote computer uses an analog modem to dial an auto answer modem at the corporate location. Security methods for protecting this connection include:

·                     Controlling knowledge of the dial-in access numbers. This approach is vulnerable to automated attacks by "war dialers," simple pieces of software that use auto-dial modems to scan blocks of telephone numbers and locate and log modems.

·                     Username/password pairs. Since an attacker would need to be tapping the telephone line, dial-in connections are less vulnerable to password sniffer attacks that have made reusable passwords almost useless over public networks. However, the use of network sniffers on internal networks, the lack of password discipline, and social engineering make obtaining or guessing passwords easy.

Advanced authentication. There are many methods that can be used to supplement or replace traditional passwords. A few examples are:

·                                Dial-back modems. These devices require the user to enter a username/password upon initial connection. The corporate modem then disconnects and looks up the authorized remote telephone number for the connecting user. The corporate modem then dials the remote modem and establishes a connection.

·                                Public key certificates. The use of public key certificates described earlier when logging on.

·                                Microsoft Challenge Handshake Authentication Protocol (MS-CHAP). This is a variant of CHAP that does not require a plaintext version of the password on the authenticating server.

·                                Microsoft Challenge Handshake Authentication Protocol version 2 (MS-CHAP v2). This provides mutual authentication, stronger initial data encryption keys, and different encryption keys for sending and receiving.

·                                Extensible Authentication Protocol (EAP). This is an extension to the Point-to-Point protocol (PPP) that works with dial-up clients.

The organization's ability to monitor the use of remote access capabilities can also become an issue. The most effective approach is to centralize the modems into remote access servers or modem pools. There should be control in allowing users to connect their own modems to their work computers. In most cases, this should not be allowed due to the fact that it becomes difficult to monitor modems that are not accessed through the firewall and are distributed throughout the organization. They are potential security risks.

Information regarding access to company computer and communication systems, such as dial-up modem phone numbers, should be considered confidential. This information should not be posted on electronic bulletin boards, listed in telephone directories, placed on business cards, or made available. The Network Services Manager should periodically scan direct dial-in lines to monitor compliance with policies and should periodically change the telephone numbers to make it more difficult for unauthorized parties to locate company

Choose the right level of encryption: Performance

The stronger the encryption algorithm, the larger the delay that will be introduced form the encryption and decryption processes. Private/Public encryption schemes, such as RSA, can be 10 to 100 times slower than secret key encryption, such as DES.While the original DES standard has been broken in brute force attacks of only three days, Triple DES standard increases the number of keys is more secure.